A sowing schedule is built backwards, from the date the plant goes into its final position to the date the seed goes into the medium. Working forwards from a calendar date produces the classic failure: transplants raised too early, held too long in small cells, stretched under inadequate light, and planted out into ground that is not ready for them.
A greenhouse changes the schedule in two directions at once. It moves the transplant date earlier, because the crop finishes in a protected environment rather than an exposed one. And it makes the raising stage easier, because the temperature and light are better than a windowsill. Both effects are worth several weeks.
The three dates every crop has
The target date is when the plant goes into its final position. Outdoors this is the last frost plus a margin for tender crops. Under row cover or in a cold frame it is roughly two weeks earlier. In an unheated greenhouse or tunnel, about three weeks earlier. In a heated house holding 45 degrees Fahrenheit, about five weeks earlier for tender crops and effectively unconstrained for hardy ones.
The sowing date is the target date minus the raising period, which is the crop-specific figure everyone quotes. It assumes good conditions; in poor light and cool medium the raising period stretches considerably, which is why a schedule that works in a heated propagator fails on a north-facing windowsill.
The hardening date is the target date minus about two weeks, and it is the one people forget. A transplant moved straight from a warm bright greenhouse into an exposed position checks badly. The process is covered in hardening off seedlings.
Run all three from your own frost date in the frost date planting calculator, which applies the protection shift automatically.
The crop table
| Crop | Sow before last frost | Germination soil temp | Transplant relative to last frost |
|---|---|---|---|
| Onions and leeks | 10 to 12 weeks | 68 to 77 degF | 3 weeks before, they tolerate cold |
| Peppers and eggplant | 8 to 10 weeks | 80 to 85 degF | 2 weeks after, they want warm soil |
| Celery | 10 to 12 weeks | 70 to 75 degF | At last frost |
| Tomatoes | 6 to 8 weeks | 75 to 85 degF | 1 week after, or earlier under cover |
| Basil | 6 to 8 weeks | 70 to 80 degF | 2 weeks after, it is very frost tender |
| Brassicas | 4 to 6 weeks | 65 to 75 degF | 3 weeks before |
| Lettuce and greens | 4 to 6 weeks | 60 to 68 degF | 4 weeks before |
| Cucumbers and melons | 2 to 3 weeks | 75 to 85 degF | 10 days after |
| Squash | 2 to 3 weeks | 75 to 85 degF | 1 week after |
| Beans and corn | Direct sow | 65 to 75 degF | 1 week after |
| Bedding flowers | 8 to 10 weeks | 70 to 75 degF | 1 week after |
Two rows on that table are worth pausing on. Peppers want ten weeks and 80 to 85 degrees in the medium, which is why they are the crop that most rewards a thermostatically controlled heat mat and most punishes a cool windowsill. And beans and corn are listed as direct sow because they resent root disturbance enough that raising them in cells rarely gains anything.
Germination temperature is a separate number from growing temperature
This is the distinction that most improves results, and it is the one general advice most often blurs.
Germination responds to the temperature of the medium, not the air, and the optimum is high: 75 to 85 degrees Fahrenheit for most warm-season crops. Growing on responds to air temperature, and the optimum is considerably lower: 60 to 70 for the same crops. Holding germination temperature after emergence is the single most common cause of stretched, weak transplants.
The practical sequence is therefore: bottom heat on until emergence, then off within a day or two, then cooler and brighter. A VIVOSUN 10 x 20.75 in Seedling Heat Mat and Digital Thermostat Set ($24.78) makes the first part reliable, because an unregulated mat commonly overshoots into the mid eighties, which suits peppers and cooks lettuce on the same bench. The full table is in the germination soil temperature chart.
Light, and why a schedule fails without it
A sowing date is a promise about how big a plant will be on a given day, and that promise depends on light as much as on temperature. Vegetable seedlings want a daily light integral of 12 to 17 mol per square meter per day, which at a 16 hour photoperiod means roughly 210 to 300 micromoles per square meter per second at the canopy.
A bright windowsill in late winter delivers a fraction of that, and the seedlings stretch: long internodes, pale leaves, thin stems that never fully thicken. This is not a cosmetic problem. A stretched transplant establishes slower, sets later and yields less.
The fix is fixtures close to the canopy. Barrina 4 ft LED Grow Lights, 5000 K, 6-Pack hung a few inches above the trays, on adjustable hangers so the height can rise with the plants, is the standard hobby answer. Check what any fixture actually delivers at working distance with the DLI and PPFD calculator rather than trusting a wattage figure, because delivered light falls off sharply with distance.
Succession sowing, which is what a greenhouse is really for
A single sowing of lettuce gives one glut and then nothing. A greenhouse makes succession sowing practical, and succession is what converts a structure into a supply.
- Salad greens: every 2 to 3 weeks through the growing season, and every 3 to 4 in the cool months when growth is slower.
- Radish and quick roots: every 2 weeks. They mature fast enough that a gap in sowing is a gap in harvest almost immediately.
- Bush beans: every 3 weeks until roughly ten weeks before the first frost.
- Brassicas: every 3 to 4 weeks for a continuous supply of smaller heads rather than a single large harvest.
- Basil: every 4 weeks. Plants decline after several cuts, so a replacement sowing keeps quality up.
- Microgreens: every week, or more often. They are the fastest succession crop there is and the one that best fills a propagation bench between transplant runs.
The discipline that makes succession work is sowing the next batch when the previous one is transplanted rather than when the previous one is harvested. Sowing on harvest guarantees a gap the length of the raising period.
The autumn schedule, which runs on different logic
Spring sowing is bounded by cold at the start. Autumn sowing is bounded by light at the end, and that is a harder constraint because no amount of heat compensates for a short day.
The practical rule is that a crop intended to be harvested through winter must reach close to full size before day length drops below about ten hours, after which growth nearly stops. Everything sown after that window is holding rather than growing, and it will resume in late winter. Missing the window by two weeks in autumn costs far more than missing it by two weeks in spring, and the whole approach is covered in winter greens in an unheated greenhouse.
Propagation equipment
Every date on a schedule assumes a plant that will transplant successfully, which is a separate skill from raising it. That is hardening off seedlings, and it is where a good schedule most often comes undone.