Global Warming in the Garden

Our guest blogger, Sheryl Canter, is an Online Writer and Editorial Manager at Environmental Defense.

If you have a garden, you know the climate is warming. In temperate zones, the last frost in spring comes earlier, and the first frost in fall comes later. The longer growing season may allow you to grow vegetables you never could grow before. But you also may have noticed your weeds are more aggressive, insect pests are more of a problem, and pollen plagues you all summer long. You’re not imagining things!

For over 40 years, gardeners have relied on the USDA Plant Hardiness Zone Map as a guide to what they can grow in their area. But the USDA zone map hasn’t been updated since 1990, and gardeners have seen detectable shifts since that time.

In 2003, the American Horticultural Society (AHS) updated the zone map with a grant from the USDA, and published a draft of the new map [PDF] in The American Gardener. Based on temperature information from July 1986 to March 2002, the map showed widespread warming, with zones edging northward.

The USDA rejected the new map without explaining why, and said they would update it themselves. Four years have passed and still they have not released a new map. But the National Arbor Day Foundation has just released one, current for 2006. Like the 1990 and 2003 maps, the Arbor Day map is based on 15 years of data. The changes between 1990 and 2006 are dramatic; the U.S. is clearly getting warmer.

Global warming is caused by elevated concentrations of greenhouse gases in the atmosphere – notably carbon dioxide (CO₂). Plants use sunlight, water, and CO₂ to synthesize the glucose they need to grow – a process called photosynthesis. Thus when CO₂ concentrations in the atmosphere increase, it acts as a fertilizer, accelerating plant growth. This may sound good at first, but there’s more to the story.

CO₂ fertilization affects different plants to different degrees. As Duke University biologists discovered, one plant that loves additional CO₂ is poison ivy [PDF]. With increased CO₂, poison ivy grows 2.5 times faster, and produces a more potent version of the rash-causing chemical urushiol. Other types of woody vines also grow much faster with higher levels of CO₂ – fast enough to strangle and topple trees.

Accelerated plant growth has some other bad side effects. One is increased pollen production, creating misery for asthma and allergy sufferers. A Harvard study [PDF] showed that elevated CO₂ concentrations caused up to a 55 percent increase in ragweed pollen production.

Another consequence is that high levels of CO₂, while increasing crop yields, decrease the plants’ nutritional value. Obviously this is bad for humans eating the plants, but it’s also bad for humans growing them. Insects eat dramatically more plant matter when the plants are less nutritious (and ironically, can still starve to death from poor nutrition). Farmers using more pesticides to control infestations will increase pollution in rivers and streams.

So the next time you hear people arguing that global warming will be good for gardeners and farmers, set them straight!