Global warming’s silver lining could be longer growing season

My cilantro bed, which usually reseeds itself about three times each year, from early spring to fall, came alive during late December when daytime temperatures got stuck in the 60s. While the heat wave didn’t last long enough for the seedlings to get more than an inch tall, I see this as a flashing neon sign of what’s to come.

The fact that soil in Northeast Kansas stayed warm and moist long enough to germinate hard-shelled seeds in December is significant. Add it to the long list of signals that global warming is having and will continue to have an impact on many facets of daily life, including gardening.

At the risk of sounding like a profiteer, the gradual increase in global temperatures may be an environmental catastrophe but, over time, it’s also likely to be a boon for the home gardener. Think vine-ripened Memorial Day tomatoes.

That said, please don’t write to tell me that I am trivializing the death of the planet; I am merely pointing out that if the microwaving of our climate has a silver lining, it might be a longer growing season and the bounty it will produce. Since we don’t appear capable of stopping global warming, we might as well eat bigger melons.

For people who grow things, though, the real fascination lies with what is happening at the lower end of the thermometer, not with how hot it might get. That’s because the potential of a garden depends more on how low the temperatures don’t go in early spring and fall.

In Kansas, for example, we have plenty of heat, but it’s crammed into a four-month period from late May through mid-September. As a practical matter, blossoms on vegetable plants are set by early June, and the growing season for most crops is done by August. Consider what might happen if tomatoes and other warm-weather crops could be planted in late March instead of late April or early May and the killing frost in the fall didn’t arrive until Thanksgiving, instead of mid-October.

With early planting of warm-weather vegetables, the plants would be further along before the heat stalled their productivity. Many home gardeners also could raise late crops of some vegetables each year by setting out seedlings for a second garden in late summer.

This is not idle fantasy. Last week the news carried reports of scientists’ warnings that 2007 was likely to be the warmest year on record. A reader also brought to my attention a New York Times story that discussed the implications of the temperature increases on home gardening on the Eastern Seaboard.

Plenty of buzz about global warming and the growing season also was generated when the Arbor Day Foundation recently released a new zone map. The U.S. Department of Agriculture has indicated that it, too, is updating its map, which is the standard and is likely to make even bigger waves when it is unveiled.

The Arbor Day people moved Northeast Kansas out of Zone 5, which indicates that the average annual low temperatures dip to minus 20, and planted us firmly in Zone 6, where the low end is minus 10. On the current USDA map, which I’ve insisted for some time is woefully out of date, Zone 6 starts in southern Kansas and continues into Oklahoma, while Northeast Kansas, which hasn’t seen 20 below for decades, is now in the same climate zone as the eastern half of Colorado. That’s just wrong.

The zone maps are important for matching crops and planting dates with climate. For example, a variety of pepper might not be recommended for gardens in locations cooler than Zone 4, but zone designations usually are most significant for overwintering plants, such as trees and perennial flowers.

For vegetable growers, the zone designation has a psychological effect. If the USDA bumps us to Zone 6, it will confirm what most of us already know, but it also will be the gardener’s equivalent to having a few more cylinders under the hood.