When to plant tomatoes in Bellingham, WA

Soil and feeding

Tomatoes do best in loose, well-drained soil with a pH of about 6.2 to 6.8. Work in a couple of inches of compost before planting and add a balanced vegetable fertilizer. Go easy on nitrogen: too much grows big, leafy plants that flower late and set less fruit, which is the last thing you want in a short Northwest summer.

Planting deep works because tomato stems grow roots wherever they're buried. A leggy transplant set in a trench or a deep hole ends up with a much larger root system, which helps it find water in August.

Why temperature matters so much

Tomato seeds sprout best in soil around 75 to 85°F and barely at all below 50°F. Once planted out, growth stalls whenever nights drop below about 50°F, which is why transplants set out in cold April soil often sit still for weeks and get passed by plants set out later.

Fruit set is the bigger issue here. Tomato pollen develops poorly when nights fall below about 55°F (and also when they stay above about 75°F or days top 90°F), so early flowers simply drop off. That's not a disease; it's the weather. Nights of 60 to 70°F give the best set. Varieties like Stupice and Oregon Spring were picked because they set fruit in cooler nights than most.

West of the Cascades, the easiest gains come from warming things up: plant against a south-facing wall, use black plastic mulch or wall-o-water protectors early, and wait to mulch with straw until the soil has warmed, since straw keeps cold soil cold.

Water, and what causes blossom-end rot

Water deeply and on a regular schedule, roughly an inch or more a week in summer, at the base of the plant. Blossom-end rot, the leathery black patch on the bottom of the fruit, is a calcium shortage inside the fruit, but it's almost always caused by uneven watering rather than a lack of calcium in the soil. Calcium moves to the fruit with water, so dry spells interrupt the supply. Steady watering and mulch fix it; adding calcium rarely does.

Cracked fruit has a similar cause: a heavy watering or rain after a dry spell makes the fruit swell faster than the skin can stretch.

Late blight, the big Northwest risk

Late blight is caused by Phytophthora infestans, a water mold (the pathogen behind the Irish potato famine). It thrives in cool, wet weather, roughly 60 to 70°F with leaves that stay wet for hours, which is exactly what arrives with the first fall rains. Spores blow in from nearby tomatoes and potatoes, and there's no cure once a plant is infected: dark, greasy blotches spread over leaves and stems, and fruit turns brown and firm.

Prevention is about keeping leaves dry. Water at the soil, space plants for airflow, remove the lowest leaves, and, if you can, grow under a simple rain cover or in a greenhouse. Varieties bred for resistance, such as Mountain Magic, Defiant PhR and Iron Lady, hold up noticeably better. Bag and throw out infected plants rather than composting them, and pull up volunteer potatoes, which can carry the disease.

Ripening and storage

Ripening is driven by ethylene, a natural plant hormone, and the red pigment (lycopene) forms best at about 68 to 77°F. Above about 85°F it barely forms, which is why fruit can stall orange in a heat wave, and in cool fall weather ripening slows to a crawl.

A tomato picked once it shows its first blush of color will ripen indoors with nearly the same flavor as one left on the vine. Before fall rains or frost, pick full-size green fruit (glossy, with a lighter color starting at the blossom end) and ripen it at room temperature. Keep ripe tomatoes on the counter: the fridge, below about 55°F, dulls their flavor and turns the texture mealy.