April 7, 2013 April 23, 2013
Two weeks ago I said that I was ready to start outdoor planting. Well, several snow storms and a lot of cold temps later we are finally expecting spring to hit this week. My south facing garden bed is now completely snow free and today I decided to see if it is ready for planting.
How does one know if a garden bed is ‘ready for planting’?
First, if you did not clear the dead plant debris last fall, now is the time. It really is best to clear dead debris in the fall so that any diseased material is removed and doesn’t incubate for the following season. It also allows the soil to warm up quicker in the spring. If you have roots, bulbs or perrienal plants that need winter protection, cover them with a layer of fresh straw or chopped leaves rather than leaving their dead stems & leaves standing.
Second, the soil needs to be dry enough to work without causing clumping. This is especially important in high clay soils like those we have in the Red River Valley. Working the soil before it has adequately dried will create clods of soil that become very brick like once they dry. You will not enjoy trying to break them up and neither will your plants.
about 4 inches deep about 2 inches deep
Third, check the temperature of the soil. Most seeds need at least 40F or greater soil temp to germinate. You can use a soil thermometer or if you like, you can do like my Mom & raid your kitchen drawer for a reliable meat thermometer. Insert the thermometer at least two inches deep (seed planting depth) and wait a minute or two to take your reading. Once your soil temp is 40F or greater, the soil is ready for the cool season seeds (peas, kale, spinach, some lettuce, etc). If you are transplanting started plants, take a deeper soil temp to be sure the deeper soil is warm enough for the established roots of the transplants.
Now that I know my south bed is ready for planting, I am looking forward to getting my fingernails dirty later this week. How about you? Do you have a garden bed ready for spring planting?
Just encase you are wondering, my main veggie garden is still under snow as well as most of my front flower bed.
Happy Spring Gardening from the Frozen North! 😉
Those of you who have some experience working your garden know how important it is to monitor the temperature of soil. Knowing the soil temperature tells you when to plant or seed. For example, planting flora or vegetables or seeding should not be performed when the soil is too cold or the plants will die.
Moreover, plants and vegetables tolerate different soil temperatures. Crops that thrive in soil temperatures down to 40°F include arugula, fava beans, kale, lettuce, pak choi, parsnips, peas, radicchio, radishes, and spinach seed.
Vegetables that tolerate soil temperatures at or higher than 50°F include Chinese cabbage, leeks, onions, Swiss chard, and turnips.
Warm-season vegetables can tolerate soil temperatures at or beyond 60°F. They include beans, beets, broccoli, Brussels sprouts, cabbage, carrots and cauliflower. However, experts caution that you should monitor the weather as well because beans will not tolerate frost and may have to be planted twice if the temperature goes below freezing.
Vegetables including tomatoes, eggplants, peppers, cucumbers, squash, corn and melon thrive in soil temperature of 70°F and higher.
Spring bulbs can be planted when soil temperature drops below 60°F.
A soil temperature thermometer is an essential tool for gardeners.
(Courtesy: MaryKlein1 at flickr.com)
It is suggested that you apply crabgrass control in the spring when soil temperatures reach 55°F for four to five days in a row.
Plant cool-season grass seed when soil temperatures are in the 50s F.
Plant shrubs before soil temperature drops below 40°F to give the roots time to grow.
So knowing soil temperature is essential to the life of your crops and plants. Ascertaining soil temperature is achieved with the use of a soil temperature thermometer.
A soil temperature thermometer generally includes a coated probe that can resist corrosion. Still, there is some maintenance that needs to be performed so that corrosion does not develop. For example, experts suggest that you wipe down the thermometer to remove soil and salts to extend the life of the probe. If you live in a wet environment, be sure to wipe the probe clean and dry it before putting it away after use. Many soil thermometers come with some kind of cover including a case or clips to protect them while they are stored.
There are quick reading thermometers that are ideal for a quick probe of soil to ascertain conditions. There are also thermometers that need some time to achieve their reading. This variety of thermometer needs to be left in place in the ground for a few seconds to generate a stable reading.
Soil temperature thermometers cost just a few dollars and can be found at local garden centers.
How To Use A Soil Temperature Thermometer
A soil temperature thermometer can perform a measurement in six steps:
1. Determine the proper depth to perform the measurement. If measuring for a mixed garden, the depth should be at least 5-inches to 6-inches.
2. Use a screwdriver to make a pilot hole. This ensures that the thermometer’s probe will not be damaged if forced into hard soil.
3. Insert the thermometer into the pilot hole and follow the directions supplied with the thermometer.
4. Provide shade if the sun is bright. This can be achieved by simply putting your hand between the sun and the thermometer. This ensures that the reading is accurate.
5. Take a reading in the morning and late afternoon, and then average out the results. If you are seeding a lawn, take measurements on all four sides of your house because some areas warm quicker than others.
6. Check the reading.
(Next time: What is and how to use a compost thermometer)
The question that many amateur gardeners like myself often ask is:
Why are my seeds not germinating and rotting in the soil?
When will the soil be warm enough to sow my seeds or transplant my plants?
Well, I don’t pretend to be an expert in this, but I suspect it has something to do so with the temperature of the soil.
You can purchase or adapt a thermometer for taking soil temperature measurements
Lets talk about soil temperature and how this compare to air temperature, a subject that is interesting beyond gardening.
