What Zone Are We?
I’m often asked what zone are we: referring to the USDA winter hardiness zone map. The hardiness zone map links areas together that have, on average, similar winter low temperatures. They are broken down into 10F degree intervals; 5F degrees into sub zones a & b. Most plants are given a hardiness zone rating to correspond with this mapped area. On existing maps we are in hardiness zone 5b at Powell Gardens so on average our winter low should be between -10F and -15F. Since we had -11F at our official weather station this past winter that looks right on.
The only thing is–if you average our winter lows for the past 15 years we would average around -5F here. In the past 15 years we’ve been as cold as -12F and as mild as +17F; with a reading of -27F in 1989! I was just in Wichita where it allegedly got to minus 17F last winter — but at Botanica, the Wichita Gardens, I saw no or little damage to tender zone 6 plants. Clearly there is more to it than the minimum low.
This Chicagoland Green Boxwood (Buxus hybrid ‘Glencoe’) is listed as hardy fully through zone 5 (-20F) but burned badly this past winter and has done the same at the Kauffman Memorial Garden in the past. It is a good example why I don’t like hardiness zones because as I get more experience under my belt I see why my mentoring professor (Robert W. Dyas) wouldn’t teach us winter hardiness zones. I followed his advice: look around and see what is doing well in a particular region, paying most attention to things that have done well for many years.This badly burned Green Bay (registered trademark) Sweetbay Magnolia (Magnolia virginiana ‘Green Shadow’) is supposed to stay evergreen to -20F as well but I wouldn’t call this evergreen. The plant will be fine as only the leaves are winter burned but it is not living up to its billing here. I’m glad we are here to do a lot of testing for the Greater Kansas City gardening community.
I had to take a picture of our ‘Taylor’ Windmill Palm (Trachycarpus fortunei). We planted this many years ago for trial as a below zero hardy palm. It has always died back but funny thing is this one always sends up a new basal shoot every summer, only to be killed back the next winter. Sure it can survive a rare below zero event in North Carolina but not the predictable and sustained below zero weather here. You could go to a lot of trouble and put winter protection around a plant like this and it might survive better.
Our Bracken’s Brown Beauty Southern Magnolias (Magnolia grandiflora) are living up to their name: brown. At least that’s the way they look from their southern side where the leaves were winter burned (consistently the most burned of our hardy southern magnolias each year). Evergreen shrub China Girl Hollies are fine at its base.The same Bracken’s Brown Beauty Magnolia is green on its shady side where the leaves weren’t burned by the abrupt change in temperatures caused by the sun’s warmth after our bitter cold. So our hardiness zone depends on how you look at it. Gardeners I have talked to say local lows ranged from the single digits below zero in the city’s heat island and wooded hilltop gardens like my own, while it was a sustained -15F below in low and outlying regions where cold air settled–a difference of more than 10 degrees across the region largely due to microclimates. Where does your garden sit? A visit to Powell Gardens will show you what does well here and what was disheveled by this colder-than-average winter. Very few things were killed by last winter but we plant things accordingly: plant more tender plants in sheltered microclimates and the tough ones in low, open places. Gardener common sense.
New USDA plant hardiness map: Wichita’s a little warmer | The Wichita Eagle
Wichita has warmed up half a zone, from 6a to 6b, under a new plant hardiness map released today by the U.S. Department of Agriculture.
The hardiness zone map, which helps gardeners determine which plants will survive the winters where they live, was last updated in 1990. Under that map, Wichita’s winters averaged lows of 10 below to 5 below zero. The new map is based on average lows from 1976 to 2005, which were 5 below zero to zero in Wichita.
The new map is much more sophisticated than the old one because of advances in technology and can’t be exactly compared to the old map, said Kim Kaplan of the Agriculture Research Service in Washington. It shows that some areas of the country have become warmer and others have become cooler since the issuance of the last map but takes into account only low temperatures and makes no claim for or against climate change, Kaplan said.
Kaplan said the map was issued because nurseries and other companies that do business in plants requested it. Home Depot, for example, wanted to know when it was best to ship, say, pansies to its stores around the country, Kaplan said.
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The maps are only a guide and don’t reflect how cold a winter can get. Wichitans have only to remember the 17 below zero last February. Even if an area’s zone changed, it doesn’t change the fact that the plants that did well in a gardener’s yard yesterday will still be doing well today, Kaplan said.
The new map is interactive and can be found on the website planthardiness.ars.usda.gov.
Zoned Out, Part 2: What’s Your Plant Hardiness Zone?
What Zone are You In?
When planting seed in the vegetable garden or picking fruit and nut trees for your yard or the back forty, its best to do some research on local climate conditions. Temperature and rainfall vary widely across the country and around the world. The average annual precipitation or the average temperature determines which types of crops will grow the best in your area. The USDA first published a plant hardiness zone map based on temperature in 1960. Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future.
The statistical average, which used to be termed as “Normal” is changing as was shown in the last blog post. By my estimate the Hardiness Zones moved north 100 miles between 1990 and 2006. The change seems to have accelerated. In Kansas we are able to grow Fig trees on the south side of our houses, an impossibility without a greenhouse 20 years ago. Here is a map that shows the Plant Hardiness Zones.
The zones are numbered from 1 – 7, and assigned a color from red, warmest, to blue and pink which are cooler. But first, let’s find your zone. By clicking on the USDA Hardiness Zone Map you can zoom in to your location. Using the USDA website you can zoom in by state and county for a detailed view. A close up view will show the differences between bottom lands along a river and the hilltops and bluffs above the river as shown in the lighter shade of green in the area around the Kaw river from Topeka to Kansas City. (my zones)
As shown below, In my area, the Zone value given is 6a. Zone 6a has a range of -10F to -5F.
Now, let’s look at some historical data from the Kansas City area. Weather Warehouse provides a good tabulation of data shown below. Note the column Lowest Temperature and then look at the Plant Hardiness Zone from the zoom-able USDA website. Although this area has a rating of -10F to -5F, the average lowest temperature between 2001 to 2013 seems to be above zero or about Zone 7a which is 0F to 5F. This correlates with our experience in this area, it gets warmer sooner.
So, When Should You Plant?
The Northern Hemisphere had it’s 12th warmest winter on record. Here is the USA, where parts of the country had the coldest winter in 20 years and Tuscon and Las Vegas had the warmest on record, what should we do in our respective zones? My conclusion:
- Use the Plant Hardiness Zone Map as a guide.
- Measure soil temperature as detailed in Part 1 of this series.
The Politics of the Plant Hardiness Zone Map
This is absolutely fascinating. It is classic case of denial. In 2002 the USDA decided to update the Zone Map. In conjunction with the American Horticultural Society a new map was developed but rejected by the USDA. It was rejected with no comment. It was obvious to the Bush administration that adding new zones due to warmer temperatures would be perceived as evidence of global warming. It was not until 2012 that a new map was issued with the new climate zones.
USDA Agricultural Research Service
The USDA 2012 Hardiness Zone Map and Some of Its Predecessors
The Polar Vortex vs. The Artctic Amplification