The Grounded Gardener: Know your zone to map out the best garden

By MARTY WINGATE
SPECIAL TO THE P-I

AN EARLY SPRING garden tour to the Southeast, with an extra stop in Texas, showed our group of Pacific Northwest gardeners just how important it is to pay attention to the way climate affects what we grow.

How is that, while Charleston, S.C., and Seattle are in the same USDA hardiness zone, Pittosporum tobira can be a small landscape shrub here but a sizable tree in Charleston -- and in full fragrant bloom in March?

Here's what I learned on my spring break in Charleston, Savannah, Ga., and San Antonio: It isn't only how cold we get that matters to our plants.

As helpful as it is -- especially in cold-weather climates -- the USDA hardiness zone system, based on the lowest average temperatures in a region, is not the whole story.

Two other zone systems can help us understand how our gardens grow, and how to choose the best plants:

The American Horticultural Society (AHS) Heat-Zone Map -- It has 12 regions, assigned according to how many days a year the temperature is above 86 degrees.

The Sunset Garden Climate Zones -- This system that takes into account the length of a region's growing season, rainfall amount and pattern, humidity and temperature highs and lows.

In the Seattle area, our zone designations in these systems are: USDA 8; AHS 2; Sunset 5.

It's good for our gardens if we can get these zone systems straight -- even if we have to write down our numbers and carry them around with us -- because more nursery plant tags are including the heat zone and/or Sunset zone along with the USDA hardiness rating.

USDA

The USDA system is the most common one found on plant tags and in catalogs. It shows us to be in zone 8, which runs in a thin strip of land along Puget Sound, and around the top of the Olympic Peninsula. In zone 8, we have an average winter low between 10 and 20 degrees, although many small pockets of our region border on zone 9 (average low of 20-30 degrees).

Zone 8 extends through western Oregon, into parts of California, Arizona, New Mexico, through Texas and on to the coastal Southeast.

This, however, doesn't answer the question why, in Charleston, Pittosporum tobira grows into a small tree and summer geraniums (Pelargonium) were in full bloom in March. The answer to those questions is how hot it gets in the coastal Southeast.

AHS

The American Horticultural Society's heat-zone map looks much like the USDA map, with colorful swaths painted across North America. It shows that Charleston receives 60 to 90 days a year above 86 degrees, that's AHS zone 7.

The Seattle area, on the other hand, gets just one to seven days a year above 86, and that puts us in zone 2.

As a comparison, the Olympic Peninsula "rain shadow" town of Sequim (USDA Zone 8/9), falls into AHS zones 3-4: up to 30 days above 86 degrees. The difference in the amount of heat shows up in plant growth and flowers.

Sunset

The Sunset Garden Climate Zones look at our gardens in a different way, with a different set of numbers. The country is divided into smaller pieces -- 45 zones, according to climate.

Until recently, Sunset zones applied to the West only, but now there are Sunset zones for all of the U.S. Seattle is in Sunset zone 5, described on the Sunset Web site as an "English-garden climate"; Charleston and Savannah, on the other hand, are in zone 28, an area characterized by high humidity and rainfall year-round.

Looking up all the zones was how I knew for sure that I couldn't grow Jatropha integerrima, which I saw in the gardens at the Alamo in San Antonio. This evergreen shrub, native to Cuba, sports clusters of red flowers. It's listed as USDA 10-11 (we're only 8) and AHS 12-9 (we're 2). Note that in the AHS listings, the zones are listed with the hottest region first.

My Sunset Western Garden Book doesn't list Jatropha, but it does include Cordia boissieri, the Texas olive. This small evergreen tree blooms in clusters of white, petunialike flowers over a long period. Cordia is USDA 10-11, AHS 12-10 and Sunset 8-24 (we're 5).

Besides excluding plants from our gardens, the various zone systems help to explain why plants grow differently. In Savannah, lovely small Foster's holly trees (Ilex x attenuata 'Fosteri'), a cross between two Native American hollies, grew outside the Ships of the Seas Museum.

Always on the lookout for another great small tree for our gardens, I snapped a few pictures and came home ready to recommend the tree, which is hardy in USDA zones 6-9 (pop quiz -- we are ... yep, 8).

I discovered, however, that while we can grow this tree, it will not grow not quite as well as in Savannah. Foster's holly is AHS zone 9-4 (we're ... yes, 2).

A quick check with Randall Hitchin, registrar at the Washington Park Arboretum, confirms this. He says the arboretum does contain a couple that are about 25 feet high, but they also are 50 years old.

Don't think I want to wait that long.

ZONE MAPS ONLINE

The USDA zone map can be found inside many national plant books. Online, you can check your zone on the Arbor Day Foundation Web site: goto.seattlepi.com/r1364

The American Horticultural Society's heat zone system is explained at: goto.seattlepi.com/r1365

A map of Sunset's Garden Climate Zones for the entire country is found at: goto.seattlepi.com/r1366

Marty Wingate, a Master Gardener, is the author of two garden books. She can be contacted at: martywin@earthlink.net.

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