Seattle, while a moderate climate, has a fairly short growing season because spring temperatures stay cool well into June, and the fall arrives early and rainy, ruining many long-season crops. Tomatoes in Seattle can be grown with good timing and proper care, keeping an eye on the weather. Since tomatoes produce better flavor in higher temperatures, Seattle gardeners sometimes struggle with growing tasty tomatoes, although size is not a problem due to the availability of water.
Choose a variety of tomato that will ripen in Seattle's conditions, such as "Northern Exposure," "Early Girl," or "Oregon Spring." The early-ripening and Northern-bred tomatoes work well because they have longer to grow, and tolerate cool temperatures better. Use seedlings, not seeds, unless starting them indoors or in greenhouse conditions.
Plant tomatoes in a good location. A spot that gets at least six to eight hours of full sun each day is best, as is a south-facing spot. Placing tomatoes against a south-facing wall or fence is a great technique near Seattle, because it makes the most of retained heat from the sun. Use good soil. Mix in plenty of compost and good, humus-rich soil. Manure is ideal for giving tomatoes a nutrient boost in the spring, getting them started well in the ground.
Add support. Tomatoes will need support later in the summer, when fruit makes them heavy, but that's a bad time to disturb the soil and roots. Add support now with wooden stakes, rebar pieces or even wire cages.
Water carefully. In Seattle weather, cool and wet soil or plants can contribute readily to mildew, mold and bug infestations. Water from the bottom, directing the water onto the soil, not splashing it on the leaves or soaking the stems, to cut down on these risks. Time the watering right to create early fruit. Water twice a week until August, then cut down on watering, to stress the tomatoes into ripening quickly before fall weather arrives, bringing more cool temperatures and sometimes early frosts in Seattle.