Saving plants from frost

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Tender tropical plants are exceptionally vulnerable to chilly winds, frosts and freezes, but cold temperatures don’t have to be a death knell for sensitive flowers, trees or shrubs. By knowing how to properly revive damaged plants, it is possible to minimize damage and save your favorite tropical plantings.

Be Proactive With Protection

First and foremost, it is best to avoid any cold damage to tropical plants if possible. Use appropriate covers, irrigate the ground thoroughly, apply an insulating layer of mulch, move container plants indoors and take other steps to protect sensitive plants when cold threatens. Anti-transpirant sprays, supplemental heating and cold frames can also prove helpful to protect plants. These measures won’t always eliminate all cold damage, but they can minimize how much vulnerable plants may suffer when the temperatures drop.

When the Damage Is Done

It can be disheartening to see tropical plants lose their vibrancy when they suffer from cold damage, but it is important to be patient before taking drastic revival measures. Depending on the type of damage and the extent of the cold, it may take 1-2 weeks or longer for the full scope of the damage to be visible.

Cold damage may show as several types of symptoms, including…

  • Wilting or drooping leaves or small branches
  • Softened or blackened foliage
  • Burn-like spots on flowers and foliage
  • Splits in woody stems or trunks
  • Excessively loose root balls

While dead bits can be pruned away, it may be advisable to leave damaged parts of the plant in place because they can provide an additional layer of protection to the remainder of the plant if extreme cold strikes again. Once the cold is completely past and no longer a threat, however, damaged leaves, shoots and branches can be pruned. Take care that only truly dead parts are removed – scrape the stem and check for green tissues beneath the outer layer that would indicate the plant is still alive and can recover naturally. If tissues are mushy, slimy or have a foul odor, however, the damage is too severe and those areas should be pruned.

When a plant is stressed from cold damage, it is critical not to prune too much tissue away, as over-pruning can further stress the plant. Furthermore, new growth will emerge from healthy areas as the plant recovers, and over-pruning can slow the plant’s recovery. In many cases, it is best to wait until the plant has begun actively growing again in order to be certain about which tissues are irrevocably damaged and which can still be productive.

When assessing cold damage on a tender plant, gently feel the root ball by moving the plant to see how loose it has become. A firm, solidly entrenched plant should still have a healthy root system that can recover and support new growth, even if a great deal of the plant’s stems or foliage has been damaged. If the root ball is very loose and moves easily, however, it has likely suffered severe damage as well and the entire plant may be in jeopardy.

Aiding Recovery

After the cold has passed, gentle watering can help rehydrate plants that are drought-stricken from frozen ice, but avoid overwatering that would stimulate new growth and stress an already weakened plant.

It is tempting to fertilize a cold-damaged plant to help stimulate new growth and vigorous recovery, but providing excess nutrition can actually be more harmful than helpful. Fertilizer will indeed stimulate new growth, but that puts the plant’s energy into more growth than it can handle after being damaged. Instead, continue to keep the plant hydrated but avoid unnecessary fertilization.

It is also very tempting to move container plants indoors or provide strong supplemental heat to warm them up after a cold shock, but doing so can actually shock plants even further. If the cold period is lengthy or even colder temperatures are predicted, it is helpful to move plants to more moderate shelter, but allow them to adjust to their new surroundings very gradually to minimize any quick changes that can irritate already stressed plants.

Even the most tender, delicate plants are hardier than we realize, and they are often able to recover from cold snaps we would assume to be deadly. By recognizing cold damage and knowing how to revive plants without causing further stress, it is possible to protect and nurture every tropical plant in your garden and landscape, even when the weather isn’t quite the tropical climate they may prefer.

Tips For Saving Cold Damaged Plants

How much cold will kill a plant? Not much, although this is usually dependent on the hardiness of the plant as well as the climate. Typically, temperatures falling below freezing will quickly damage or even kill many types of plants. However, with prompt care, many of these cold damaged plants can be rescued. Better still, protecting plants from freezing cold and frost before damage occurs is generally a good idea.

How Much Cold Will Kill a Plant?

