Like many a Seattle-area gardener, when Marcia Dillon first tried growing tomatoes here, she was shocked by how hard it was.

“When I first moved here, it was quite humbling,” she says. “Back in Ohio, you plunked the plant in the ground and it grew.” 

She learned that heat-loving tomatoes need some help to fruit in our comparatively chilly, wet summer conditions. After several years of trial and error, and training as a master gardener, she became known as Seattle’s “Tomato Lady.” If you’ve bought a tomato start at the King County Master Gardener plant sale, Dillon probably helped select and tend to it. 

Dillon manages the tomato beds and the greenhouse operations for Bellevue’s Master Gardener Demonstration Garden, which grows more than 5,000 seedlings for annual plant sales. Each season, she grows about 125 varieties between her home, a Bellevue P-Patch and the Demonstration Garden, with delicious results. 

Here are her tips on wrangling luscious, flavorful fruit — no matter what the weather brings this season.

Hacking the weather

Tomatoes require 1,300 heat units to mature and ripen — these are calculated as a day’s mean temperature minus 50 degrees (the minimum growth temperature for many plants). A mild Seattle summer day, with a high of 70 degrees and a low of 50, yields 10 heat units. So we need 130 of those, or warmer, days from May through October. 

Advertising

The gardener’s job is to “maximize every aspect of tomato growing,” Dillon says. Managing temperature is job one. You are aiming for the sweet spot for tomatoes — air temps between 55 and 85 degrees. Lower than 55, tomato plants hunker under the covers, and higher, they drop blossoms or get sun-scalded fruit. 

While Mother’s Day is generally considered tomato planting time, a soil thermometer will better tell you if your ground is ready for transplants; look for temperatures consistently close to 60 degrees. You can improve your odds by choosing a naturally warmer spot, such as against a south- or west-facing wall, or by pre-heating your soil with plastic about three weeks before planting. Dillon says clear plastic won out over black in the Demonstration Garden’s tests, retaining twice as much heat. 

Remember to harden off, or acclimate, homegrown starts by giving them a few midday hours of sunbathing for about five days before planting them outside.

Season extenders, such as plastic rings filled with water or PVC hoops covered in frost cloth, can raise ambient temperature significantly and protect tender starts from spring chills.

Mulching your plants with breathable straw or wood chips is invaluable, Dillon says. It keeps roots warmer in spring and cooler in autumn, retains moisture and reduces soil splashing onto the plant while watering, an act that can foster disease. 

If a heat wave hits, you can use bedsheets or row cover to shade the plants.

Advertising

Choosing varieties and decoding tags

Selecting the right varieties is another key to success, and a plant’s tags can help you. “Determinate” tomatoes ripen all at once, which is perfect for sauce-making gardeners. They also tend to be shorter plants, so are good for growing in pots. “Indeterminate” types keep putting on new flowers and can grow up to 8 or 10 feet. 

The biggest factor for success, however, is the days to maturity (DTM) number on the tag, which estimates time from transplanting to harvest. 

Bell peppers can be grown in the Pacific Northwest, but are challenging because of their long growing season. (Getty Images)
How to grow successful peppers and other garden questions

“Days to maturity are probably more important here than anywhere else in the country,” Dillon says.

In Seattle, she says, it’s smart to keep most of your plants in the 65–75 DTM range. Small fruits generally ripen faster. Large 2-pounders needing 90 days to mature are a risk in our climate. 

You might be tempted by super-early varieties under 60 DTMs, but Dillon has found they tend to be bland.

Advertising

“Hybrid” or “F1” means two tomato seeds have been crossed by bees or humans — not genetically modified — and that plant’s seeds will not produce the same plant. “Open-pollinated” means you can save seed from that plant and grow it again next year. An “heirloom” is an open-pollinated plant that has been saved for 40–50 years. 

 Because they are used to chilly nights, tomatoes hailing from Russia, Ukraine and Canada are often well-suited to Seattle summers. Some examples are Black Krim, Moskvich and Midnight Sun.

