Everyone’s heard about the unusually rough winter we had in the Midwest this year. I lost several plants that were marginally hardy here in Zone 5, which had survived for a number of previous winters, but gave up the ghost this time around. Something especially interesting happened with my redbuds. I had four kinds: generic, Forest Pansy, Burgundy Hearts and The Rising Sun (shown here). The Rising Sun (my fav!) and the generic redbud grew out this spring unscathed. Forest Pansy and Burgundy Hearts—well established specimens—were killed. This illustrates the natural variability within plant species, and why the variety and source matters. When you’re growing a plant near its hardiness limit, it doesn’t take too severe of a winter to push it over the edge.
There are two lessons here: first, do your homework and choose varieties known to be relatively winter hardy. Sometimes these exist, sometime not, but it’s worth checking, especially for high value plants like trees. The grower of The Rising Sun, Green Leaf Nursery, told me that they believed it was slightly hardier than the average redbud. Sure enough, it lived where others died. (Japanese maple is another example of a species that varies a lot in hardiness.)
Second, the geographical origin of the tree’s genetics, known as its “provenance”, matters. A flowering dogwood growing in the forests of Missouri will most likely have better cold tolerance than one growing naturally in, say, Florida. Each is adapted to its environment, and a colder environment means that trees from there will be better adapted to cold. Thoughtful growers act on this by seeking plant and seed sources from northern areas, when possible, and conscientious garden retailers try to stock plants from such growers. On the other hand, many retailers stock trees and shrubs grown in warmer regions. You can tell because they’re selling leafed-out specimens when the native landscape is still bare, a dead giveaway that the plant just arrived on a truck from, perhaps, hundreds of miles south.
If a plant is rated for one or more Zones colder than yours, this probably doesn’t matter. But if a plant is rated only to your Zone, it may pay to be choosy.