How much cold will kill a plant is not an easy question to answer. Be sure to look up the cold hardiness for the plant in question before leaving the plant outside. Some plants can survive sub-freezing temperatures for months while others cannot take temperatures below 50 F. (10 C.) for more than a few hours.

What Happens to Cold Damaged Plants?

While many people ask how much cold will kill a plant, the real question should be how much freezing will kill a plant. Freeze damage to plant tissue can be detrimental to plants. Light frost typically doesn’t cause major damage, with exception to very tender plants, but hard frost freezes water in plant cells, causing dehydration and damage to cell walls. Cold injury is more likely to occur as the sun comes up. As a result of these damaged cell walls, the plant defrosts too quickly, killing leaves and stems.

Young trees or those with thin bark can also be affected by cold temperatures. While not always visible until spring, frost crack results from sudden drops in nighttime temperature following the daytime heating from the sun. Unless these cracks are ragged or torn, however, they usually heal themselves.

Saving Frozen Plants

In less severe cases, cold damaged plants can be saved. Frost crack damage in trees that require repair can usually be saved by carefully cutting away the torn or loose bark. Smoothing out the edges with a knife will allow the tree to form a callous on its own. To help minimize frost damage to other woody plants, lightly mist foliage before the sun hits them. Likewise, potted plants can be moved to another location away from direct sunlight.

Unless damaged plants are moved indoors or another sheltered area, do not attempt to prune damaged leaves or stems. This actually offers additional protection should another cold spell occur. Instead, wait until spring to cut away the damaged areas. Prune dead stems all the way back. Live stems, however, need only the damaged areas cut back, as these will eventually regrow once warm temperatures return. For soft-stemmed plants suffering from cold injury, immediate pruning may be necessary, as their stems are more prone to rotting. Cold damaged plants can be watered and given a boost of liquid fertilizer to help aid in their recovery.

Protecting Plants from Cold and Frost

While saving frozen plants is possible, freeze damage to plant tissue and other cold injuries can often be prevented. When frost or freezing conditions are expected, you can protect tender plants by covering them with sheets or burlap sacks. These should be removed once the sun returns the following morning. Also, potted plants should be moved to a sheltered location, preferably indoors.

Keep your houseplants warm this winter

The majority of popular houseplants are natives of tropical or subtropical climates. That means they aren’t fond of cold weather. If introduced to lower temperatures gradually, most adapt nicely. But if, all of a sudden, they find themselves chilled, the results aren’t pretty.

This can happen several ways:
1. You take a winter vacation and turn the thermostat back to 55 degrees F., or lower.

2. The heat goes off during a winter storm.

3. Plants are left on windowsills – often behind sheer curtains – when temperatures drop far below freezing.

4. You take an indoor plant outdoors during a “warm spell” in January or February, when 50 degrees F. feels like a heat wave to you (but not to a tropical plant).

So what can you do to prevent damage to your plants from the chill?
1. Set your thermostat no lower than 60 degrees F in winter, if possible.If you can’t, gather all houseplants in the warmest room in your house — or the one that’s warmed most by the sun. Grow lights may also provide some warmth.

2. Ask someone to walk through your house each day while you’re gone. If the power goes off or the furnace fails, he or she can try to get someone in to fix the problem quickly.

3. If you can, remove houseplants from windowsills when temperatures dip below freezing, particularly if you don’t have double-paned or storm windows and/or you can feel cold wind blowing right through. Other options include placing cardboard or Bubble Wrap between the glass and the plants, pulling down the shade between the window and the plants, and keeping curtains or draperies open at night.

4. Don’t take houseplants outdoors, even temporarily, until after your spring frost-free date. Even if the plant doesn’t get too cold, it will receive way too much sun on its “day out” and then be thrust back into much lower light levels, which will require adjusting (and probably some yellowing and dropped leaves). In mild climates, you may find that if you leave plants outdoors when you’re gone, one night the temperatures dipped much too low for the plants’ comfort.

(NOTE: To go to the Monitor’s main gardening page — which contains articles and blog posts on many topics — click here.)

Treating Cold-Damaged Plants

January and February are the coldest months in Florida, and plants can be damaged by low temperatures. But with your help, cold-damaged plants can often recover.

Water

After a freeze, check the soil around your plants. Plants may not be getting the water they need if the soil has dried out or if the water in the soil is frozen.