“If you are a new gardener with room for one tomato, make it Sungold,” Dillon says, referring to the hugely popular, orange, cherry-tomato hybrid whose flavor is candy-sweet with a tropical note. Its only flaw is that the skin can split under excessive rains.

Planting and care

Tomatoes are a full-sun crop, needing a minimum of six hours of direct sun. Give your biggest-fruit varieties or those with the longest DTMs the primo hot spots, Dillon says. 

Next, bury your starts almost to their necks — it’s terrifying, but it works. Tomatoes form roots all along their stems, and this super-charges root development and creates a sturdy plant. 

Dillon likes to add a slow-release, granular, organic fertilizer at planting, which usually suffices if the plants are in the ground. Potted tomatoes, which lose nutrients with watering, may benefit from one or two monthly applications of liquid fertilizer with a balanced or lower nitrogen count (one labeled 5-10-5, for example). Finally, add mulch and water well.

Sponsored

Those little starts will grow big — and fast. Allow them 2 square feet in the ground. If you are growing in pots, use at least a 10-gallon pot (14-inch diameter) for indeterminate varieties, with a strong trellis or cage. There are dwarf and even micro-dwarf varieties, like Rosella Purple and Tumbling Tom respectively, which stay under 3 feet and can grow in a 5- or 10-gallon pot. 

Dillon also removes the bottom 8–12 inches of leaves as the plant grows. She prunes some suckers (the 45-degree shoots coming out between the stem and a larger shoot) for air circulation and to keep the plant from becoming top-heavy. This, she says, produces fewer but larger, tastier fruits.

Water your plants deeply and consistently — inconsistent watering is the cause of blossom end rot, which causes soft, dark spots to form on the fruit’s base. Water the soil rather than the leaves to forestall fungal disease. 

When flowers appear, Dillon uses a sonic fertilizing device called a VegiBee to gently shake them, mimicking the frequency of bumble bee pollination, saying it’s been shown to boost flowers — and thus fruit — up to 30%. 

Your best tools, however, are your eyes, says Dillon. It’s easier to course-correct if you watch your plants closely for changes. If you have questions, Dillon recommends taking advantage of Master Gardener clinics, now in-person and online. 

King County Master Gardeners is not holding its sale this year, but the Bellevue Master Gardener Demonstration Garden will be selling tomato starts on Wednesdays in May from 9 a.m. to 1 p.m., while supplies last. And the garden is open daily to offer inspiration for growing a beautiful and productive vegetable garden.

Advertising

Seattle’s best

Dillon says she grows every shape and color available in the tomato family, some varieties are easier than others to grow in our climate. These are a few of her proven — and delectable — favorites and their estimated days to maturity (DTM). All are indeterminate unless noted.

Cherry/grape

Sungold (60): Orange-fruited hybrid cherry with a sweet, tropical taste. Wonderful fresh or roasted. 

Sweet Million (65): Highly productive red hybrid cherry.

Juliet (60): Juicy red grape hybrid.

Small

Tiger Series (Blush, Green Tiger, Lucky Tiger and Pink Tiger) (70): Gorgeous striped, elongated fruit with flavor for days. Open-pollinated.

Stupice (60): Czechoslovakian, cold-tolerant heirloom with clusters of round red fruit.

Matina (65): Vigorous German, potato-leafed heirloom with red fruit. Very productive.

Bronze Torch (65): Large grape-style fruit in brick red with bronze striping. Hybrid.

Bloody Butcher (55): Exceptionally rich flavor for such an early fruit. High-yielding heirloom thought to be from Germany.

Slicers

Black Sea Man (75): Determinate. Richly flavored Russian heirloom with brown-pink fruit with olive shoulders. 

Goose Creek (75): Pink-fruited heirloom with complex flavor. Passed down by enslaved Caribbean people in North Carolina.

Big Beef (73): Round, full-flavored red fruit. Disease-resistant hybrid.

Sweet Tangerine (68): Determinate. Disease-resistant hybrid with meaty fruit and few seeds.