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!

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  1. Posted June 17, 2007 at 2:02 pm | Permalink

    I keep reading how global warming will negatively affect us but people who write those articles are being short sighted and self-centered. Global warming may well negatively affect some people but it will also positively affect many people. I would love a longer growing season. I wouldn’t mind the winters being a bit less harsh. From my personal perspective global warming is good.

    The reality is that our planet has gone many cycles of warmer and cooler periods than it is now. The plants and animals adapt. Perhaps people won’t but I bet they will. Some species will die off. Some will evolve. Some will thrive. That’s the natural order of things and we are a part of that.

    There is the whole question of is global warming really happening, is it caused by people or is it caused by a warming cycle in the sun (Mars is also undergoing global warming – don’t blame humans for that), is this just a temporary blip (I’ve seen warmer years around here), would we be going into an ice age as they used to predict if it weren’t for the global warming, etc. It is all rather complex.

    On the CO2 reducing the nutrients, I read that article and I am very dubious of it. It doesn’t make a whole lot of sense.

    All that said, I’m all for reducing pollution. That is a separate issue and it is unfortunate that people confuse the two.

    Time will tell. Adapt.

  2. Posted June 18, 2007 at 10:48 am | Permalink

    Hi pubwvj,

    If you are still questioning whether global warming is really happening, you are standing alone.

    If you think it might be a good thing for some people, read this post:

    A Good Side to Global Warming?

    If you think warming on Mars “proves” that the warming on earth is caused by changes in the Sun or anything but human activity, read this:

    Extraterrestrial Global Warming

    Finally, don’t confuse weather with climate. If you think we’re experiencing a short term blip, read this:

    Brrr… this is global warming?

  3. Posted February 17, 2010 at 8:59 am | Permalink

    if you are looking for more information on USDA plant hardiness zones, there is a detailed and interactive USDA plant hardiness zone map at which allow you to locate your USDA zone based on zipcode or city.