Start tender crops such as tomatoes, peppers and squash under cover several weeks before your local last frost, count back from the planting-out date by the number of weeks each crop needs to reach transplant size, and harden the seedlings off over about a week before they go into the open. Find your last-frost date from your own temperature records, since no date applies everywhere. Give seedlings strong light, or they stretch and weaken, and ventilate to prevent damping-off.
Spring-started transplants are seedlings begun in a protected place, such as a cold frame, greenhouse or windowsill, and moved to the garden when the weather allows, to gain weeks on the growing season.
Counting Back From The Last Frost Gives The Sowing Date
A transplant buys time: the crop spends its first weeks in shelter instead of cold ground. The carrier guide (Four-Season Growing & Season Extension) puts onion starts indoors in February and early spinach in the cold frame in the same month, and warm-season crops outdoors after the last frost in May. Its monthly table is written for one climate (it labels itself Zone 6). The zone and dates are not yours; use the method below.
What You Need
- Your last-frost date, found from several years of your own records (Season Extension Crops gives the method).
- Seed and the weeks each crop needs to reach transplant size, from the seed source or your own records.
- A protected place with strong light: a cold frame, a south-facing window, a greenhouse.
- Seed-starting mix that is clean and fine, and trays or pots with drainage.
Method
Step 1: Count back. Sowing date = planting-out date − weeks to transplant size. As an illustration, if a crop needs 6 weeks and you intend to plant out on 15 May, sow on 3 April (6 × 7 = 42 days before 15 May). The 6 weeks is a placeholder; use your crop’s own record.
Step 2: Sow in clean, moist mix at the depth the seed requires, and keep it warm until it sprouts; many tender crops germinate poorly in cold mix.
Step 3: Give light as soon as seedlings emerge. A seedling in dim light stretches into a thin, pale stem that flops. Put trays right against the glass or provide a reflecting surface.
Step 4: Ventilate and avoid overwatering. Damp, still, crowded mix causes damping-off, a fungal rot that topples seedlings at the soil line.
Step 5: Harden off. Over about a week, give the seedlings more sun, wind and cooler nights each day, starting with a few hours outside in shelter. Do not move them from a warm room into open ground in one step.
Step 6: Plant out after your last-frost date, or earlier under a cover that you have measured (Cold Frames & Low Tunnels). Plant on a cloudy day or in the evening to reduce the shock.
Common Mistakes
- Sowing too early — seedlings that outgrow their pots weaken and set poorly.
- Skipping hardening off — leaf scorch and stunting follow.
- Using the date from a printed calendar — it belongs to another place.
What’s Next
- Learn next: Succession Planting — how to keep the beds producing
- Combine with: Thermal Mass Heating — warming the start