When to Plant: Reading Your Frost Dates

Almost every sowing instruction on a seed packet is written relative to a date the packet cannot know: your last frost. "Sow indoors six weeks before last frost" is useless until you put a number on it, and slightly dangerous once you put the wrong number on it.

What Last and First Frost Dates Really Mean

Your last frost date is the average date in spring after which air temperature no longer drops to 0 C (32 F). Your first frost date is when it starts again in autumn. The gap between them is your frost-free season, and it decides what you can realistically grow.

Two things people misread. These are air frost figures measured at about 1.5 m, so a clear still night can put frost on the soil while the thermometer reads 2 C. And frost is not the only cold limit: tomatoes stop setting fruit below about 10 C at night, and beans rot in soil under 12 C. A frost-free date is a floor, not a green light.

Finding Your Dates (and Why the Average Lies)

Take your dates from a national meteorological service using the nearest station, or from a local horticultural society, which usually knows the microclimate better than a national map. In the US the USDA hardiness zone gives winter minimums rather than frost dates, so look those up separately. Averages also drift: a 1981-2010 data set may be a week out, and three or four years of your own records beat any published figure.

What matters is the probability behind the number. A published last frost date is usually the 50% date: half of years have a later frost, which is a coin flip on tender seedlings.

• Use the 50% date to build your sowing calendar
• Use the 10% date, typically two to three weeks later, for putting tender crops out unprotected
• Keep fleece to hand for the fortnight in between

Counting Backwards: Sowing Windows

Take last frost as day zero and count backwards in weeks.

Two rules keep you out of trouble. Sowing early gains nothing if the plant then sits root-bound on a windowsill for a month; a tomato sown eight weeks before last frost beats one sown at twelve weeks almost every year. And soil temperature governs direct sowing more than the calendar does: carrots want 7 C, beans 12 C, sweetcorn 15 C.

Autumn works in reverse: take first frost, subtract days to maturity, add two weeks because growth slows as day length shortens, and that is your last useful sowing date.

Hardy, Half-Hardy and Tender Crops

Hardy crops tolerate frost and often improve after one: kale, leeks, parsnips, broad beans, garlic, spinach. Many go out weeks before last frost.

Half-hardy crops survive cold but not freezing: lettuce, peas, beetroot, carrots, potatoes, brassica transplants. Potato tubers are safe underground but emerged shoots die at 0 C, so earth them up when frost threatens.

Tender crops die at the first touch of frost, sometimes at 2-3 C: tomatoes, peppers, aubergines, courgettes, cucumbers, sweetcorn, beans, basil. The 10% date exists for these.

Harden off everything raised indoors. A week of increasing outdoor exposure, starting with two hours in shelter, prevents the checked growth that follows a move straight from windowsill to bed; the seed starting guide covers it in more detail.

Microclimates Change Your Dates

A published date describes a weather station, not your garden.

• A south-facing wall can advance your effective last frost by one to two weeks
• A low-lying corner collects cold air and may frost a week later than the rest of the plot
• Urban gardens typically run 1-2 C warmer than surrounding countryside
• Every 100 m of altitude costs roughly 1 C, often a week at each end of the season
• Coastal sites have later last frosts but much later first frosts, so the season is longer overall

Bare soil under clear skies frosts before mulched soil, and black plastic or fleece laid two weeks before sowing raises soil temperature by 3-5 C, often the difference between beans germinating and beans rotting.

Building a Calendar You Will Follow

Plans fail because they are too detailed to maintain. A calendar you actually use has three columns: the week, what to sow, and what to move outside. Anchor it to your last frost week and accept that you will slip a week here and there.

Two habits make it better every year: write down what you actually did rather than what you intended, and record the real frost dates each season. After three years you have a local data set that beats any average. If you would rather not rebuild that spreadsheet each January, Vegarden generates sowing and transplant weeks from your own frost dates and keeps last year's notes attached to each crop.