Changes to the USDA Zone Map and Your Garden

The U. S. Department of Agriculture created a map of North America in the mid-1900s that separated the continent into eleven growing zones based on the lowest annual minimum temperature. This was done to aid farmers and gardeners in selecting plants that would do well in their region. This map is especially helpful when choosing trees, shrubs, and perennial plants, but is also useful as a guideline when planting seeds for crops and gardens. Since growing seasons vary by region, it is beneficial to know what seeds require longer periods to reach maturity, and produce grain, fruits, and vegetables for harvesting.

The map has been modified several times since its inception, due to changes in climate and to give it more granularities in certain areas. The AHS (American Horticulture Society) created a plant heat zone map in 1997 to be used along with the USDA zone map, to help aid in selecting plants according to their ability to sustain high temperatures. Recently there was funding granted to AHS to update the USDA map to accommodate global climate changes. The updated map is expected to be released this year.

According to a Minnesota Pollution Control Agency report, "parts of northern Minnesota have warmed five degrees F or more in winter" in the last 100 years; and "precipitation has already increased 20 percent in the southern half on the state since 1900." It is generally thought that for every 1.8 degrees F that the climate heats up, the vegetation zone shifts north approximately 60 miles.

Scientists predict that Minnesota weather will become more humid and wetter, and may potentially be more like the climates of Nebraska, or Missouri. For Minnesota gardeners, this could mean that plants and shrubs that were previously off-limits in your backyard, would now flourish in the warmer climate. While this might seem like a wonderful thing, it generates concerns for the effect that it will have on not only plants, but insects, birds, and the rest of the food chain as it depends on vegetation for nourishment.

For those gardeners that like to experiment with different annuals, perennials, and bulbs, the climate changes provide a continuous laboratory in the backyard garden. Vegetation, much like humans, is very adaptable, and if the changes are gradual, most will adjust and be resilient enough to not only survive, but possibly even flourish. However, it is safe to say that some plants (much like insects, birds and humans) will move on to climates that are more conducive to their survival, or just die out completely.

To be sure, capturing evidence of climate change patterns and modifying plans for successful growing seasons is an ongoing challenge. We can be confident that farmers and gardeners will continue to do what they've done for centuries. That is, through trial and error, determine what grows best in their fields and gardens, and build on their experience, working to refine and adjust it to accommodate the weather changes year after year. And continue to hope that the rest of the food chain can succeed in doing the same.