Updated USDA planting zone maps a hot topic

[XemaSab]

Westerners may swear by the Sunset maps, but for most of the country the U.S. Department of Agriculture map of plant hardiness zones is the gold standard. Why hasn't it been updated since 1990? A new version was prepared five years ago, under the auspices of the American Horticultural Society, but the USDA shot it down. Although what happened could be attributed to miscommunication, political or economic pressure may also have been involved.

The USDA has been producing hardiness zone maps, showing average low winter temperatures, at irregular intervals since 1960. The 1990 version was prepared by consulting meteorologist Mark Kramer and former AHS President Marc Cathey, based on data over a 13-year period ending in 1986.

In 2001, Kramer's firm, Meteorological Evaluation Services, contracted with AHS to create a new map using the most recent 15 years of reported temperatures. The USDA funded the project but did not deal directly with Kramer. The new map, released as a draft early in 2003, looked considerably different from its predecessors. Cathey and Kramer had added four new subtropical zones, eliminated the previous "a" and "b" zone divisions, and dropped coverage of Canada and Mexico. But what drew most attention was the rise in temperature averages over the new 15-year span. The 2003 map showed zones creeping northward.

"We sent the map back to USDA but got no feedback," says David Ellis, now director of communications for AHS. Kim Kaplan, spokesperson for USDA's Agricultural Research Service, says the technical review team recommended rejecting the 2003 draft: "It wasn't GIS/GPS compatible. The scientists said it couldn't be incorporated into existing models. The 'yes or no' decision was made at that point, even though other issues were being raised."

http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2008/07/30/HO9G11TRQE.DTL

[Karl_Bonner_1982]

1. They need to reintroduce the a/b divisions for more precision

But the zone creep is very interesting, and there could definitely be political reasons for not making new maps readily available.

I also think that the Sunset zone system needs to be updated, or at least the zone recommendations for various plants as they become more suitable for "colder-winter" climates. Also they need to eliminate the precision bias in the Sunset zones that favors nitpicky detail in California while making broad generalizations in the Northwest, and especially the intermountain West.

[XemaSab]

2. As a Californian, I have never really found much use for the USDA zones

I would imagine they would be more useful in parts of the country where cold weather means more. Sometimes it gets below freezing here, but we're just on that tempting edge where citrus, avocados, lantana, and other species might just make it through the winter. We get a little snow about once every 5 years, and usually if it's going to get truly cold we just throw blankets over the more delicate plants and hope for the best.

The real problem for our area is summers are cripplingly hot. 100+ for weeks on end, spiking up into the one-teens. Some plants love it, but other plants get SO drought stressed they just collapse. And we can't grow many succulents here because the soils are too rich and the winters are too cool and wet. We get as much winter rainfall here as Seattle in many years (though they get more summer rain, which keeps their total higher).

So Sunset is possibly broader in your area because you get crushing cold weather that we just don't have to worry about so much here, and plants are going to live or die on that alone. :shrug:

I think my Sunset description is dandy though:

ZONE 9. Thermal Belts of California's Central Valley
Growing season: late Feb. through Dec. Zone 9 is located in the higher elevations around Zone 8, but its summers are just as hot; its winter lows are slightly higher (temperatures range from 28 degrees to 18 degrees F/-2 degrees to -8 degrees C). Rainfall pattern is the same as in Zone 8.

(We're basically like the rest of teh valley but we don't have so-called cold air basins.)

http://www.sunset.com/sunset/garden/article/1,20633,845218,00.html

[Karl_Bonner_1982]

3. Zone 6: Willamette Valley

Long, wet winters, dry and comfortably warm summers, limited winter cold spells, etc.

But there are a TON of variations within Sunset Zone 6. Eugene is a frost pocket because cold air drains into the south valley from the surrounding hills. The "thermal belts" are mostly up around Portland, and a few down around Drain, Elkton and the Roseburg area. They don't follow the same regular geographical pattern as they do in zones 8 and 9.

Same goes for zone 1. On the USDA map it spans several hardiness zones and there are immense ranges in summer heat units within this zone. The only unifying factor seems to be short growing seasons.

Also, Sunset zone 3 in the Columbia Basin largely coincides with USDA zone 7 even by the old map. There are many plants recommended for "USDA 7-10, Sunset 4-24." I think a lot of gardeners in the inland Northwest have not experimented as much with less-hardy plants as they would if the Sunset plant recommendations were a little more liberal when it came to winter tolerance. The Sunset book should at least list which (borderline) plants would be worth experimenting with in your zone.