The first thing to keep in mind is that the temperatures you hear about on TV or read about in the newspaper are for the air temperature, in shade, at a height of roughly 2 meters (6 feet). So when meteorologists talk about surface air temperatures…that is what we mean. And such temperatures SHOULD be taken above native vegetation, which is increasingly rare.
Now the temperature of the ground surface is often quite different than the air temperature. During sunny days, ground temperatures can be much warmer than the air temperature (10-40F is not unusual), and on a cold, clear winter nights, when the ground radiates heat to space, the ground temperature can be 1-8F cooler than air temperature. After relatively warm periods, heat conduction from the warmed soil below can keep the surface temperature warmer than the air temperature at night.
Several of these characteristics are evident in this plot of surface and ground temperatures at the WSDOT road weather site at Silica Road (near Quincy in eastern Washington, along I90) for Sunday. Time advances to the left in this figure. The road temperature zoomed up to roughly 105F, while the air temperature was in the low to mid 60s! Wow. And last night the air temperature was cooler than ground temperature as the former dropped in the upper 30s.
Such super-large temperatures changes just above road surfaces lead to the famous water on the road mirage (see my book for details on this).
Now what about the soil temperatures? The deeper you go into the soil the weaker the daily temperature variation becomes, with the soil temperature increasingly reflecting the average temperatures of the weeks and months before as one descends. Here an example of the soil temperatures at roughly 1 inch (2.4 cm), 6 inches (15 cm), and 12 inches (30 cm). Lots of daily variation in the top inch (in this case a range of 30F), but only a 2F range at 12 inches. And you can see it takes a while for the warming to propagate down into the soil. Go down 5 feet or so and the temperature hardly varies throughout the year.
So what are the current soil temperatures around the State? Lets look at soil temps at 8 inches down from the highly useful Washington State University AgweatherNet . Here are the values today…mid 50s over western Washington and the mid to upper 60s over the western portion of eastern Washington (60 and above are in yellow).
Now lets examine the 8-inch deep soil temperatures in Seattle since January 1 (see graph). A steady rise until the cold spell this week, and now it is rising again with the warmer days. Roughly upper 50s F.
Ok, so what does this have to do with the germination of vegetable seeds….and particularly why my bean seeds are just rotting in the soil?
Take a look at the typical soil temperature required for germination of various seeds (see below). Big variations. Here in western Washington we are now good to go for corn, spinach, carrots, and peas (not shown, but ok at 50F). But beans need soil temps in the 60s. Boy did I make a mistake planting those too early! Next time I will check the soil temperatures before I sow.
Vegetables or not, soil and ground temperatures are fascinating, and during the winter knowledge about them can save your life when roadway icing is threatening.
- Surface and Air Temperature
- SURFACE TEMPERATURE
- AIR TEMPERATURE
- TEMPERATURES CLOSE TO THE GROUND
- ENVIRONMENTAL CONTRASTS: URBAN AND RURAL TEMPERATURES
- THE URBAN HEAT ISLAND
- HIGH-MOUNTAIN ENVIRONMENTS
- TEMPERATURE INVERSION
- TEMPERATURE INDEXES
- Soil Temperature Gauges – Tips For Determining Current Soil Temperatures
- What is Soil Temperature?
- How to Check Soil Temperature
- Ideal Soil Temperatures for Planting
- Realistic Soil Temperatures
- Ideal Soil Temperature for Your Plants
- Research at Glacier Creek
- Six-Inch Soil Temperature Network
- Other Soil Temperatures
- Agricultural and Horticultural Decision Support Tools for Minnesota
- Climate and Agriculture
- Weather Forecasts
- Useful to Usable Project (U2U)
- Temperature, Freeze/Frost Dates, Growing Degree Days
- Soil Temperature
- Weather and Climate Data from University of Minnesota Research and Outreach Centers
- Snow Management
Surface and Air Temperature
This chapter focuses on air temperature—that is, the temperature of the air as observed at 1.2 m (4 ft) above the ground surface. Air temperature conditions many aspects of human life, from the clothing we wear to the fuel costs we pay. Air temperature and air temperature cycles also act to select the plants and animals that make up the biological landscape of a region. And air temperature, along with precipitation, is a key determiner of climate, which we will explore in more depth in Chapter 7.
Five important factors influence air temperature:
- Latitude. Daily and annual cycles of insolation vary systematically with latitude, causing air temperatures and air temperature cycles to vary as well. Yearly insolation decreases toward the poles, so less energy is available to heat the air. But because the seasonal cycle of insolation becomes stronger with latitude, high latitudes experience a much greater range in air temperatures through the year.
- Surface type. Urban air temperatures are generally higher than rural temperatures. City surface materials—asphalt, roofing shingles, stone, brick—hold little water, compared to the moist soil surfaces of rural areas and forests, so there is little cooling through evaporation. Urban materials are also darker and absorb a greater portion of the Sun’s energy than vegetation-covered surfaces. The same is true for areas of barren or rocky soil surfaces, such as those of deserts.
- Coastal or interior location. Locations near the ocean experience a narrower range of air temperatures than locations in continental interiors. Because water heats and cools more slowly than land, air temperatures over water are less extreme than temperatures over land. When air flows from water to land, a coastal location will feel the influence of the adjacent water.
- Elevation. Temperature decreases with elevation. At high elevation, there is less atmosphere above the surface, and greenhouse gases provide a less effective insulating blanket. More surface heat is lost to space. On high peaks, snow accumulates and remains longer. The reduced greenhouse effect also results in greater daily temperature variation.
