Seed starting calculator
Enter your local average last frost date and choose a crop. You get the date to sow indoors and the date to plant out, using published timings for 13 common vegetables and herbs, plus a full planting calendar.
Frost date and crop
About the seed starting calculator
Enter your local average last frost date and choose a crop. You get the date to sow indoors and the date to plant out, using published timings for 13 common vegetables and herbs, plus a full planting calendar.
Part of SmartCalcy’s garden tools. It runs in your browser with no account, uses your own currency and units (change them at the top of the page), and gives estimates for information only, so check official guidance before you rely on a result.
How to use the seed starting calculator
What it calculates and when to use it
Works out when to sow seeds indoors and when to plant out, counting from your local average last frost date, for 13 common vegetables and herbs or your own crop.
How to use it, step by step
- Enter your average last frost date from a local source.
- Choose a crop, or enter the weeks from the seed packet.
- Read the sowing and planting-out dates.
- Use the calendar for every crop in the list.
What each input means
- Last frost date – Starts at 20 April.
- Crop – Tomatoes to start with.
How to read the results
- Sow indoors – The date and how many days away it is.
- Plant out – Before or after the frost date, by crop.
- Planting calendar – Every crop’s dates for your frost date.
The formula
Plant-out date = last frost + transplant offset. Sowing date = plant-out date − weeks indoors. Timings follow NDSU Extension’s seed-starting chart.
A worked example
With a last frost on 20 April, tomatoes are planted out two weeks after it, on 4 May; sowing six weeks earlier gives 23 March.
Assumptions and limits
- Frost dates are averages; a late frost can still happen.
- Timings are approximate and vary with your conditions.
Common mistakes
- Sowing too early, which gives leggy seedlings.
- Planting out without hardening seedlings off for a week.
Frequently asked questions
Where do I find my last frost date?
From your national weather service, a local gardening body or an extension service.
What is hardening off?
Putting seedlings outside for longer each day for about a week before planting out.
Related tools
- Weeks indoors
- 6
- Plant out
- Tue, 4 May 20272 weeks after the last frost
- Last frost
- Tue, 20 April 2027
Planting calendar for your frost date
| Crop | Sow indoors | Plant out |
|---|---|---|
| Tomatoes | 23 Mar | 4 May |
| Peppers (sweet and chile) | 9 Mar | 4 May |
| Aubergine (eggplant) | 9 Mar | 4 May |
| Broccoli | 19 Jan | 30 Mar |
| Cabbage | 19 Jan | 30 Mar |
| Cauliflower | 19 Jan | 30 Mar |
| Kale | 19 Jan | 30 Mar |
| Leeks | 26 Jan | 6 Apr |
| Lettuce | 26 Jan | 23 Mar |
| Onions | 26 Jan | 6 Apr |
| Basil | 13 Apr | 4 May |
| Oregano and marjoram | 23 Mar | 4 May |
| Parsley | 19 Jan | 30 Mar |