Planting by Frost Dates: Working Out Seed-Starting, Transplant and Harvest Dates
Almost every vegetable timing question comes back to two dates: the average last frost in spring and the average first frost in fall. Seed packets say "start indoors 6 weeks before last frost" or "sow when danger of frost has passed" because those two dates move from garden to garden, so a packet cannot print a calendar date that works for everyone. Once you know your own pair, the rest is counting.
The GardenStuffs Planting Calendar does that counting. You enter your last spring frost date, optionally your first fall frost date, pick a crop and a growing method (Start Indoors, Direct Sow, or Show Both Methods), and it returns the dates to start seeds, transplant or sow outdoors, and expect a harvest, plus how many days you have to spare before the first fall frost. This guide walks through one example garden so you can see what the numbers mean and where they stop.
Step one: find your frost dates (and treat them as averages)
Your local extension service (in the US), your national weather service, or a long-running local gardening club can give you the average last spring frost and average first fall frost for your area. Two things matter about those figures:
- They are averages, not promises. On the average date there is still a real chance of frost, which is why the calendar itself suggests giving frost-tender crops a couple of weeks to spare. Keep frost cloth or old bedsheets handy for the first few weeks after planting out.
- They vary by location and year. A garden in a low spot where cold air settles can frost later in spring and earlier in fall than a garden a short drive away on a slope. Use the date for your area, then adjust from what you see in your own yard.
The calendar uses exactly the dates you type. It does not look anything up for you, and it has no idea whether this spring is running late.
The example garden
For the figures below we used an example last spring frost of April 20, 2027 and an example first fall frost of October 15, 2027. The calendar counts that as a 178-day growing season. Your dates will differ, so run your own.
| Crop and method | Start indoors | Plant out or sow | Expected harvest | Days to spare |
|---|---|---|---|---|
| Tomatoes (both methods) | March 9 | May 4 | July 18 | 89 |
| Peppers (indoors) | February 23 | May 4 | July 23 | 84 |
| Lettuce (both methods) | February 23 | March 23 | May 7 | 161 |
| Broccoli (indoors) | February 23 | April 6 | June 15 | 122 |
| Onions (indoors) | January 12 | March 23 | July 1 | 106 |
| Herbs (indoors) | March 9 | April 20 | June 19 | 118 |
| Cucumbers (both methods) | March 30 | May 4 | July 3 | 104 |
| Squash (both methods) | March 30 | May 4 | August 2 | 74 |
| Melons (both methods) | March 23 | May 11 | August 4 | 72 |
| Beans (direct) | none | May 4 | June 28 | 109 |
| Carrots (any method) | none | April 6 | June 15 | 122 |
All dates are 2027 and come straight from the calendar with the example frost dates above.
Reading the pattern: cool-season and warm-season crops
The table splits neatly into two groups, and that split is the most useful thing to understand about frost-date planning.
Crops that go out before the last frost
Lettuce, onions and broccoli go out ahead of the frost date: lettuce and onions four weeks before it (March 23 in the example) and broccoli two weeks before (April 6). Carrots are sown direct two weeks before it. These are cool-season crops that tolerate light frost and grow best before summer heat arrives. Starting them late is the common mistake; by the time the soil feels warm, lettuce and broccoli are already heading toward bolting.
Crops that go out after the last frost
Tomatoes, peppers, cucumbers, squash and beans go out two weeks after the frost date (May 4), and melons three weeks after (May 11). These are frost-tender: a single frost can kill them outright. Waiting past the average date is not caution for its own sake; cold soil also slows them down, so an early planting often gains nothing.
How the calendar counts indoor starts
The indoor start date counts back a set number of weeks from whichever comes first, the last frost or the transplant date. Tomatoes get six weeks, peppers eight, onions ten, broccoli six and lettuce four. So tomato seeds start on March 9, six weeks before the April 20 frost date, and spend about two more weeks indoors or hardening off before the May 4 transplant. Lettuce, which goes out before the frost date, counts its four weeks back from its own transplant date of March 23, which puts the sowing on February 23.
Hardening off is not in the calendar, but it belongs in those last days before planting out: a week or so of moving seedlings outside for a few hours a day, longer each day, so they meet sun and wind gradually.
Crops the calendar always sows direct
Pick carrots or beans with "Start Indoors" and the calendar still gives a direct sowing date, with a note that they are sown straight into the garden. Carrots fork and stunt when their taproot is disturbed, and beans grow so fast from large seeds that an indoor start gains little. In the example, carrots are sown on April 6 and beans on May 4, whatever method you pick.
Harvest dates and the days-to-spare figure
The expected harvest is the planting date plus the crop's days to maturity in the calendar's table (tomatoes 75, peppers 80, lettuce 45, broccoli 70, beans 55, carrots 70, squash 90, melons 85, onions 100). For tomatoes, peppers, broccoli and onions the days run from transplanting, which is how most seed catalogs quote them for those crops. If you sow one of those four direct instead, the calendar adds its seedling weeks on top, as a rule of thumb, and says so.
"Days to spare" is the gap between that harvest date and your first fall frost. In the example every crop has more than two months in hand, which tells you two things: the first harvest is safe, and there is room for later sowings. A long margin on lettuce (161 days) or beans (109 days) is the calendar telling you a single sowing wastes most of the season. Our guide to succession planting lettuce and beans uses that margin.
A short-season check
Margins shrink fast in a cold climate. With an example last frost of May 25, 2027 and first frost of September 15, 2027 (a 113-day season), melons started indoors go out on June 15 and are expected on September 8, just 7 days before the first frost. Squash sown direct on June 8 is expected on September 6, 9 days ahead. The calendar still marks both as safe because the harvest falls before the frost date, but a week is thinner than the uncertainty in an average frost date. In a garden like that, choose faster varieties, start melons indoors and plant out under cover, or both.
If you leave the first fall frost blank
The first fall frost field is optional. Leave it empty and the calendar assumes a first frost 200 days after your last spring frost, labels the season "(assumed)" and tells you so. With the example April 20 last frost, that assumed date is November 6, 2027, three weeks later than the October 15 we used, and the tomato margin grows from 89 to 111 days. Many gardens freeze well before 200 days are up, so for anything that ripens late in the season, type in your real date.
What the calendar does not cover
- Variety. It uses one days-to-maturity figure per crop. A 55-day cherry tomato and a 90-day beefsteak are both "tomatoes" to the calendar. Check your seed packet and adjust the harvest date.
- Soil temperature. Beans, squash and melons rot or sit still in cold soil even after the frost date. A soil thermometer is a better trigger than the calendar date in a cold spring.
- Weather this year. A late cold snap does not move the calendar. Watch the forecast in the weeks either side of your frost date.
- Space and layout. Dates tell you when, not where. Use the Plant Spacing Calculator and the Garden Layout Planner for how many plants fit.
- Heat. In hot climates the limit on lettuce and broccoli is summer heat, not fall frost, and the calendar has no heat check.
A simple routine
- Write down your average last spring frost and first fall frost from a local source.
- Run each crop through the Planting Calendar and copy the dates onto a wall calendar.
- Mark the indoor start dates first: onions in deep winter, then peppers, lettuce and broccoli, then tomatoes and herbs, then cucumbers, squash and melons.
- Plan hardening off for the week before each transplant date.
- Keep frost protection ready for the first two or three weeks after planting tender crops out.
- Check days to spare. Where it is large, plan a second sowing; where it is under about two weeks, plan protection or a faster variety.