Everything is counted from the last frost
A planting calendar that gives fixed calendar dates is useless outside the region it was written for. "Sow tomatoes in March" is right in zone 8 and six weeks early in zone 4. The only calendar that travels is one counted in weeks either side of the last frost date, which is what this one does.
Each crop carries three offsets. How many weeks before the last frost it should be sown under cover; how many weeks after the last frost it can safely be planted out; and, for crops that prefer it, how many weeks either side of the last frost it can be sown straight into the ground. Change your zone and every date moves together.
The offsets themselves come from the biology of the plant rather than from tradition. Peppers get eight weeks indoors because they germinate slowly and grow slowly at first. Cucumbers get three, because they grow so fast that a six-week-old cucumber plant is already root-bound and will sulk for a fortnight after planting out. Sowing earlier is not the same as harvesting earlier.
Does the crop actually fit your season?
This is the question most planting calendars quietly avoid, and it is the one that decides whether you get a harvest. A tender crop has to complete its entire run between your last spring frost and your first autumn frost. If the days to maturity exceed the days available, no amount of good gardening fixes it.
The calculator checks this and says so plainly. In zone 4, with roughly 138 frost-free days, a 100-day winter squash sown after the last frost is finishing right on top of the first autumn frost with nothing to spare. That is not a comfortable margin, and it is exactly the situation where people conclude they are bad at growing squash when in fact the arithmetic never worked.
There are three honest answers when a crop does not fit: start it earlier under cover, choose a variety bred for a shorter season, or grow it under protection. A short-season tomato at 55 days and a beefsteak at 90 days are the same crop as far as a seed catalogue is concerned and completely different propositions in a cold garden.
| Crop | Under cover | Plant out | To first picking |
|---|---|---|---|
| Tomato | 6 wks before | 2 wks after | 75 days |
| Sweet pepper | 8 wks before | 2 wks after | 80 days |
| Courgette | 3 wks before | 1 wk after | 55 days |
| Sweetcorn | 2 wks before | 2 wks after | 80 days |
| Lettuce | 4 wks before | 2 wks before | 55 days |
| Kale | 6 wks before | 3 wks before | 60 days |
| Pea | direct only | 5 wks before | 65 days |
| Carrot | direct only | 2 wks before | 75 days |
Hardy, half hardy, tender
The frost date only constrains crops that frost actually harms, and that is far fewer than most people assume. Hardiness is the reason half the calendar sits before the last frost date rather than after it.
Tender crops — tomatoes, peppers, beans, squash, basil, sweetcorn — are killed outright by a frost. Nothing in this group goes outside until the danger has genuinely passed, which is the safe date rather than the median.
Hardy crops are a different proposition entirely. Peas can be sown five weeks before the last frost. Spinach and radish four. Broad beans and garlic go in the autumn and overwinter. Kale and leeks are not merely tolerant of frost but improved by it — the plant converts starch to sugar as a natural antifreeze, which is why kale picked after a hard frost tastes noticeably sweeter than kale picked in September.
The practical consequence is that a garden planned only around the last frost date sits empty for the two most useful months of the year. The hardy crops are what make a short season productive.
Common questions
When should I start tomato seeds?
Six weeks before your last frost date, under cover. In zone 6 that is around mid-March for an April 21 last frost, with plants going outside about two weeks after the frost date.
What can I sow before the last frost?
Peas about five weeks before, spinach and radish four, carrots and lettuce two. Kale, cabbage and broccoli raised indoors can be planted out two to three weeks before the frost date because they tolerate light frost.
Why does the calculator say my crop does not fit?
Because its days to maturity are longer than the frost-free days your zone has. Start it earlier under cover, pick a shorter-season variety, or grow it in a greenhouse or polytunnel.