Thursday, March 10, 2011

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 that if you average our winter lows out 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. From gardener's I have talked to; 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 that was 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.

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