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14 February 2012
For the first time since 1990, the United States Department of Agriculture has published a new Plant Hardiness Zone Map. Released last month, the guide for the nation’s 80 million gardeners reflects a warmer trend across the country. Of 34 cities listed in the map’s key, 18 are in warmer zones, including San Francisco, which now falls into the 10b (35-40 degree) category. Some entire states, like Nebraska, Ohio and Texas have been classified in warmer zones and two new climate zones, 12 and 13, have been created – for areas with average lows of 50-60 and 60-70 degrees, respectively.
The map is the result of a $500,000 project of the US Department of Agriculture and Oregon State University tracking the last 30 years of weather. Based on average annual winter low temperatures at 8,000 weather stations nationwide and a few in Canada and Mexico, the new map designates 26 climate zones, divided into 5-degree (F) increments. The 1990 PHZM included data from 1974–1986; while the 2012 PHZM covers data between 1976 and 2005.
Winds of Change
The larger trend of the map is that zones have shifted upward; winters aren’t as cold as they used to be and spring comes earlier. Berkeley, for example, now falls into a warmer zone suitable for the types of plants grown in Southern California, Florida and Hawaii. However, San Francisco’s new designation, although alarming to many, may not offer much new insight into gardening given our microclimates. As residents here know well, localized climates can encompass entire neighborhoods – like the cool spots caused by hills and valleys or heat islands caused by concrete or blacktop – or even occur within a singular garden. This kind of variation is not common in other parts of the country and most do not appear on the map, so while the new zoning may suggest planting more adventurously, nothing replaces your experience and knowledge of your own garden.
"It's one thing for zones to change on a map," Griff Hulsey of Berkeley Horticulture said to the San Francisco Chronicle. "But we're not going to start recommending plants that we haven't had years of experience growing in this area."
Many gardeners look to Sunset’s “Western Garden Book” as a planting guide, which has been publishing its own zoning map since 1954, focusing on the microclimates of the western U.S., specifically California. Sunset’s update to their 2007 zone guide is due out later this month. (See Sunset’s San Francisco Bay Area zone classification here).
Warmer winters + warmer zones = global warming?
“It is great that the federal government is catching up with what the plants themselves have known for years now: the globe is warming and it is greatly influencing plants (and animals)," Stanford University biology professor Terry Root wrote in an email to AP writer Michael J Crumb.
Boston University biology professor Richard Primack echoed a similar sentiment,"People who grow plants are well aware of the fact that temperatures have gotten more mild throughout the year, particularly in the wintertime. There's a lot of things you can grow now that you couldn't grow before. People don't think of figs as a crop you can grow in the Boston area. You can do it now."
Agricultural Research Service spokeswoman Kim Kaplan, however, insists that the map is not proof of global warming, just more sophisticated technology. "It is not a good instrument for trying to prove global warming. It does not cover a long enough period," Kaplan said. "Climatologists like to talk in terms of a minimum of 50 to 100 years. We only have 30 years here. In some cases where areas changed zones there was less than a one-degree change in temperature.” She pointed out that data in the 1960 version reflected warmer temperatures, similar to that of the 2012 update; however the 1990 map tracked colder temperatures, which people at the time suggested was the advent of another ice age.
Changes were to be expected given the amount of time elapsed since the last publication of the map. For the first time, more detailed factors like changes in elevation, proximity to large bodies of water, variations in urban vs. rural temperatures, and prevailing winds have been accounted for, which the USDA points to as a reason for the warmer trends. Despite the sophistication of the new map, one problem is that it does not account for high temperatures or humidity. A plant that thrives in a dry zone 10 in California wouldn’t make it in Florida’s humid zone 10, although that doesn’t stop people in “zone denial” from trying.
Seasoned gardeners use these zoning maps as a guide, but look to their own planting history first and foremost in choosing their spring garden plans. They may experiment with plants recommended for the new zone, but don’t expect them to thrive just because the map indicates success. The USDA concedes “no hardiness zone map can take the place of the detailed knowledge that gardeners pick up about their own gardens through hands-on experience.” Key conditions like wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine should be taken into account just as much as the general plant hardiness map.
“The sophisticated gardener can’t just rely on the zone map, and what’s the fun of that?” Todd Forrest of the New York Botanical Garden said in response to the update, which put his hometown in the same zone as Washington, D.C. “If you can’t plant things you’re not supposed to grow, you’re not having fun as a gardener.”
Resources:
USDA Interactive Plant Hardiness Zone Map
The Sun Sentinel
San Francisco Chronicle
The Huffington Post
The Washington Post
Sunset Magazine Climate Zones
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