- Atmospheric and oceanic circulations. Local temperatures can rise or fall rapidly when air from one region is brought into another. Temperatures of coastal regions can be influenced by warm or cold coastal currents. (We will investigate this factor more fully in Chapter 5.)
Temperature is a familiar concept. It is a measure of the level of kinetic energy of the atoms in a substance, whether it is a gas, liquid, or solid. When a substance receives a flow of radiant energy, such as sunlight, its temperature rises. Similarly, if a substance loses energy, its temperature falls. This energy flow moves in and out of a solid or liquid substance at its surface—for example, the very thin surface layer of soil that actually absorbs solar shortwave radiation and radiates longwave radiation out to space.
The temperature of a surface is determined by the balance among the various energy flows that move across it. Net radiation—the balance between incoming shortwave radiation and outgoing longwave radiation—produces a radiant energy flow that can heat or cool a surface. During the day, incoming solar radiation normally exceeds outgoing longwave radiation, so the net radiation balance is positive and the surface warms. Energy flows through the surface into the cooler soil below. At night, net radiation is negative, and the soil loses energy as the surface temperature falls and the surface radiates longwave energy to space.
Energy may also move to or from a surface in other ways. Conduction describes the flow of sensible heat from a warmer substance to a colder one through direct contact. When heat flows into the soil from its warm surface during the day, it flows by conduction. At night, heat is conducted back to the colder soil surface. Latent heat transfer is also important. When water evaporates at a surface, it removes the heat stored in the change of state from liquid to vapor, thus cooling the surface. When water condenses at a surface, latent heat is released, warming the surface.
Another form of energy transfer is convection, in which heat is distributed in a fluid by mixing. If the surface is in contact with a fluid, such as a soil surface with air above, upward- and downward-flowing currents can act to warm or cool the surface.
In contrast to surface temperature is air temperature, which is measured at a standard height of 1.2 m (4.0 ft) above the ground surface. Air temperature can be quite different from surface temperature. When you walk across a parking lot on a clear summer day, you will notice that the pavement is a lot hotter than the air against the upper part of your body. In general, air temperatures above a surface reflect the same trends as ground surface temperatures, but ground temperatures are likely to be more extreme.
In the United States, temperature is still widely measured and reported using the Fahrenheit scale. In this book, we use the Celsius temperature scale, which is the international standard. On the Celsius scale, the freezing point of water is 0°C and the boiling point is 100°C. Conversion formulas between these two scales are given in Figure 3.4.
Air temperature measurements are made routinely at weather stations. Although some weather stations report temperatures hourly, most only report the highest and lowest temperatures recorded during a 24-hour period. These are the most important values in observing long-term trends in temperature.
Temperature measurements are reported to governmental agencies charged with weather forecasting, such as the U.S. Weather Service or the Meteorological Service of Canada. These agencies typically make available daily, monthly, and yearly temperature statistics for each station using the daily maximum, minimum, and mean temperature. The mean daily temperature is defined as the average of the maximum and minimum daily values. The mean monthly temperature is the average of mean daily temperatures in a month. These statistics, along with others such as daily precipitation, are used to describe the climate of the station and its surrounding area.
TEMPERATURES CLOSE TO THE GROUND
Soil, surface, and air temperatures within a few meters of the ground change through the day (Figure 3.6). The daily temperature variation is greatest just above the surface. The air temperature at standard height is far less variable. In the soil, the daily cycle becomes gradually less pronounced with depth, until we reach a point where daily temperature variations on the surface cause no change at all.
ENVIRONMENTAL CONTRASTS: URBAN AND RURAL TEMPERATURES
On a hot day, rural environments will feel cooler than urban environments. In rural areas, water is taken up by plant roots and moves to the leaves in a process called transpiration. This water evaporates, cooling leaf surfaces, which in turn cool nearby air. Soil surfaces are moist because water seeps into the soil during rainstorms. It is drawn upward and evaporates when sunlight warms the surface, again producing cooling. We refer to the combined effects of transpiration and evaporation as evapotranspiration.
There are other reasons why urban surfaces are hotter than rural ones. Many city surfaces are dark and absorb rather than reflect solar energy. In fact, asphalt paving absorbs more than twice as much solar energy as vegetation. Rain runs off the roofs, sidewalks, and streets into storm sewer systems. Because the city surfaces are dry, there is little evaporation to help lower temperatures. Another important factor is waste heat. In summer, city air temperatures are raised by air conditioning, which pumps heat out of buildings and releases it to the air.
In winter, heat from buildings and structures is conducted directly to the urban environment.
THE URBAN HEAT ISLAND
As a result of these effects, air temperatures in the central region of a city are typically several degrees warmer than those of the surrounding suburbs and countryside, as shown in Figure 3.8. The sketch of a temperature profile across an urban area in the late afternoon shows this effect. We call the central area an urban heat island, because it has a significantly elevated temperature. Such a large quantity of heat is stored in the ground during the daytime hours that the heat island remains warmer than its surroundings during the night, too. The thermal infrared image of the Atlanta central business district at night demonstrates the heat island effect.
