MARCH 24, 1997
Gardening: Last frost dates
Dustings of snow recently in south Pierce County, Olympia, and Snohomish County lead many gradeners to wondering if spring is ever going to arrive. The soil is still cold and wet. Spring bulbs are emerging slowly, but they are cheering us by coming up. Overnight lows, even in the urban areas, are still down in the thirties. This doesn't add up to a very promising flower or vegetable gardening scene!
Patience is in order. Seed packets often say "Sow outdoors after all the danger of frost is past." And when, we ask, will that likely be? It depends on your location, on what the specific microclimate of the garden is. Records have been kept for over 60 years at the University of Washington tracking first and last frost days. This location is particularly warm and sheltered, as are other fortunate areas of King and Pierce Counties. They suggest March 23 as the last frost date, but since it's a "mean," the frosts could come later. The "mean" predicts that there is a 50% chance of frost on this date.
The closer to the water the garden is, the milder the temperatures. The moderating effect of Puget Sound or Lake Washington, for instance, which results in milder winter temperatures, extends inland for some distance. If your garden is more than a mile or so from water, that moderating influence could vary. The last frost date for Vashon Island is April 5; for the Sea-Tac area, April 9. Again, add at least a week and check your own garden temperatures and patterns. North of Everett and in the Olympic area, frosts often come quite late.
Elevation also affects cold. Think of the common weather prediction "ice or snow at the higher elevations." Cold air drains downhill like water; gardens at the bottom of hills or in low valley areas will have later spring freezes. Individual gardens will have warm spots and cold pockets depending on slope and sun orientation. A gentle, southwest facing slope will warm sooner and stay warmer than a shaded north lowland area.
Snoqualmie is listed as having its last spring freeze on May 6; the Bothell area, May 13. None of these dates can be relied upon specifically; they simply delineate a pattern in your garden. A minimum-maximum thermometer is an excellent garden tool. Even with these many variations, it's easy to see that a gardener in a sheltered area might experience the last frost on March 22, while one in a higher, more exposed part of the Puget Sound region could still see frosts 6 weeks later.
Texture of the soil affects warming. After a long, wet winter soils dry and warm gradually. Do you know what texture your garden soil has? Sandy and gravelly soils will dry and warm faster than heavy clay soils. Digging in and working with soils taht are too wet, particularly those with heavy texture, can create lasting clods and cause soil structure damage. If the soil is very heavy, don't mulch over the vegetable garden until it is well warmed, which may be as late as June or early July.
If you want to determine soil texture in a rough way, put a handful of soil in a quart jar. Then fill the jar with water, adding a teaspoon of liquid soap to help distribute the soil. Shake and set it aside for a week or so. The heaviest soil particles will fall to the bottom, and the lighter ones will distribute themselves in layers. Clay and silt particles will settle very slowly. THe percentages of sand to clay to silt determine texture.
To check for "readiness to dig" pick up a handful of soil. Give it a good squeeze, making a firm ball or wad. Press against the soil ball with your thumb. If it holds together and resists breaking, the soil is probably too wet to dig, walk on, rototill, or drag heavy equipment over. If the ball crumbles, the soil can be dug. Experience with your particular garden site will help in determining when to work.
Since the spring of 1997 is arriving slowly, there's still plenty of time to plant peas, cole crops, chard, collards, and cool weather flowers like sweet peas and calendulas. Prune apple and pear trees now if it hasn't been done, because they have not yet begun to leaf out. Complete the pruning before bloom. Pull out and destroy winter weeds like bittercress and chickweed. But if the soil is very wet, walk on it as little as possible. Start vegetable seeds indoors now.
Gardeners often want to modify the site to help enhance heat availability. A plastic or fiber row cover can add 4 to 7 degrees of warmth, protecting early crops from late frosts. Plastic tunnels, gallon plastic botles, and the shelter of the covered "cold frame" are other ways to help enhance the heat. Planning contributes to success with early spring gardens, especially when vegetables or tender annual flowers are grown.
For more information on climate and starting gardens, send a self-addressed stamped envelope to WSU Cooperative Extension, 700 Fifth Avenue, 37th Floor, Seattle, Washington 98104-5037. Request Community Horticulture Fact Sheet #8, "Starting Garden Crops," Fact Sheet #19, "Row Covers," and Fact Sheet #40, "Frost Dates/Climatic Data." Each Fact Sheet requires 32 cents postage.
Visit WSU's Agriculture Site on the World-Wide Web at http://www.cahe.wsu.edu.