A Half Zone

The Chicago area is now officially a half-zone warmer. The U.S.D.A.’s new Plant Hardiness Zone Map, issued last week, puts the city of Chicago in zone 6a and most suburbs in zone 5b.

You might want to think twice before you start celebrating, however.

Yes, the map is more detailed and accurate than the 1990 version. And the USDA’s Website now has an interactive version. Just enter your zip code and it will tell you your zone. That’s especially handy for new gardeners, who can use it to choose plants appropriate for their area.

Like any map, though, it has its limitations. The long-awaited changes are only based on the average lowest winter temperatures. As the USDA press release noted, the zone designation “does not reflect the coldest it has ever been or ever will be at a specific location.”

In other words, if there's a very cold winter, you are still likely to lose some plants. And as any one who has gardened in the Chicago area knows, our winters can be extremely volatile. Witness this year’s light snowfall and mild termperatures, a major contrast from just a year ago.

The map’s changes are a bit dated as well. It’s based on 30 years of data ending in 2005. That means the most recent temperatures included are six years old. The previous map, released in 1990, was based on 13 years ending in 1986. (See this article by Tony Avent of Plant Delights Nursery, who was involved in the map-making process, for a description of the latest effort. For his history of U.S. hardiness maps, click here.)

Cold temperatures probably aren’t even the biggest limiting factor for Chicago area gardeners. It’s the heavy clay soil, which makes it difficult to grow plants that need good drainage or prefer acidic soil.

For example, I’d love to grow pieris japonica, also called lily-of-the-valley shrub, because of its fragrance. I’ve tried several times, though, and failed. Rhododendrons and azaleas also are difficult to grow for the same reason.

That said, area gardeners can feel slightly more comfortable “pushing" the zone – trying zone 6 plants in a 5b garden – now that the new map has been issued. I think most experienced gardeners already do this, using the “micro-climates” or more protected areas of their garden to try marginal plants.

So I’ll half-heartedly take another half-zone. But I'm not planning to throw out my winter coat any time soon.

By Karen Geisler