The urban heat island effect has important economic consequences. Higher temperatures demand more air conditioning and more electric power in the summer. The fossil fuel burned to generate this power contributes CO2 and air pollutants to the air. The increased temperatures can lead to smog formation, which is unhealthy and damaging to materials. To reduce these effects, many cities are planting more vegetation and using more reflective surfaces, such as concrete or bright roofing materials, to reflect solar energy back to space.
The heat island effect does not necessarily apply to cities in desert climates. In the desert, the evapotranspiration of the irrigated vegetation of the city may actually keep the city cooler than the surrounding barren region.
We have seen that the ground surface affects the temperature of the air directly above it. But what happens as you travel to higher elevations? For example, as you climb higher on a mountain, you may become short of breath and you might notice that you sunburn more easily. You also feel the temperature drop, as you ascend. If you camp out, you’ll see that the nighttime temperature gets lower than you might expect, even given that temperatures are generally cooler the farther up you go.
What causes these effects? At high elevations there is significantly less air above you, so air pressure is low. It becomes harder to catch your breath simply because of the reduced oxygen pressure in your lungs. And with fewer molecules to scatter and absorb the Sun’s light, the Sun’s rays will feel stronger. There is less carbon dioxide and water vapor, and so the greenhouse effect is reduced. With less warming, temperatures will tend to drop even lower at night. Later in this chapter, we will see how this pattern of decreasing air temperature extends high up into the atmosphere.
Figure 3.10 shows temperature graphs for five stations at different heights in the Andes Mountain Range in Peru. Mean temperatures clearly decrease with elevation, from 16°C (61°F) at sea level to ?1°C (30°F) at 4380 m (14,370 ft). The range between maximum and minimum temperatures also increases with elevation, except for Qosqo. Temperatures in this large city do not dip as low as you might expect because of its urban heat island.
So far, air temperatures seem to decrease with height. But is this always true? Think about what happens on a clear, calm night. The ground surface radiates longwave energy to the sky, and net radiation becomes negative. The surface cools. This means that air near the surface also cools, as we saw in Figure 3.6. If the surface stays cold, a layer of cooler air above the ground will build up under a layer of warmer air, as shown in Figure 3.11. This is a temperature inversion.
In a temperature inversion, the temperature of the air near the ground can fall below the freezing point. This temperature condition is called a killing frost—even though actual frost may not form—because of its effect on sensitive plants during the growing season.
Growers of fruit trees or other crops use several methods to break up an inversion. Large fans can be used to mix the cool air at the surface with the warmer air above, and oilburning heaters are sometimes used to warm the surface air layer.
Temperature can also be used with other weather and climate data to produce temperature indexes—indicators of the temperature’s impact upon environmental and human conditions. Two of the more familiar indexes are the wind chill index and the heat index.
The wind chill index is used to determine how cold temperatures feel to us, based on not only the actual temperature but also the wind speed. Air is actually a very good insulator, so when the air is still, our skin temperature can be very different from the temperature of the surrounding environment. However, as air moves across our skin, it removes sensible and latent heat and transports it away from our bodies. During the summer, this process keeps us cool as sweat is evaporated away, lowering our skin temperature. During the winter, it removes heat necessary to keep our bodies warm, thereby cooling our skin and making conditions feel much colder than the actual measured temperature.
The wind chill index, which is used in the United States and measured in °F, can be very different from the actual temperature (Figure 3.12). For example, an actual temperature of 30°F (?1°C) and a wind speed of 30 mi/hr (13.45 m/s) produce a wind chill of 15°F (?26°C).
The heat index gives an indication of how hot we feel based on the actual temperature and the relative humidity. Relative humidity is the humidity given in most weather reports and indicates how much water vapor is in the atmosphere as a percentage of the maximum amount possible. Low relative humidity indicates relatively dry atmospheric conditions, while high relative humidity indicates relatively humid atmospheric conditions.
Why does relative humidity influence how hot the temperature feels? One of the ways our bodies remove excess heat is through the evaporation of sweat from our skin. This evaporation removes latent heat, which cools our bodies. However, when the relative humidity is high, less evaporation occurs because the surrounding atmosphere is already relatively moist, and the cooling effect is reduced.
The heat index is given in °F, and, like the wind chill, it can be very different from the actual temperature (Figure 3.13). For example, if the actual temperature is 90°F (32°C) and the relative humidity is 90 percent, the heat index indicates that the temperature will feel like 122°F (50°C)—a difference of 32°F (18°C)!
The ideal or optimal soil temperature for planting and growing most vegetables is 65° to 75°F.
Vegetable seeds and seedlings require minimum soil temperatures to germinate and grow. Seeds and seedlings require optimal soil temperatures to thrive.
Soil temperature triggers not only seed germination but is an important factor in soil chemistry. Soil chemistry includes the release (dissolution) of mineral nutrients in soil moisture. Mineral nutrients are essential for vegetable plant growth and maturation to harvest.
The ideal or optimal soil temperature for planting and growing most vegetables is 65° to 75°F (18°-24°C).
Taking Soil Temperature
Soil temperature can be measured with a soil thermometer or gauge. Most home vegetable gardeners use a soil thermometer—a thermometer attached to a metal probe several inches long which is inserted into the soil.
The soil temperature for seed sowing should be taken between 1 and 3 inches deep. The soil temperature for transplants should be taken at 4 to 6 inches deep.
Commonly the temperature reading is taken after the thermometer has been in the soil for a couple of minutes.
Soil temperature is best taken in the early morning when the soil is coolest and not yet warmed under the day’s sun.
