You're in the zone, but which one?

Perhaps your desk, like mine, is buried in drifts of plant catalogs that talk about "zones." Three different "zoning systems" currently exist, and untangling them can be confusing at first.

The oldest, and most commonly used, is the USDA Zone Hardiness map which calculates 11 zones and tells what level of winter cold a plant could experience in a location.

This system looks at Canada, the United States and Mexico, using data collected between 1974 and 1986 based on "average annual minimum temperatures."

Seattle falls into zone 8b with expected winter minimums of 15 to 20. It's useful to know about this USDA system if you order from nationally distributed catalogs.

Another zone system, the Heat Zone Map, developed by the American Horticultural Society in 1995, divides the United States into 12 zones indicating summer heat: the number of days in a year exceeding 86 F.

Both the USDA Zone Hardiness Map and the AHS Heat Zone map, because they look at only one factor, give Seattle some odd-couple zone partners.

On the USDA Zone map, Seattle is paired with Atlanta (similar winter temperatures) and on the Heat Zone Map the number of "heat days" puts Seattle in Zone 2, the same as Duluth, Minn., (only one to seven days per year exceeding 86 degrees.)

Any local gardener knows our climate resembles neither Atlanta nor Duluth though we may be able to grow plants that can thrive in either location.

It's obvious that the lowest winter cold temperature or summer heat days alone aren't the only limiting factors on plant growth. How much water's available, soil conditions such as pH, and elevation are also important.

Any gardener west of the Rocky Mountains can choose a more specific system developed decades ago by Sunset magazine editors, which charts 24 Western zones using winter minimums, summer highs, elevation, rainfall, humidity or aridity, and growing season.

The Sunset zone system links with their plant encyclopedia, which lists more than 8,000 plants and their zone needs.

Once you find your Sunset zone, you can determine which plants will fit in. Many local nurseries rely on the Sunset Western Garden Book to help get plants in the right places.

But you can't expect where your garden stands in the Sunset system to help you use a national catalog filled with USDA hardiness levels. Seattle, which is zone 8b in the USDA map, occupies Zone 5 in the Sunset system.

Your best approach is to use local sources, talk to local nurseries and keep your own records. Knowing what's happening in your garden helps you make wise plant choices.

Don Morrow, WSU Master Gardener and Consulting Rosarian, uses weather monitors to track rainfall and temperature, as well as keeping a log of plant performance in his Renton garden.

His temperature records for 2001 and 2002 indicate winter lows of 23 and 27. In May 2002 he noted one day with 35 and another with 84 — which may in itself illustrate the conundrums of Western Washington gardening.

In 2002 his first tomatoes, set out on May 18, produced on July 10, and his first blueberries on July 4. (Journal records make your garden that much more personal.)

Only 24 miles west of Renton, in North Bend, Beth Burrow reports that "everything is more, more rain when it rains, colder when cold, hotter when hot."

When I asked her about planting tender crops such as tomatoes, she said, "Old gardening wisdom says to plant tomatoes when the snow's melted off Mount Si. This has worked for one gardener for the past 15 years."

A distance of 24 miles won't show up on a national map like the USDA; and that gets back to the value of using a gauge like Sunset's with more detail, and keeping your own records.

Knowing thyself may be difficult, but knowing thy garden can be done — with some measurement tools and patience.

Mary Robson is area horticulture agent for Washington State University/King County Cooperative Extension. She shares gardening tips every Wednesday. Her e-mail is gardeningtips@seattletimes.com.