Watering the area can help defrost the soil and provide your plants with an available source of moisture. Even injured plants need water.

Fertilizer

While you may be tempted to add a little fertilizer to your plants to help speed their recovery hold off. If you fertilize too early you could encourage new growth before cold weather has gone. It’s best to wait until spring to begin fertilizer application. Once the danger of frost has passed, an application of fertilizer can help speed recovery.

Pruning

Don’t prune cold-damaged plants right away. The dead foliage looks bad, but will help insulate plants from further injury. In the spring, assess the extent of the damage by scraping the bark with your fingernail. Cold-injured wood will be black or brown under the bark. To be certain where to prune, wait until plants begin to sprout new growth.

Herbaceous plants like impatiens and begonias that are damaged by the cold may collapse. If this happens, it’s best to cut them down and remove the plants to prevent fungal or bacterial problems from arising as they decay.

Lawn

Seeing your lawn turn brown during the winter can be worrying for some homeowners; however, this is a normal part of your lawn’s winter dormancy. Come spring time your lawn should rebound and begin producing new green growth.

But when hard freezes hit, your turfgrass may be injured. If temperatures suddenly fall below 20 degrees Fahrenheit, your lawn may be permanently damaged. The grass may initially appear wilted, and then turn to a whiteish or brown color. It may mat to the ground and smell putrid. If your lawn does not recover in the spring, you may have to replace some of the grass with sod pieces or plugs.

  • Learn more about Cold Damage to Turf

With a little care and patience you may be able to recover most or all of the plants in your landscape.

UF/IFAS Publications

  • Cold Damage on Palms

Also on Gardening Solutions

  • Cold Damage to Turf
  • How to Help Your Lawn Survive the Chill
  • Your Florida Lawn in Winter

The First Frost

Nothing sends gardeners running faster than a weather forecast of FROST. Cool air, clear skies and light or calm winds are necessary for frost to occur. Cool air permits temperatures to drop low enough to freeze moisture in the air which would otherwise form dew. When skies are clear, heat from the soil is able to rise, allowing the cool air to settle close to the ground and chilling the plants as they lose heat. Calm winds allow the cool air to settle without mixing it with warm air.

Frost (the sparkling ice crystals that form on all surfaces) can occur without severely damaging plants. The critical feature is the internal temperature within plant tissues. If temperatures within these tissues are cold enough to break cell walls or disrupt cell constituents beyond repair, damage, wilting, and dying will occur in those tissues affected.

Some plants are more tolerant of frost than others. Woody plants are less affected than succulent plants. Fruits and flowers may be more sensitive than leaves. Sudden and prolonged freezing will be more damaging than gradual cooling of short duration. Plants already exposed to cool temperatures will be more resistant. Within our own properties we can find variations on different sides of the house, under trees, on south or north facing slopes, or low lying areas. Cool air settles at the bottom of slopes because it is heavier than warm air. Frost pockets will then form in valleys where cool air becomes trapped. Hilltops are also susceptible to cool temperatures. Hillsides often remain frost free until a more severe frost occurs.

How can we protect plants from that first cold snap? The two most common methods are covering to keep the plants warm or to warm the plants by sprinkling with water. Covering is the most effective for most people. Covering plants the night before with a sheet, blanket, or tarp will trap the warmth from the soil over the plants thus preventing freezing. This type of covering will usually protect plants when temperatures drop into the upper 20’s. Plastic used as a covering usually doesn’t work as well as the other coverings mentioned. Sprinkling the plants with water is often used as a “morning after” solution. When water cools and crystallizes into ice, heat is released which may prevent internal damage before freezing occurs within plant cells. The time when the internal plant temperature is coldest is in the morning. If the drop in temperature is not too great (more than a few degrees), watering plants in the early morning may protect tender plants that were left uncovered. Of course it never hurts to wish for cloud cover and a good breeze on those first cool nights of autumn to help prevent damage.

This article originally appeared in the October 11, 1996 issue, p. 163.

To water or not to water a plant right before near frost temps in mid-Spring?