Take the soil temperature for at least three consecutive days and then average the results. Don’t depend on just one reading.
Planting and Soil Temperature
Vegetable seeds can be sown in the garden early in spring before the soil has warmed to optimal germination temperatures. If you sow early before temperatures are ideal, you cannot expect optimal germination.
Optimal germination and growing temperatures may not come until late spring or early summer. In regions where the growing season is short, waiting for optimal soil temperatures may not be practical or realistic. A 70 percent germination rate is often considered both practical and realistic.
You can use the minimum soil temperature for germination and the optima soil temperature for germination to decide at about what soil temperature you want to get started sowing seeds and setting out transplants.
Minimum Soil Temperatures for Seed Sowing and Germination:
Soil Temperature Needed for 70% Germination:
Optimal Soil Temperature for Germination (near 100% germination):
For specific soil temperature requirements for crops you are planting see the How to Grow articles for each vegetable and herb. There are more than 60 crops covered in the How to Grow category.
Workable Soil Test for Direct Seed Sowing and Transplants:
Before soil thermometers were used in gardens and farms, the common method of determining when to plant was soil workability. (This is the tried-and-true old fashioned way to know when to plant.)
The soil is workable and ready for seed sowing or planting if it passes the Workable Soil Test. Here’s the test: squeeze a handful of soil in the palm of your hand; when you open your hand if the soil remains a wet or very moist clump, it is not workable. Let the soil dry. If the soil crumbles from your hand with a touch, it is workable.
When the soil is workable in spring, you can:
Of course, once the soil is workable in spring, it will continue to warm. Often, old-time farmers would look to lilacs and other spring flowering plants to decide when to plant. For more on this is see the articles on Phenology: Nature Planting Signals for Vegetables.
More tips at: Pre-Warm Your Soil Before Planting Vegetables
See also: Soil: Making the Kitchen Garden
Soil Temperature Gauges – Tips For Determining Current Soil Temperatures
Soil temperature is the factor that drives germination, blooming, composting, and a variety of other processes. Learning how to check soil temperature will help the home gardener know when to start sowing seeds. Knowledge of what is soil temperature also helps define when to transplant and how to begin a compost bin. Determining current soil temperatures is easy and will help you grow a more bountiful and beautiful garden.
What is Soil Temperature?
So what is soil temperature? Soil temperature is simply the measurement of the warmth in the soil. Ideal soil temperatures for planting most plants are 65 to 75 F. (18 to 24 C.). Nighttime and daytime soil temperatures are both important.
When are soil temperatures taken? Soil temperatures are measured once soils are workable. The exact time will depend upon your USDA plant hardiness zone. In zones with higher numbers, the soil temperature will warm up quickly and earlier in the season. In zones that are lower, the soil temperature may take months to warm up as winter chill wears off.
How to Check Soil Temperature
Most people don’t know how to check soil temperature or what tools are used for taking accurate readings. Soil temperature gauges or thermometers are the common way to take the reading. There are special soil temperature gauges used by farmers and soil sample companies, but you can just use a soil thermometer.
In a perfect world, you would check nighttime temperatures to ensure they are not so cold your plant’s health will be impacted. Instead, check in the early morning for a good average. The night’s coolness is still mostly in the soil at this time.
Soil readings for seeds are done in 1 to 2 inches of soil. Sample at least 4 to 6 inches deep for transplants. Insert the thermometer to the hilt, or maximum depth, and hold it for a minute. Do this for three consecutive days. Determining soil temperatures for a compost bin is also done in the morning. The bin should maintain at least 60 F. (16 C.) bacteria and organisms to do their work.
Ideal Soil Temperatures for Planting
The perfect temperature for planting varies dependent upon the variety of vegetable or fruit. Planting before it is time can reduce fruit set, stunt plant growth and prevent or reduce seed germination.
Plants such as tomatoes, cucumbers and snap peas benefit from soils at least 60 F (16 C.).
Sweet corn, lima beans and some greens need 65 degrees F. (18 C.)
Warmer temperatures into the 70s (20s C.) are required for watermelon, peppers, squash, and at the higher end, okra, cantaloupe and sweet potatoes.
If you are in doubt, check your seed packet for ideal soil temperatures for planting. Most will list the month for your USDA zone.
Realistic Soil Temperatures
Somewhere between the minimum soil temperature for plant growth and the optimum temperature is the realistic soil temperature. For instance, plants with higher temperature needs, such as okra, have an optimum temperature of 90 F. (32 C.). However, healthy growth can be achieved when they are transplanted into soils of 75 F. (24 C).
This happy medium is suitable for beginning plant growth with the assumption that optimum temperatures will occur as the season progresses. Plants set out in cool zones will benefit from late transplanting and raised beds, where soil temperatures warm up more quickly than ground level planting.
Ideal Soil Temperature for Your Plants
Seed planting is fun and rewarding. Not to mention it can be a great educational experience for the whole family. Many beginner gardeners are scared of staring their plants from seed, but the good news is that the process doesn’t have to be complicated. There are many plants you can start from seeds easily, which will provide you with not just a valuable experience but with a beautiful garden.
When starting your plants from seed, keep in mind that one of the most important things for a success is that the seed is of a good quality. This will provide you with stronger and healthier plants. Other important things needed for growing healthy plants are rich, fertile soil and favorable environment ideal for making your plants thrive.
