Seed Starting Calendar
Work back from your last frost date to a sowing week.
Everything is scheduled from the last frost date
Seed packets rarely give dates, because a date meaningful in one region is wrong by two months in another. Instead they give an offset — "sow 6–8 weeks before last frost" — and the whole calendar hangs off that single local anchor. Finding your area's average last spring frost is therefore the one piece of information that makes every other instruction usable.
That average is a probability, not a guarantee. It typically means a 50% chance of frost after that date, so planting tender crops exactly on it is a coin flip. Waiting one to two weeks past it, or having fleece ready, is the standard hedge.
Sowing too early is worse than sowing late
The most common beginner error is starting seeds indoors weeks ahead of schedule out of enthusiasm. The result is leggy, pale, root-bound transplants that have outgrown their pots and stalled, and which then sit in a cold greenhouse waiting for conditions to allow planting out. A seedling started two weeks later in better light routinely overtakes one started early and checked, because a stressed transplant takes weeks to recover.
Indoor light is usually the limiting factor. A windowsill provides far less light than seedlings need, and stretching towards the glass is the visible symptom.
Soil temperature governs germination, not air temperature
Most seeds have a soil temperature range below which they simply will not germinate, and sitting in cold wet soil for weeks invites rot rather than delayed sprouting. Peas and brassicas will move at 7–10°C; beans, squash, and cucumbers want 15–18°C or more; tomatoes and peppers germinate reliably around 21–24°C. A cheap soil thermometer resolves more sowing questions than any calendar. Hardening off — exposing indoor-raised plants to outdoor conditions gradually over a week to ten days — is the other step most often skipped, and skipping it can undo months of work in a single windy afternoon.
How to use it
- Find your local average last frost date and enter it.
- Read the suggested indoor sowing and transplanting weeks for each crop group.
- Treat the frost date as a 50% probability and hold tender crops back a week or two.
- Allow a week to ten days for hardening off before planting out.
Before you plant
Where do I find my last frost date?
National meteorological services and regional extension or horticultural bodies publish average last-frost dates by area or postcode. Local gardening groups often have more accurate figures for microclimates such as valleys and urban areas.
Is it safe to plant out on the frost date?
Not really. The average last frost date typically carries about a 50% chance of frost afterwards. Waiting one to two weeks, or keeping fleece to hand, protects tender crops.
What happens if I sow too early?
Seedlings become leggy and root-bound waiting for conditions to allow planting out, and a stressed transplant takes weeks to recover. A later sowing with adequate light usually overtakes an early one.
Does soil temperature matter more than air temperature?
For germination, yes. Beans and squash want soil around 15–18°C and tomatoes closer to 21–24°C; sowing into colder soil produces rot rather than a delayed sprout. A soil thermometer is a worthwhile purchase.
Disclaimer: These are general planning estimates. Seed packets, product labels, and regional frost, water, and pesticide guidance always take priority over any figure produced here. Local soil, climate, and variety differences can change the result substantially.