Watered believe it or not. The best solution is to WATER with an oscillating sprinkler BEFORE morning. Watering like this actually slows the already frozen cells in thawing. Fast thaw is what kills most plants or plant material subjected to a freeze or frost. Watering slows the thawing down. Healthy plants will be able to recover. So yes, water them. Get up in the dark mornings to check your garden for freezing. If there was a freeze and you’ve got frozen plant material start watering. Seriously, this slows the thaw process down so that there are less plant cells bursting when those cells go from frozen to thawing.. I barely got out there to water but only the tops froze. Plenty of foliage to continue to make potatoes. I just lopped the ruined foliage off.

BUT WAIT! You’ve got to get some ‘row cloth’ to wrap your babies in BEFORE night time and the chance of freezing. This stuff, white, permeable to water and light…is a gardener’s best friend. Also stops insects during the time they want to lay eggs on or near your plants. Just dump soil all around the edges so the flies or moths are unable to crawl beneath.

This row cloth is purported to maintain temps that are 10 to 20 degrees higher than the outside temperatures. Plants in pots are very susceptible to cold and freezes…the weakest part of plants are first; the deciduous leaves which usually abscise to fall and depart the plant to protect the plant. The next are the roots. In a pot, plant roots are terribly unprotected from cold. If the frost or freeze is no big deal plant roots should do fine. For very sensitive plants or plants planted in pots left out side in an exotic zone (tropical plants trying to endure non tropical zones and temperatures) I have used heated wraps, Christmas lights and lots of burlap to allow those plants to survive a few too cold nights…wind. Row cloth on the potatoes in my picture would have not allowed those leaves to freeze. I forgot to deal with freezing that night. Bummer but not a deal breaker. ROW CLOTH. Gotta have it to extend your seasons and protect from sporadic freezes!

In Irish forests the most damaging frosts are those that occur at the end of spring and beginning of autumn. Young trees are susceptible to frost damage from the early seedling stage until they reach a height of approximately 1.5 – 2 metres. Trees come into leaf in spring when they have experienced a certain accumulated temperature. This mechanism ensures that warm days do not cause the trees to come into growth too early in the season. Similarly, shortening days in autumn is the mechanism that causes the trees to stop growing and become dormant, as a warm period in autumn could cause the trees to grow late into the season making them vulnerable to early autumn frosts. Native species are generally well adapted to the climatic conditions in this country and, with their growth and dormancy cycles synchronised with the seasons, they tend to avoid unfavourable weather conditions. Exotic or introduced species, however, are adapted to the climatic conditions that occur in their native habitat, moving them to Ireland may upset their growth pattern leaving them vulnerable to damage by frosts and winter cold. In introducing new non-native species into Irish forestry it was essential that the early foresters test the species adaptability before planting them on a wide scale.

There are considerable differences in phenology (bud break and dormancy) both between and within species and foresters can use this to their advantage. For example, Norway spruce is a late flushing species which can be planted on low lying midland sites where late spring frosts frequently occur. Its late growth avoids the danger of frosts while Sitka spruce planted on the same sites can be badly affected and a year’s growth lost, or in severe cases killed outright.

Late spring frosts

Spring frost damage on spruce (photo B. Hartzler)

Late spring frost are a feature of the oceanic climate that Ireland experiences. Frosts can occur as late as early June which cause severe damage to young plantations. At that time the newly flushed shoots are at a vulnerable stage and are readily killed by the freezing temperatures. This can result in trees loosing a year’s growth and repeated frost damage can kill trees or hold them in check for many years. Some trees, however, are able to eventually grow to a height above the frost line and then grow normally.

Late spring frosts are the most damaging frosts to trees in Ireland affecting both nursery stock and young plantations. Their occurrence is very unpredictable and little can be done in the field other than planting frost resistant species such as Norway spruce and pines (Scots, lodgepole, Corsican pines) in low lying areas that are likely to experience late frosts. Many of the main broadleaves such as oak, ash, beech are particularly susceptible to late frosts while some of the minor species such as birch, alder, rowan, sycamore, and hornbeam are frost tolerant.

Early autumn frosts

Early autumn frosts are generally not a problem in young plantations in Ireland. These can occur in early September, but by that stage many species will have completed their year’s growth and are entering dormancy. Southern seed origins of Sitka spruce and Douglas fir, however, can remain growing late into the season can be affected, but these are rarely planted and only in the milder parts of the country.

