Seed starting schedule: how to count backwards from your last frost
Build a seed starting schedule that fits your garden: how to count backwards from your last frost date, a weeks-before-frost chart, and a full worked example.
A seed starting schedule is built backwards from one number: your average last spring frost date. Call it Week 0. Every seed packet tells you when to sow in weeks before or after that date, so you subtract, and you get a real calendar for your own garden.
That is the entire method. The reason it is written in weeks rather than dates is that spring arrives on wildly different days across the country, so a fixed calendar copied from a magazine is wrong for almost everyone reading it. Yours will be right.
Step 1: get your Week 0
Look up your average last spring frost date by ZIP code, using a NOAA-based frost date lookup or your state’s cooperative extension service. Do not use your hardiness zone for this. Zones measure winter lows, not spring warm-up, and two gardens in the same zone can have last frost dates weeks apart. Our guide to garden planning by zone explains why that mix-up trips up so many people.
While you are there, grab your average first fall frost date too. You will need it for fall crops.
One caveat worth internalizing: published frost dates are probabilities from historical records, usually at a 30 or 50 percent threshold. Your last frost date is not a wall that frost cannot cross. It is the point where the odds have dropped enough to plan around.
Step 2: the weeks-before-frost chart
Here is where common crops fall relative to Week 0. Your seed packet always wins if it disagrees, because it knows the variety and this chart does not.
| Crop | Start indoors | Move outdoors |
|---|---|---|
| Onions, leeks | 10 to 12 weeks before | 2 to 4 weeks before frost |
| Celery | 10 to 12 weeks before | Around last frost |
| Peppers, eggplant | 8 to 10 weeks before | 1 to 2 weeks after frost, warm soil |
| Tomatoes | 6 to 8 weeks before | After frost, warm soil |
| Broccoli, cabbage, cauliflower | 4 to 6 weeks before | 2 to 4 weeks before frost |
| Lettuce, chard | 4 to 6 weeks before | 2 to 3 weeks before frost |
| Basil | 4 to 6 weeks before | 1 to 2 weeks after frost |
| Cucumbers, squash, melons | 2 to 4 weeks before | After frost, warm soil |
And the crops that should skip the indoor stage entirely:
| Direct sow | When |
|---|---|
| Peas, spinach, radish | 4 to 6 weeks before frost, as soon as soil is workable |
| Carrots, beets, parsnips | 2 to 4 weeks before frost |
| Beans, corn | After frost, warm soil |
| Cucumbers, squash (alternative) | 1 to 2 weeks after frost |
Root crops hate having their roots disturbed, and beans, peas and corn come up so fast outdoors that an indoor start buys you nothing but work.
Step 3: a worked example
Suppose your lookup gives you a last frost date of May 10. That date is only an example. Yours is what matters, and the arithmetic is identical whatever it says.
Write the date, then subtract:
| Task | Count | Your date |
|---|---|---|
| Sow onions indoors | 11 weeks before | Feb 22 |
| Sow peppers indoors | 9 weeks before | Mar 8 |
| Sow tomatoes indoors | 7 weeks before | Mar 22 |
| Sow broccoli indoors | 5 weeks before | Apr 5 |
| Direct sow peas | 5 weeks before | Apr 5 |
| Transplant broccoli out | 3 weeks before | Apr 19 |
| Start hardening off tomatoes | 1 to 2 weeks before | ~Apr 29 |
| Last frost (Week 0) | 0 | May 10 |
| Direct sow beans | 1 week after | May 17 |
| Transplant tomatoes out | 1 week after, if soil is warm | May 17 |
| Transplant peppers out | 2 weeks after, warm soil | May 24 |
Now run the same subtraction with your own date. If your last frost is April 1, everything above shifts about six weeks earlier. If it is June 1, everything shifts three weeks later. The shape of the schedule does not change, only the anchor.
Notice what the layout reveals: onions and peppers start long before anything else, and there is a real gap before the tomato rush. Most people discover this backwards, in a panic, in March.
Step 4: build in the two steps everyone forgets
Hardening off. Indoor seedlings have never met real sun or wind. Moving them straight outside can scorch or snap plants you spent two months raising. Take 7 to 10 days: an hour in the shade on day one, building up exposure and time daily. Put it on the calendar as its own task, ending at your transplant date, or it will not happen.
Warm soil. Your frost date is the floor, not the starting gun. Tomatoes and peppers stall in cold soil and get overtaken by plants set out a week or two later into warm ground. Beans, cucumbers, squash and melons rot in cold wet soil rather than germinate. A cheap soil thermometer answers the question, and the packet gives the germination range. Patience here consistently beats enthusiasm.
Step 5: succession and the fall half
A schedule with one sowing date per crop wastes the season. Two additions fix that:
Succession sowing. For fast crops like lettuce, radish, bush beans, spinach and cilantro, sow a small amount every 2 to 3 weeks instead of everything at once. You get a steady supply instead of a glut followed by nothing.
Fall crops, counted from the other end. Take the days to maturity on the packet, count back from your first fall frost, then add roughly two weeks. Growth slows as daylight shortens in autumn, so a 60-day crop sown in August takes longer than the same crop sown in May. Miss that adjustment and your fall brassicas run out of season half-grown.
Step 6: write down what actually happened
Your first schedule is a hypothesis. The garden is what grades it.
Keep a note next to each sowing: what you sowed, when, the variety, how germination went, and what the weather did. After one season you will know that your peppers really wanted ten weeks not eight, that the tomatoes you started in week 8 got leggy and the week 6 batch caught up anyway, and that the bed by the fence stays cold two weeks longer than the rest of the garden. None of that is in any chart, including this one, because it is specific to you.
This is exactly what GardenOS is built around: your beds and crops, your sowing and harvest timing, and a garden journal that carries across seasons so last year’s notes are open in front of you while you plan this year’s. It runs as a single offline HTML file, so your records stay on your own device. Whatever you use, use something. Last season’s notes are the raw material for a good plan; without them you are guessing from averages again every spring.
Do this next: look up your last frost date, write it at the top of a page, and subtract the weeks for the five or six crops you actually intend to grow. Not twenty. Five. A short schedule you follow beats a complete one you abandon in April.
Frequently asked questions
How do I make a seed starting schedule?
Look up your average last spring frost date and call it Week 0. Every seed packet lists a sowing time in weeks before or after last frost, so subtract that number of weeks from your date to get the day you sow. Do this for every crop on your list and you have a schedule built for your garden rather than someone else's.
How many weeks before last frost do I start tomatoes?
Tomatoes are usually started indoors 6 to 8 weeks before your last frost date, and transplanted outdoors after the frost date once the soil has warmed. Starting earlier than that is the most common beginner mistake: you end up with tall, weak, root-bound plants waiting weeks for the weather to catch up.
What is hardening off and how long does it take?
Hardening off is gradually exposing indoor seedlings to real sun, wind and cooler air before transplanting. It takes about 7 to 10 days, starting with an hour in the shade and building up daily. Skipping it can scorch or snap plants you spent two months growing, so build it into your schedule rather than treating it as optional.
Which seeds should not be started indoors?
Root crops like carrots, radishes, beets and parsnips resent transplanting and should be sown directly where they will grow. Beans, peas and corn germinate fast outdoors and gain nothing from an indoor start. Cucumbers, squash and melons can be started indoors but only about 2 to 4 weeks ahead, since older seedlings transplant poorly.
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