Gardening in the Puget Sound area presents challenges that defy conventional national planting calendars. The region’s marine influence creates unique microclimates, making standard guides unreliable for vegetable timing. This guide provides a localized framework specifically tailored for the Seattle gardener to successfully plan the vegetable patch. Correct planting timing is crucial here, ensuring crops are started when conditions are most favorable to maximize the growing season.
Defining Seattle’s Unique Climate
The mild, temperate nature of Seattle’s weather is governed by the maritime effect, which acts as a thermal moderator, keeping winters mild and springs cool. This influence means that while the region rarely experiences prolonged deep freezes, the necessary warmth for many summer vegetables is significantly delayed. The average last frost date for the Seattle area falls between late March and mid-April, and the average first frost typically does not arrive until mid-November. Successful planting relies less on the USDA Plant Hardiness Zone (9a) and more on recognizing the cool, slow-to-warm pattern of the spring soil.
Timing for Cool Season Vegetables
Cool-season vegetables thrive in Seattle’s characteristic weather and represent the region’s greatest gardening strength. These crops can be planted as soon as the soil is workable, often by late February or early March. The initial spring planting window is dictated by soil moisture, as waterlogged, cold ground can cause seeds like peas to rot. Direct sowing of hardy crops like peas, radishes, spinach, and lettuce can begin in late February or early March, provided the soil is not overly saturated.
For slightly more frost-sensitive crops, such as kale, collard greens, cabbage, and beets, the window typically opens in mid-March through April. Root vegetables like carrots and parsnips can also be sown directly into the ground from early spring onward. This extended cool-weather period allows for multiple staggered plantings of leafy greens to ensure a continuous spring harvest before the summer heat causes them to bolt.
The second major planting window occurs in late summer for a fall and winter harvest. From late July through August, sow seeds or transplant starts for fall brassicas, including broccoli, cauliflower, and Brussels sprouts. Hardy greens like kale, chard, and fall-maturing lettuce varieties can be planted throughout August and early September. These crops must reach near-maturity before the shorter days of late fall significantly slow their growth.
Timing for Warm Season Vegetables
Warm-season vegetables, which require both warm soil and sustained high air temperatures, present the greatest timing challenge in the Seattle climate. Plants like tomatoes, peppers, eggplant, squash, and corn must bypass the cool spring to ensure they have enough time to mature before the season ends. This requires a two-part strategy: starting seeds indoors under controlled conditions and delaying the outdoor transplanting.
Seeds for long-season, heat-loving crops like tomatoes and peppers should be started indoors approximately six to eight weeks before the target outdoor transplant date, typically in mid-March. These seedlings need warmth and light to develop a robust root system, which will allow them to quickly establish once moved outside. The outdoor planting date for all warm-season vegetables is generally mid-May to early June, well after the average last frost date.
The soil must be consistently warm—ideally 60 degrees Fahrenheit or higher—before transplanting heat-sensitive plants like peppers and squash. Cucurbits, such as squash and cucumbers, can be direct-seeded in late May or early June, or started indoors in late April for a head start. Gardeners often employ season-extending tools like row covers or cold frames to raise the soil temperature and protect tender transplants from cool evenings. Choosing short-season or early-maturing varieties is also necessary for a successful harvest within Seattle’s brief summer growing window.
Succession Planting for Continuous Harvest
Succession planting maximizes output by staggering planting times. This strategy ensures that as one crop finishes its harvest cycle, another is ready to take its place, preventing gaps in food production. For fast-maturing crops, such as radishes, lettuce, spinach, and bush beans, re-sowing a small batch every two to four weeks from early spring through mid-summer is effective.
The technique involves planting a portion of the garden area rather than the entire crop at once, which spreads the harvest over a longer period. For instance, space cleared after the spring pea harvest can be replanted with a quick-maturing summer squash or a fall green. Planning the final succession of fall crops is tied to the first expected frost date in November. By calculating backward from this date using the “days to maturity” on the seed packet, the gardener determines the final possible sowing date for crops like carrots and beets.