Early autumn frosts are more a problem for the nurseryman than the forester. Mild weather conditions at the end of the summer can cause young seedlings and transplants to continue growing or produce Lammas growth which is a second flush of growth around late August. This delayed dormancy leaves the plants very vulnerable to early autumn frost and winter cold. However, as plants become older they develop a more predetermined growth pattern which is less affected by prevailing weather conditions at the end of the season. Hence the reason why plantations are less affected than nurseries.

Like late spring frosts, the occurrence of early autumn frosts is unpredictable. Ireland has usually mild weather conditions in the autumn and early autumn frosts are rare. The most severe damage to trees is from late spring frosts.

How Badly Does Late Spring Frost Damage Trees?

One of the most common misconceptions about the weather is that frost only happens during the winter season. As much as we want this to be true, frost can actually also occur during the spring season. During the early stage of spring, the temperature starts to warm up again as trees start to bloom.

​However, a frost can unexpectedly arrive in the late period of spring and lead to plant shock. So, how much can a late spring frost damage trees? More importantly, what can you do about it? Here, we look at the possible actions you can take to protect your trees from the onset of a late spring frost.

How Badly Does Late Spring Frost Damage Trees?

​Identifying The Trees

As we now know, spring season could either be beneficial or bad for your trees. On one hand, we’ll have warm temperatures and long days indicating that the winter cold is over. Then again, a sudden frost and storm will stress out your trees as they are already moving away from their dormancy stage. However, which trees should you be concerned about the most?

​You see, not all trees will be severely damaged from a late spring frost. Because some leaves that bud and grow fast enough are relatively unaffected by the sudden dip in temperature. These hardy trees include crab apple trees, silver maple trees, lilac trees, and linden trees.

​Still, some trees do not bloom as fast and do not develop frost-tolerant leaves quickly. These trees prone to late spring frost include apple trees, magnolia trees, dogwood trees, and flowering cherry trees.

​Extent Of The Damage

So, how bad is the late spring frost? While it can cause damage to the leaves and flowers that have just begun to grow and blossom, respectively, the sudden arrival of a late spring frost won’t likely to kill your trees. Likewise, the chances of it causing long-term damage are not high.

Extremely cold weather for a long time is bad, but a late spring frost won’t last enough to wreak havoc on your trees. As long as your trees are not recovering from previous frost damage, they should be able to recover from a late spring frost.

​Signs Of The Damage

But how to realize your tree is damaged? In general, trees do not react the same way when they are exposed to late spring frost. However, there are some common signs that you should take note of. Some of these signs include dry leaves, discolored leaves, shriveled leaves, and leaves that fall earlier than expected once the temperature becomes warmer.

​Do not be gravely concerned about your trees if they have been hit by late frost. It’s common for homeowners to be shocked once they see the effect of it on their trees. Luckily, the damage is primarily on the aesthetic appeal of your trees. Moreover, growth and recovery will occur as soon as possible. Your trees might not grow at their most optimal state, but the effect won’t last long to go over the next season.

​What To Do With Damage Trees?

If you notice any of the aforementioned signs of late spring frost damage, there’s actually nothing that you can do. Your trees will recover on their own as the aesthetic damages are reduced in time. Still, you can strive to take better care of your property to ensure the good health of your trees all year long. As long as they are healthy, the trees will become better prepared for frost.

​One of the things you can do is to prevent yourself from overwatering your trees, especially if they are located in heavy clay soil. In addition, you should begin to prune any dead branches, but only if you are sure that no more frost will arrive. Once you’re done pruning, you can begin to apply a fertilizer to help the trees grow. Also, you must water your trees deeply. The roots of trees need the water to reach their depths in the soil.

In conclusion, late spring frost is indeed something that happens every once in a while. However, you should not be incredibly worried. The damage is not going to last and it is mostly just at the aesthetic level. Furthermore, there are trees that are quite tolerant to late spring frost.

We hope that this article helped you in understanding the effects of late spring frost. If you have any questions, do give us a comment.​

Injury of trees by subfreezing temperatures is complex. We generally distinguish between injury during the growing season (frost injury) vs. the winter (winter injury).