When it comes to these three parameters, two are under your control: seed quality and soil quality. While not everyone starts with a great soil, and while it’s not always possible to choose the best seeds (especially if you are a beginner gardener), with a bit of effort and practice, you can give the best soil to your plants and learn how to choose the best seeds.
The third requirement – climate conditions, however, is not always easy to achieve. A lot will depend on your climate zone and the micro-climate of your garden. Luckily, it’s still possible to achieve good results, even if the conditions are not the best. All you need is to understand your garden, climate and conditions present in it, and to try to make the best environment for your plants.
One of the most important things you need to ensure, regardless of your climate conditions, is a good soil temperature. Soil temperature is crucial for making seeds sprout and this is why you should pay a close attention to it. Regardless of your climate conditions and micro-climate of your garden, chances are that you will be able to grow plants as long as you make them sprout. To make this happen, you need to provide your plants with the best soil temperature.
Ideal Soil Temperature
So, what’s the ideal soil temperature you want to achieve? There is no one, universal answer to this question. Each plant has its own ideal temperature. Some plants don’t require soil to be too warm to sprout, so you can plant them earlier in the season. Others require very warm soil, so you can’t plant their seeds until it’s hot enough to do so.
All gardeners are eager to start planting, but you have to be patient. Sowing seeds too early, before the soil temperature is adequate, won’t do any good to your plants.
When it comes to starting seeds and soil temperatures, it’s important to understand when and where you can start your seeds. Some seeds can be sown outdoors, directly in the garden, when the temperature is consistent and the soil is warm enough. Other plants have to be started indoors first and later transplanted to your garden.
One warning: soil temperature and the outside temperature you see on your thermometer don’t have to be the same. Soil holds the temperature depending on various factors. In most cases, dark soil is the healthiest and it can hold both heat and water very efficiently. However, to tell the exact temperature of your soil, it’s best to use a soil thermometer. These thermometers are inexpensive and will help you a lot when it comes to determining if the soil is ready and warm enough for your plants.
Seeds You Can Start Outdoors
You can start these plants outdoors, when the soil temperature is warm enough.
Chard is an interesting plant when it comes to soil temperature. Its seed actually prefers cooler temperatures to germinate. It means you can sow the seeds while it’s still cold outside. As soon as you can work the soil, start your Chard seeds to provide the best results.
These seeds require soil temperatures of at least 40 degrees F to germinate:
The following seeds require soil temperatures of at least 45 degrees F to germinate:
The following vegetables require a bit warmer soil. Start these seeds when the soil temperatures reach 50 degrees F:
The following plants require 60 degrees F to germinate:
The following plants require soil temperatures of at least 65 degrees F. It’s important to wait for warm weather to start them! Warm-lowing plants are:
Finally, these plants really like it hot! Start their seeds when Soil temperatures are at least 70 degrees F:
- Kohlrabi (fall crop)
Seeds You Should Start Indoors
Keep in mind that there are some vegetables that are best started indoors. For these vegetables, you should always start seeds indoors early in the season, and then transplant them to your garden. These seeds germinate the best when started indoors, and they also produce the best crops if started this way. This is because they need warm indoor temperatures and later have to be hardened-off before transplanting in the garden.
Here is a list of the most popular vegetables you have to start indoors:
- Peppers. Both sweet and hot peppers are best started indoors. They require temperatures around 80 degrees F. Start the seeds about 8 weeks before you want to transplant peppers to the garden.
- Tomatoes. They are best to be started indoors. The seeds require soil temperatures of about 70 to 90 degrees F. Transplant them to the garden when the outside temperatures are consistently above 50 degrees F.
- Celery. It requires temperatures of about 75-85 degrees F, and it’s best started indoors. Keep in mind that the seeds are very small and you will probably over-plant them. This is why you will have to thin them to the strongest plants later. Use a handheld seeder to make the seeding process easier.
- Broccoli. Start the seeds about 6 weeks before you want to transplant it to your garden. The seeds germinate at temperatures of about 85 degrees F.
- Kohlrabi. For a spring crop, start it indoors. For a fall crop, you can sow the seeds outdoors, as long as the temperature is are at least 70 degrees F.
Photo credit: ralphhogaboom via photopin cc
Research at Glacier Creek
More Detail on Past, Present, and Future Research:
Studies on the vegetation of Glacier Creek Preserve are highlighted by the long-term nature of both ongoing research and the potential for new long-term research. Highlights of present and potential future research include the following:
Long-Term Prairie Restoration Dynamics: Established in 1970, the 140-acre Allwine Prairie Tract of Glacier Creek Preserve represents decades of post-restoration plant and soil development. Preserve-wide plant community surveys of the Allwine Prairie Tract conducted in 1979, 1993, and 2009 provide a long-term quantitative data set on changes occurring during this time. Future surveys can extend this data set adding further to what we might know about long-term plant dynamics following restoration.
Long-Term Effects of Burning and Mowing on Prairie: A suite of replicated research plots established within the Allwine Prairie Tract in 1978, are among the longest-running, continuously managed burn and mow plots known in the region. The plots, replicated at a second site south of Mead, Nebraska, include annual and quadrennial spring, summer, and fall burning or mulch-mowing plots with vegetative data collected in the plots across the years. These plots provide a unique setting to expand our understanding of long-term management effects on belowground biota or soil processes.
