When to Start Seeds Indoors by Zone: A Countdown

The sources do not agree on tomatoes

Counting back is the method every source here uses. Ohio State’s BYGL says find your frost-free date, read the packet for the weeks until transplant, and count backward; its Morrow County example puts tomatoes in 8 weeks before a May 9 frost-free date. Cornell says about 6 weeks before your transplant date, which for warm-season crops it puts 2 to 3 weeks after the average last frost. University of Minnesota Extension’s schedule starts them 5 to 6 weeks before planting out, in early April. Our tool uses 6 weeks before the last freeze.

Cornell’s advice settles the difference: err on the side of too late, because a small plant catches up and an overgrown, leggy one may never recover. University of Illinois Extension says the same of starting too early, tomatoes in particular.

Setting up

• Not on the windowsill. Minnesota says a windowsill can be the coldest spot in the house at night and the hottest by day, and seedlings there grow too tall with thin, bent stems. Ohio State’s BYGL adds that many new windows do not let enough light through. If you are starting only a few plants and have roomy sills, Minnesota allows that a south-facing window may do.
• Lights. Fluorescent or LED, Minnesota says, no more than 4 inches above the seedlings — 2 inches is ideal — for 12 to 16 hours a day, with a dark period each night, on a timer. Hang them on chains so you can raise them. Lack of light is the main cause of long, skinny stems.
• Bottom heat. A heat mat made for seed starting helps seedlings and can prevent damping off (Minnesota). Do not plug it into the light timer. Ohio State’s BYGL moves trays off the heat mat once the seeds have sprouted.
• Containers and mix. A sterile soilless seed-starting mix, usually peat and vermiculite (Minnesota); containers about 3 inches deep with drainage holes, reused ones washed in a 10% bleach solution (Cornell).

Sowing and growing

1. Sow shallow. Cornell says 2 to 3 times as deep as the seed is wide, a few extra seeds per container, and label each with the variety and date.
2. Cover, then uncover. Keep the mix evenly moist; take off the plastic or dome once seeds sprout (Ohio State) or when seedlings touch it (Minnesota).
3. Feed lightly. Minnesota says a water-soluble fertilizer at ¼ strength once a week, plain water the rest of the week.
4. Toughen the stems. Ohio State’s BYGL brushes the seedlings with a hand every day, which mimics the wind, and turns trays if the plants lean toward the light.
5. Pot up. Move seedlings that outgrow their cells into containers 1 to 2 inches wider, holding them by a leaf, never the stem (Minnesota).
6. Harden off. Two weeks before planting out, a few hours a day in shade and out of the wind, a little longer and sunnier each day (Minnesota).

Common mistakes

• Starting too early. Leggy plants may never recover (Cornell).
• Relying on a window. Too cold at night, too hot by day, too little light (Minnesota).
• Lights too high. More than 4 inches up gives stretched seedlings (Minnesota).
• Lights on all night. Seedlings need a dark period (Minnesota).
• Skipping hardening off. Sun and wind can scorch leaves, and plants may wilt and die (Minnesota).

Where the countdown does not work

Where a station never freezes there is no last freeze to count back from: in zone 10, 19% of the stations record none, so the zone row is a median of the rest. Our seed-starting tool says so for those ZIP codes rather than inventing an anchor; in the low desert, the Arizona calendars on our Arizona page give local sowing months instead.

Each crop’s own page carries its full calendar by zone, from seed to last planting: Tomato, Pepper, Lettuce, Onion, Cucumber, Zucchini and Squash, Broccoli, Kale, Cabbage and Cauliflower.

Next: January garden jobs by zone and how to plan next year’s vegetable garden.