Frost

Spring frost injury in Quercus gambelii (Gambel oak). The leaves that were expanding at the time of the frost are dead. The live leaves were produced after the frost. Spring frost injury to flowers of Symphoricarpos oreophilus (mountain snowberry). Flowers and leaves of snowberry develop at about the same time in spring. Sometimes both are killed by a frost, but in this case the stage of development and severity of frost were such that the leaves survived. Such an event can lead to complete absence of fruit in the area, with significant implications to wildlife.

Usually, and preferably, the term frost damage is restricted to damage due to subfreezing temperatures during the growing season, or while the tree is not dormant. This can be after bud break, due to a late spring frost. Spring frost damage is fairly common. Young tender shoots are killed, droop and discolor, then eventually fall off. The damage is soon difficult to detect. The featured image above shows a closeup of spring frost damage in Picea pungens (blue spruce). Frost killed the tender, expanding shoots, which lost their turgor and drooped.

Frost damage tends to occur most commonly and severely in low areas where draining of cold air downslope is largely blocked by the topography. These frost hollows or pockets may even be treeless because the frost-free season is often too short for trees to survive. In Colorado, such treeless drainages and valleys, locally termed “parks”, are common. Though not all are due to frost, it appears that most are. Often one can see that the lower treeline of the area corresponds to the elevation of the topographic feature that blocks air drainage at the lower end.

An overstory tends to protect seedlings and saplings, which are most susceptible to frost damage. This may account for regeneration failures after cutting in low areas, and should be considered before cutting in them ​​.

It seems to be a paradox, but there is abundant evidence that climate warming has increased the risk of spring frost damage ​​. Warmer, earlier springs induce premature bud break and foliage and flower development, while timing of late frosts has not changed appreciably.

Less commonly, frost may occur prior to hardening off of shoots and buds, due to an early autumn frost. An early winter in central Alaska led to an unusual situation where the winter came while green leaves were still on the deciduous trees. Leaves persisted through the winter and into the following summer. Apparently buds were not completely set so it probably led to some shoot damage.

Frost Cracks

Frost cracks are axial openings in a stem that may penetrate radially some distance into the wood. They may spiral somewhat, like lightning scars, presumably following the grain of the wood.

According to Boyce ​​, frost cracks are formed during the dormant season, when there is a large, sudden drop in temperature. The outer wood contracts rapidly as it cools, while the inner wood remains warmer. Wood tends to shrink tangentially more than it does radially, so the result is a radial crack.

However, such cracks may have other origins as well. A stem that is twisted under the force of the wind will tend to have such tangential strain that may result in cracking, and internal decay makes this much more likely.

Winter Injury

Damage due to low temperature during winter dormancy is a separate phenomenon, though related. It may actually be two phenomena. In one scenario, which may be called winter freezing, warm weather during dormancy induces partial deacclimation. If it is followed by a rapid and large drop in temperature, there may be freezing damage to shoots and/or buds. Repeated cycles of extreme temperature fluctuation make such trees much more susceptible to such injury. The result is reddening and browning of foliage in the spring (evergreens) and dieback of shoot tips.

Evidence suggests that acid deposition, such as acidity generated by SO2 and NOx in fog, can increase susceptility of trees to winter injury ​​.

In some cases, cambium and phloem are killed.

Winter Sunscald

Sunscald is killing of cambium and phloem due to temperature changes associated with solar exposure. It is especially common when exposure of tree stems to the sun suddenly increases, such as by harvest, road construction, or a severe blowdown event.

Sunscald due to actual overheating and drying of bark during the summer is rare on forest trees ​​. It is especially common in winter when solar exposure heats up the bark, which then freezes. The rapid and large temperature drop apparently kills the tissues.

Sunscald is recognized long after by long, narrow scars, mostly below the live crown, on the southern or southwestern side (in the northern hemisphere). There is often evidence of a recent disturbance near the affected trees on that side that increased exposure, such as harvesting of adjacent trees. It typically affects groups of trees and is most severe on trees with thin bark.

Canker and decay fungi may enter through such lesions.

More general information is available on abiotic diseases and injury.

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