There are two basic climates in the Pacific Northwest. Those parts of Oregon and Washington west of the Cascade Mountains generally have mild, wet winters and earlier planting times. Idaho, Montana, and Oregon and Washington, east of the Cascades, have cold winters, so their spring growing season is later. Minimum soil temperatures must be followed, but there's no need to worry about springtime soil being too hot for seeds to grow in the Pacific Northwest.
Taking Soil Temperature
The Oregon State University Extension Service recommends that you take the temperature of your soil in the early spring. Insert your thermometer two inches into the soil. As the spring progresses, push the thermometer in four inches. Calculate the average temperature taken at midday each day for several days. You will generally have to wait longer before you plant seeds east of the Cascade Mountains. You need to know the lowest soil temperatures (below) in which certain seeds will still germinate.
Arugula, fava beans, kale, lettuce, pac choi, parsnips, peas, radicchio, radishes and spinach will germinate at soil temperatures of 40 degrees Fahrenheit or above.
Chinese cabbage, leeks, onions, Swiss chard and turnips will germinate in soil temperatures of 50 degrees or above.
Beans, beets, broccoli, Brussels sprouts, cabbage, carrots and cauliflower need soil temperatures of 60 degrees or above in order to germinate. If you get a frost, you'll likely have to replant the beans.
Cucumbers, corn, eggplants, melons, squash, and tomatoes need soil temperatures of at least 70 degrees to germinate. Eggplants, peppers and tomatoes grow slowly, often taking weeks to grow large enough to plant outdoors; you can buy them as starts at a garden center.
Make sure your seed bed is well-aerated and has plenty of organic matter. Buy varieties of seeds that are cold-tolerant or have short growing seasons. Warm the soil in the spring with a cloche, plastic mulch, cold frame or a fabric or a "floating" row cover of fabric or spun fiber. Be prepared to cover your crop if it's threatened by a predicted late frost.
If you plant your seeds before the soil is warm enough, the seeds can rot. They can also germinate slowly, grow poorly and be subject to disease.