When to start tomato seeds

How many weeks before the last frost?

University of New Hampshire Extension gives tomatoes 6 to 8 weeks from seed to transplant and says they are safe to set out 1 to 2 weeks after the last frost date. Put those together and you sow roughly 4 to 7 weeks before your last frost. For example, with an average last frost of May 10, you would sow indoors March 22 to April 5 and set plants out May 17 to May 24.

University of Minnesota Extension uses a shorter window of 5 to 6 weeks indoors and warns that plants started earlier are hard to manage and don't necessarily give an earlier or bigger harvest. If you have limited light or space, start at the shorter end. A stocky 6-week-old plant beats a leggy 10-week-old one.

Germination: warmth matters more than anything

Tomato seed sprouts slowly in cool soil. In J. F. Harrington's germination trials (published by Oregon State Extension), seedlings took about 43 days to appear at 50°F, 14 days at 59°F, 8 days at 68°F and 6 days at 77°F. The minimum soil temperature listed is 50°F and the optimum is 85°F.

Iowa State Extension lists 70–75°F as the target for indoor germination, with seedlings appearing in 6 to 12 days. UMN recommends a heat mat that keeps the tray at 75–85°F until the seedlings come up, then about 70°F soil afterward. Heat mats dry the mix quickly, so check moisture daily.

Sow seeds about a quarter inch deep in a sterile, soilless seed-starting mix (UMN). Iowa State notes tomato seeds germinate in darkness, so cover them.

Growing strong seedlings

• Light: UMN advises keeping fluorescent or LED lights 2 to 4 inches above the seedlings for 12 to 16 hours a day. A windowsill usually gives tall, bent stems.
• Spacing: after true leaves appear, thin or pot up so seedlings stand about 2 inches apart.
• Feeding: seedlings don't need fertilizer until they have several true leaves. Then use a quarter-strength soluble fertilizer once a week (UMN).

Hardening off and transplanting

When plants are about 5 inches tall and 6 to 8 weeks old, UMN suggests reducing watering and starting to move them outdoors: first to a spot with wind protection and a couple of hours of sun, then more sun each day over a week or two. Bring them in if nights approach freezing.

Transplant after all danger of frost has passed and the soil has warmed, ideally in late afternoon or on a calm, cloudy day. Tomatoes can be planted deeper than they grew in the pot: UMN describes laying the plant in a trench 3 to 4 inches deep, after removing the lowest leaves, so roots form along the buried stem. Put stakes or cages in at planting time, and allow 2 to 3 feet between vining plants.

Rotate: UMN recommends a spot where you haven't grown tomatoes, potatoes, peppers, eggplant or tomatillos for the past three or four years.

Can you direct sow tomatoes?

UNH lists tomatoes among crops that should be started indoors and set out as transplants. In short-season areas, a seed sown outdoors after frost rarely has time to ripen much fruit. If you skip seed starting, buy sturdy transplants and plant them on the same schedule.

Common mistakes

• Starting too early. Ten-week-old tomatoes in small cells get root-bound and leggy. Count back from your set-out date, not from when you get spring fever.
• Cold germination. At 59°F, seeds take two weeks or more to emerge. Use a heat mat or the warmest spot in the house.
• Not enough light. Keep lights close and raise them as plants grow.
• Planting out on the frost date. The average last frost is a midpoint, and tomatoes are tender. Wait the extra week or two.
• Skipping hardening off. Indoor-grown leaves can scorch in full sun and wind.