Sowing schedule guide
The short answer: build your sowing schedule backwards from the harvest date. Take the day you want your first harvest, subtract the plant's days to maturity (DTM, listed in every seed catalogue), then subtract the seedling's time in trays — that's your sowing date. DTM counts from transplanting, not from sowing, so the tray weeks must be added separately. Below is the full method with a worked tomato example.
How many days do common crops need?
| Cool-season plants | Days to maturity | Warm-season plants | Days to maturity |
|---|---|---|---|
| Spinach | 50–60 | Tomatoes | 80 |
| Broccoli | 60–70 | Melons | 80–90 |
| Onions / scallions | 60 | Summer squash | 70–80 |
| Carrots | 60–70 | Eggplants | 80–90 |
| Radishes | 40–50 | Peppers | 80–90 |
| Lettuce | 40–50 | Cucumbers | 75–80 |
| Parsley | 60–70 | Basil | 60–80 |
Cool-season crops: plant in fall for winter harvest, or in early spring for early summer. Warm-season crops: plant in late spring, harvest through fall.
Worked example: 45 kg of tomatoes from August 15 to October 19
Sample calendar for tomato sowing planning.
Working backwards: first harvest August 15 → minus DTM (72 days after transplant) = transplant by June 4 → minus 6 weeks in trays = sow on April 23. Each plant yields for about 3 weeks, and the target window (Aug 15 – Oct 19) is 9 weeks — so you need 9 ÷ 3 = 3 staggered sowing dates:
| Sowing date | Transplant to greenhouse | Harvest window |
|---|---|---|
| April 23 | June 4 | Aug 15 – Sep 5 |
| May 14 | June 25 | Sep 6 – Sep 27 |
| June 4 | July 16 | Sep 28 – Oct 19 |
Why keep a sowing schedule at all?
However many varieties you start each season, a schedule — simple or detailed — keeps track of seedling varieties, makes the most efficient use of greenhouse space, and becomes a multi-year reference showing which varieties succeed best for you. If you start seeds in trays and transplant later, choose varieties that tolerate transplanting well.
What to record in your seedling log
For each variety, track: seed name, planting date, days to germination, number of trays, transplant date, seedlings potted, date moved to the greenhouse, and comments. After a season or two the log tells you exactly which varieties and dates work in your conditions.
Frequently Asked Questions
What does "days to maturity" (DTM) actually count?
The time from transplanting to first harvest. It does not include germination or the seedling's weeks in trays — add those separately when planning backwards.
How do I get a continuous harvest instead of one big glut?
Stagger sowing: divide your desired harvest window by each plant's productive period (about 3 weeks for tomatoes) to get the number of sowing dates.
Does a sowing schedule change watering needs?
Yes — staggered plantings mean seedlings, flowering and fruiting plants share one greenhouse, each needing different amounts. Zoned, sensor-based watering like the GRAAS iRigator handles each stage separately.