Wait for warmth, not just frost
Tomatoes need soil above about 60°F (15°C) to grow well, not just a frost-free night. Planting into cold soil stalls them and invites disease. Harden seedlings off for a week — a few hours outside, increasing daily — before transplanting, and plant them deep, burying part of the stem, so they root along it.
Use the seed starting calendar to work back from your last frost date.
The two conditions that matter
Tomatoes need two things before transplanting: no frost, and warm soil. Frost kills them outright, but cold soil — below about 60°F (15°C) — stalls growth and invites disease even without frost. In many regions, soil warms 1–2 weeks after the last frost date, so the last frost date is a minimum, not a planting date. A soil thermometer, or simply waiting until nights are consistently above 50°F (10°C), is a better guide than the calendar alone.
From seed to transplant, week by week
- Weeks 1–2: sow indoors in seed compost, keep warm (65–75°F / 18–24°C) and evenly moist until germination.
- Weeks 3–5: give strong light, pot on into larger pots when roots fill the first pot, and feed lightly once true leaves appear.
- Week 6: begin hardening off — a few hours outdoors in shade, increasing daily, bringing plants in at night.
- Transplant week: plant deep, burying part of the stem, which roots along its length and gives a stronger plant. Water well and mulch after the soil warms.
Choosing varieties and sites
Determinate (bush) varieties stop growing at a set height and ripen over a shorter period, suiting small spaces and shorter seasons. Indeterminate (vining) varieties keep growing and fruiting until frost, but need staking and pruning. All tomatoes need full sun — at least 6 hours of direct sun — and good air circulation. A sheltered, south-facing spot against a wall is often several degrees warmer than an open bed, which can make the difference in a cool climate. Use the seed starting calendar to set your sowing date from your last frost date.
Frequently asked questions
Can I plant tomatoes earlier under cover?
Yes, with a cloche, cold frame or greenhouse, you can gain 2–3 weeks, provided you can protect plants on frosty nights. Cover alone does not warm cold soil much, so wait for soil warmth even under cover.
Why are my tomato leaves yellow after transplanting?
Usually transplant shock, cold soil, or overwatering. Plants usually recover once soil warms and watering settles to deep, less frequent watering. Persistent yellowing with stunted growth may indicate nutrient deficiency — a soil test helps.