When to Start Seeds
Skip the math: the Seed-Starting Calculator turns your zone, crop and block size into exact sow and transplant dates — spring or fall.
Every sowing date on this page counts back from one number: your last spring frost date. Find yours, write it at the top of your seed list, and the whole season falls into order. One soil-block twist — blocked seedlings often reach transplant size sooner than the charts on the seed packet expect.
There’s a morning in late winter when the seed box comes off the shelf and suddenly it’s all happening at once — the peppers should already be started, the zinnias absolutely should not be. This page is the sorting-out.
When should I start seeds indoors?
Working back from your last spring frost date:
| Start indoors | Crops |
|---|---|
| 10–12 weeks before | lisianthus, snapdragons started for early blooms, slow perennials, onions and leeks from seed |
| 8 weeks before | peppers, eggplant |
| 6–8 weeks before | tomatoes, dahlias from seed |
| 4–6 weeks before | zinnias, cosmos, celosia, marigolds; broccoli, cabbage, kale |
| 3–4 weeks before | lettuce and greens |
| 2–4 weeks before | cucumbers, squash, melons — in a large block; they hate root disturbance |
Two rules keep this honest. Fast, heat-loving flowers like zinnias started too early become root-bound plants you’ll end up composting — four to six weeks is genuinely enough. And when in doubt, start later: seasoned growers routinely trim a third off the packet’s recommendation and are happier for it.
Do soil blocks change the timing?
Yes — in your favor. Soil-blocked seedlings tend to reach transplant size roughly a quarter faster than the same crop in a plug tray, because they never stall from root disturbance. If you’re working from a plug-tray chart, expect to transplant earlier than it says. Treat that as a rule of thumb to check against your own trays, not a promise — every greenhouse runs a little different.
The block itself also sets a ceiling, whatever the crop:
| Block size | Plan to transplant or pot up within |
|---|---|
| 0.9″ | ~3 weeks |
| 1.125″ | ~3–4 weeks |
| 1.5″ | ~4–6 weeks |
| 2″–2.25″ | ~6 weeks |
Past that point the block — not the plant — becomes the limit: it dries faster and runs out of root room. Slow growers like lisianthus and delphinium will outrun even the largest block; plan to pot those up partway (the Dahlia Blocker’s oversized cells exist for exactly this).
What is succession sowing?
One tray of zinnias gives you one glorious week of buckets, then a gap. Sowing a short row of blocks every two to three weeks — zinnias, sunflowers, cosmos, celosia; lettuce and cilantro on the vegetable side — turns the glut into a steady supply. Stop summer sowings about 70 days before your first fall frost so the last round matures in time.
Can I plant anything in fall?
Hardy annuals — snapdragons, larkspur, sweet peas, bells of Ireland — can be transplanted out six to eight weeks before your first fall frost, overwinter as young plants, and bloom weeks earlier than anything spring-sown. Flower farmers call them cool flowers, and they’re the closest thing to a head start the calendar offers.