Washington State’s geography creates a wide range of microclimates, meaning there is no single date for planting tomatoes across the region. Successful gardening requires acknowledging the differences between the mild, maritime conditions west of the Cascades and the hotter, continental climate to the east. For heat-loving crops like tomatoes, timing is determined not by a fixed calendar date but by specific temperature thresholds in the air and the soil. Planning a successful tomato season begins indoors, using local frost data to calculate the optimal seed-starting timeline.
Calculating the Indoor Start Date
The first step is accurately scheduling when to sow seeds indoors, a calculation universal regardless of location. Tomato seeds need a head start of approximately six to eight weeks before they are ready for the final outdoor transplant. To determine this indoor start date, identify your area’s average last spring frost date and count backward. This period allows the young plants to develop the robust root systems necessary to thrive once moved outside.
By the time the weather permits outdoor planting, your seedlings should have reached a specific stage of maturity. The ideal size for transplanting is generally between six and ten inches tall, exhibiting a sturdy stem structure. More importantly, the plants must have developed several sets of true leaves, which are the second, distinct set of leaves that grow after the initial cotyledon leaves emerge. Starting the seeds too early can result in tall, spindly, or “leggy” plants that are more susceptible to transplant shock and wind damage.
Timing for Western Washington
Gardeners in Western Washington, encompassing the Puget Sound region, contend with a cooler, maritime climate where spring soil temperatures warm up slowly. The primary challenge is not the risk of a hard frost, which typically passes by mid-April, but the persistently cool nights and soil. Tomatoes, originating from tropical regions, will stall and struggle if planted into cold earth, regardless of how warm the daytime air feels.
For optimal root development, the soil temperature must consistently register at least 60°F, measured six inches deep, before transplanting. Waiting for this temperature is non-negotiable, as cold soil leads to stunted growth and nutrient deficiencies, often causing the leaves to turn purple. Furthermore, the nighttime air temperature must reliably stay above 50°F; temperatures below this threshold can cause tomato blossoms to drop, preventing fruit set. This combination means the safe outdoor transplant window for most of Western Washington is late May, often coinciding with Memorial Day weekend, or even later into early June.
Planting a week or two later is always preferable to planting too early. Many successful West Side gardeners utilize soil warming techniques, such as covering the planting area with black plastic mulch for several weeks before their target date. Choosing short-season or determinate tomato varieties is also highly recommended to ensure the fruit has enough time to mature fully during the cooler, shorter growing season.
Timing for Eastern Washington
The strategy for Eastern Washington, including areas like Spokane, Tri-Cities, and Yakima, contrasts with the maritime West Side due to its continental climate. This region experiences harsher winters but often sees spring soil warm up more rapidly once the snow melts and the sun intensifies. The average last frost date is typically later than the west, falling closer to mid-May for many inland areas, but the ensuing summer heat arrives quickly.
While the soil may reach the necessary 60°F minimum sooner than on the coast, the variable nature of the last hard frost demands careful monitoring of the local forecast. A late, unpredictable frost event can wipe out an entire bed of tender tomato seedlings, making the general May 15th to May 20th window a guideline, not a guarantee. The necessity of waiting for the 60°F soil temperature remains the same to ensure active root growth and nutrient uptake.
The benefit of the Eastern Washington climate is the rapid onset of hot summer temperatures, which allows for a slightly earlier outdoor transplanting compared to the West Side’s typical June start. This earlier window is necessary because the intense summer heat accelerates the plant’s growth and maturity, allowing a wider selection of tomato varieties to be grown. Gardeners must be prepared to provide consistent deep watering once the summer heat arrives to manage the drier conditions.
The Critical Step Before Transplanting
Once the date is determined and temperatures are suitable, the most important step before the final transplant is called “hardening off.” This 7- to 10-day transition process acclimates the pampered, indoor-grown seedlings to the harsher realities of the outdoor environment. Without this preparation, the delicate plants risk severe “transplant shock” from exposure to wind, direct sunlight, and fluctuating temperatures.
The process begins by placing the seedlings in a shaded, protected location outdoors for just one or two hours on the first day. Over the next week, the time spent outside is gradually increased each day. The plants are also slowly introduced to more direct sunlight, moving them from full shade to dappled light and then to morning sun. This slow progression allows the plant’s cuticle—the waxy, protective layer on its leaves—to thicken and its stem structure to become more resilient.
After approximately one week, the seedlings should be able to tolerate a full day outside, but they must still be brought indoors or placed in a protected, unheated space, such as a garage, overnight if the temperature drops below 50°F. By the end of the hardening-off period, the tomato plant will be tougher and ready to face the permanent conditions of the garden bed without suffering a growth-stunting setback.