When should I start sowing seeds?

Seed packets give a month, and a month means something very different in Cornwall and in Aberdeenshire. Every useful sowing decision is made relative to your own last frost, which is the number worth finding once.

Step by step

1. Find your local last frost date.The average date after which frost is unlikely where you live. It varies by weeks across the UK and by several days between a town and open country nearby. Local gardening groups know it better than any national map.
2. Work backwards from that date.Seed packets state how long before the last frost to sow indoors. Six to eight weeks is typical for tomatoes and chillies; two to four for courgettes and squash. Count back from your date, not from March.
3. Separate hardy from tender crops.Hardy things — broad beans, peas, onions, spinach, most brassicas — tolerate cold and go in early or direct. Tender things — tomatoes, chillies, courgettes, beans, sweetcorn — are killed by frost and must wait.
4. Resist sowing early.The most common mistake by a distance. Seedlings sown too early outgrow their pots and their light before it is safe to plant them out, and become leggy and weak. A plant sown three weeks later often overtakes them.
5. Check you have enough light.A windowsill in February gives too little light for the length of the day. Without a grow light, sowing tender crops before about March produces stretched seedlings whatever the packet says.
6. Consider soil temperature for direct sowing.Seed sown into cold wet soil rots rather than germinating. Most direct-sown crops want soil above about 7-10°C. A cheap soil thermometer is more reliable than the calendar.
7. Warm the soil before sowing outside.Covering a bed with black plastic or a cloche for two or three weeks raises the soil temperature and lets you sow a fortnight earlier than bare ground allows.
8. Sow in succession rather than all at once.Salad, radish, beetroot and beans sown every two or three weeks give a continuous supply rather than a glut. It also insures against one sowing failing.
9. Harden off before planting out.A week or two of increasing time outdoors before anything raised inside goes into the ground. Plants moved straight out are checked badly, and it undoes the head start.
10. Watch the actual weather, not the date.The last frost date is an average, so about half of years have a later one. A late frost forecast means fleece, a cloche, or bringing plants back in.
11. Keep a record of what you sowed and when.A note each year, with what worked. After two seasons you have a sowing calendar for your own garden, which is worth more than any general guidance.
12. Check the packet's own dates against your climate.Packet months are usually written for southern England. Further north, later is generally right — often by two to three weeks.

Tips

• If in doubt, sow later. A late sowing catches up quickly in warm soil; an early one struggles all season.
• Sow a few seeds two weeks apart rather than the whole packet at once. It is the cheapest insurance against a cold spell.
• Most seed keeps for two or three years properly stored, so there is no need to use a whole packet in one season.

Common mistakes

• Sowing tender crops in February — There is not enough daylight to support them, and they outgrow their pots long before it is safe to plant out. They end up weaker than seedlings sown a month later.
• Following the packet month without adjusting for where you live — Packet dates are generally written for southern England. Further north the same sowing can be two to three weeks too early.

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