Long-Term Biomass of Restored Tallgrass Prairie: Above-ground biomass sampling of burned and unburned upland and lowland tallgrass prairie has been conducted annually in the fall since 1994. As an example, a comparison of these data with annual weather data, may provide insight on plant-fire-weather interactions.
Vascular and Non-Vascular Plants of Glacier Creek Preserve: A survey of vascular plants, mosses, and lichens at Glacier Creek Preserve (previously Allwine Prairie Preserve) was initiated in 1970’s and continues today. Verified specimens are labeled and maintained in the Glacier Creek Herbarium.
Potential Long-Term Prairie Restoration Practices: The recent addition of 148 ha (365 acres) of mostly cropland to Glacier Creek Preserve provides opportunities for superimposing long-term research on prairie restoration as these new acquisitions are restored over the years.
Habitat Diversity of Glacier Creek Preserve: The recent expansion of the preserve increased habitat diversity available for research by adding a second, small creek (North Creek) as well as slope wetlands, wetlands that developed along mid-slope springs, and a historic wetland. These additional habitats provide many opportunities for new studies at the Preserve.
Birds: Quantitative data have been collected on grassland birds as part of thesis projects or faculty research, but the Preserve provides many opportunities to expand, particularly with increased size and habitat diversity. See Publications and Theses for listing of completed research projects.
Mammals: A study of the habitat distribution of small mammals was conducted from 2012-2014 in the newly acquired Papio Tract and Barbi Hayes Overlook. In 2016, a long-term, small mammal survey was initiated in the Allwine Prairie Tract and the Barbi Hayes Overlook to assess composition and to begin to assess long-term changes in small mammals in farmed and prairie habitats.
Herpetofauna: A study conducted from 2015-2017 on habitat distribution of herpetofauna in the Allwine Prairie Tract. A more detailed study would be able to expand on results from this preliminary assessment.
Invertebrate Fauna and Other Biota
Butterflies: Since 1998, twenty weekly Pollard Transect butterfly population censuses have been conducted each year from early June to mid-October by Dr. Ted Burk of Creighton University. In addition, since 2001, data also have been collected on nectar plant visits by butterflies observed during each census. Results to date provide a characterization of the butterfly community of the Allwine Prairie Tract across the seasons, quantify nectar plants use by the butterfly community, and assess the effects of preserve management, which includes prescribed burning.
Other Invertebrates: While butterflies have been and continue to be well studied, the Preserve would benefit from additional work on other above-ground, below-ground, and aquatic invertebrate groups.
Other Biota: The soil bacterial and fungal communities in burned, mowed, and untreated long-term research plots were evaluated in 2014-2015. Preliminary results show noticeable differences among treatments suggesting an area ready for further study, including on other soil biota.
Six-Inch Soil Temperature Network
The MDA Six-Inch Soil Temperature Network provides real time soil temperatures at locations across Minnesota. The network was established to assist in following best management practices for fall nitrogen fertilizer application which refer to a soil temperature at a six-inch depth (6 inch soil temp).
In areas of the state where fall nitrogen fertilizer application is appropriate, Minnesota’s nitrogen fertilizer best management practices (BMPs) for nitrogen use recommend that fall application of urea (46-0-0) and anhydrous ammonia (82-0-0) be delayed until soil temperatures at a six-inch depth stabilize below 50 degrees F.
Ammonia (NH3) is one of the primary forms of nitrogen applied in the fall. It reacts immediately upon contact with soil water to form ammonium which binds tightly to the soil. If average soil temperatures are above 50 degrees F soil microbes can transform ammonium into nitrite, and then into nitrate. The nitrate form is highly mobile with water and can leach from the soil. This results in less available nitrogen for crops in the following growing season. The leached nitrate can travel to groundwater and cause an increase in nitrate levels. Nitrate levels exceeding 10 mg/L are above Minnesota’s drinking water standard and unfortunately these levels are being found in well water in several areas of the state.
The MDA is responsible for the development, promotion and evaluation of BMPs for pesticide and nitrogen fertilizer use. The Six-Inch Soil Temperature Network is part of meeting that responsibility. It is a cooperative effort between the Minnesota Department of Natural Resources (DNR) and the MDA. All of the current Six-Inch Soil Temperature Network sites are co-located at DNR cooperative stream gauging sites. MDA soil temperature measuring probes are connected to DNR data logging systems. Soil temperature information is collected by DNR data logging equipment every 15 minutes and up-linked to the G.O.E.S. satellite and down-linked to the National Weather Service. The Minnesota Pollution Control Agency assists in data transfer as another cooperator.
Soil temperature information from the Six-Inch Temperature Network is provided on a map that also provides soil temperature information from the North Dakota NDAWN agricultural weather network sites located in Minnesota, University of Minnesota research centers, and other soil temperature sites. Most of these other sites do not report soil temperature at a six-inch depth, but still provide useful information that can estimate when soil temperatures are cool enough for fall application of nitrogen fertilizer to begin.
There are soil temperature sites on the map in southeast Minnesota which is an area where BMPs do not recommend fall application of nitrogen fertilizer due to the high leaching potential. The soil temperature sites are included to assist in the timing of fall manure applications, which like nitrogen fertilizer, are best done after soil cools to under 50 degrees F.
- Minnesota Department of Natural Resources
- Minnesota Pollution Control Agency
- City of Hutchinson
Other Soil Temperatures
Soil temperatures in Minnesota
- Southwest Research & Outreach Center
- Southern Research & Outreach Center
- West Central Research & Outreach Center
Soil temperatures in adjacent states
- South Dakota Climate and Weather
- North Dakota Agricultural Weather Network
- Iowa Environmental Mesonet
- Wisconsin State Climatology Office
- High Plains Regional Climate Center
- DNR stream gauging
Agricultural and Horticultural Decision Support Tools for Minnesota
Climate and Agriculture
- Climate and Agriculture Fact Sheets, Videos, and Other Publications
(Iowa State University)
- Localized Agricultural Weather Forecasts
(University of Kentucky and National Weather Service)
Useful to Usable Project (U2U)
- Agricultural Climate Data Decision Dashboard
- Growing Degree Day Decision Support Tool
- Climate Patterns Viewer
Temperature, Freeze/Frost Dates, Growing Degree Days
- Normal May through September Growing Degree Days
- Last Spring/First Fall Freeze/Frost Dates and Plant Hardiness Zone Map
- Last Spring/First Fall Freeze/Frost Date Probabilities
- Plant Hardiness Zone Map
(U.S. Department of Agriculture)
- Spring Frost-Free Date Map
- Greatest Number of Consecutive Hours At or Below Freezing
(Midwestern Regional Climate Center)
- Lowest Minimum Temperature Since August 1
(Midwestern Regional Climate Center)
- This Year’s Date of First Frost/Freeze from Midwestern Regional Climate Center’s Climate Watch Page
(Midwestern Regional Climate Center)
- Real-time Six-Inch Soil Temperatures
Minnesota Department of Agriculture
- Daily Soil Temperatures for Northwestern and West Central Minnesota
North Dakota Agricultural Weather Network (NDAWN)
- Average Date of Initial Soil Freeze
Weather and Climate Data from University of Minnesota Research and Outreach Centers
- Regularly Updated Weather Data – St. Paul
University of Minnesota Agricultural Experiment Station
- Regularly Updated Weather Data – Lamberton
University of Minnesota Southwest Research and Outreach Center
- Regularly Updated Data – Waseca
University of Minnesota Southern Research and Outreach Center
- Historical Weather Data – Crookston
University of Minnesota Northwest Research and Outreach Center
- Regularly Updated Weather Data – Morris
University of Minnesota West Central Research and Outreach Center
- Climate Resources for Master Gardeners
(CoCoRaHS and NOAA Office of Education)
- Minnesota Department of Transportation Snow Climatology Project
(controlling drifting snow)
The Minnesota Department of Agriculture (MDA) advises farmers and applicators to check soil temperature and delay fall application of anhydrous ammonia and urea fertilizer until soil temperature stays below 50 degrees F.
To assist tracking soil temperature, the MDA provides real-time soil temperatures at 48 locations across the state at https://app.gisdata.mn.gov/mda-soiltemp/. The website includes a map with MDA sites with soil thermometers at a six-inch depth, North Dakota Ag Weather Network sites at four-inch depths, and research sites at various depths.
“There are areas of the state where fall application of nitrogen fertilizer is simply not recommended due to groundwater contamination concerns,” said Bruce Montgomery, manager of the MDA Fertilizer Management Section. “Those would be areas with coarse-textured soils that drain quickly or areas underlain by fractured bedrock karst geology. In other areas of the state where fall nitrogen fertilizer application is a recommended practice, the MDA encourages delaying application until soil temperatures cool down.”
Map showing average date that 50 degree F soil temperature is reached in Minnesota. On average soil temperatures reach 50 degrees F during the first week in October in northern Minnesota and the fourth week of October in southern Minnesota.
Waiting until soil temperature stays below 50 degrees F before applying anhydrous ammonia and urea increases the availability of nitrogen to next season’s crop and decreases the amount of nitrate that could potentially leach into groundwater or tile drainage. At cooler temperatures microbial activity in the soil slows down, slowing the conversion from ammonium to nitrate. Ammonium is stable in the soil whereas nitrate moves with water and may leach out of the root zone over winter and early spring.
Although the soil temperature network was established to support application of commercial fertilizer it is equally useful for those applying manure in the fall. University of Minnesota Extension recommends delaying fall manure applications until soil temperatures at six-inch depth are below 50 degrees F to prevent leaching losses. Research from the University of Minnesota at Waseca showed liquid dairy and hog manures injected in November produced yields 10 bushels per acre higher than manures injected in September and October.
In addition to delaying application until soil temperature stays below 50 degrees F, best management practices for nitrogen use developed by the University of Minnesota Extension for south-central Minnesota recommend using a nitrification inhibitor when fall applying anhydrous ammonia and not to apply urea in the fall. In drier western Minnesota fall application of both anhydrous ammonia and urea are recommended practices. In southeast Minnesota’s karst region and statewide on coarse-textured soils, fall application of nitrogen fertilizer is not recommended regardless of soil temperature. Fall application of 28% liquid nitrogen is not recommended anywhere in the state due to its high leaching potential. Specific nitrogen fertilizer use recommendations by region of the state can be found at www.mda.state.mn.us/nitrogenbmps.
The MDA is proposing a rule that by 2019 may restrict fall nitrogen fertilizer application in areas vulnerable to groundwater contamination. The rulemaking is part of the state’s Nitrogen Fertilizer Management Plan. More information on the proposed rule can be found at www.mda.state.mn.us/nfr.