First frost is coming: a last garden afternoon with the children

Every garden has one last good afternoon of the year, and almost nobody notices it until it has gone. Here is how to see it coming, what to do in it with the children, and why the thing to watch for is a warm, dry Saturday rather than the frost itself.

Updated 16 September 2026.

What the frost actually is

The National Weather Service describes frost as the formation of thin ice crystals on the ground or other surfaces when temperatures of the Earth's surface and earthbound objects falls below 32°F, and issues a frost advisory during the growing season when widespread frost formation is expected over an extensive area, when surface temperatures are usually in the mid 30s Fahrenheit. NWS glossary: frost. The practical point in that definition is the word surface: the ground and the leaves get colder than the air a thermometer measures, which is why a forecast low of 36° can still blacken the tomatoes. When the overnight lows in your forecast start reading in the mid-thirties, the first frost is a week or two away, not a month.

Why you watch for the warm afternoon, not the cold night

The frost is the deadline. It is not the day you want. The day you want is the last one before it that is warm enough to spend two hours outside with a child, dry enough to lift potatoes without making mud, and still enough to plant bulbs without the labels blowing across the lawn. Those two things do not arrive together. The cold night comes on the heels of a clear day; the warm dry Saturday is often a fortnight earlier, and you only know it was the last one afterwards.

So the alert we would set for October is not “frost tonight.” By then it is too late to do anything but throw a sheet over the beans. It is a warm, dry Saturday afternoon — 58° to 75°, rain chance under 20%, wind under 12 mph, three hours in a row — watched for from the middle of September, so that when one turns up you spend it in the garden on purpose.

What to do with the afternoon

• Lift the potatoes. If you planted them together in spring, this is the payoff, and there is nothing in gardening a child likes better than tipping out a bag and counting.
• Cut the sunflower heads and hang them to dry for the birds, or for next year's seed. Measure the tallest first; somebody will want to know.
• Pick everything that will not survive a frost: the last tomatoes (green ones ripen on a windowsill), peppers, beans, courgettes, basil. Let the children decide what counts as ready.
• Plant bulbs for spring. Daffodils, tulips, crocuses, alliums: a bulb is a promise a child can hold in one hand, and planting them is the easiest job in the garden. Mark where they went; everyone forgets.
• Put the beds to bed. Pull the spent plants, spread compost or leaves, and tuck the garden in. Children understand this one immediately.
• Leave something standing. Seed heads for the birds, a few stems for the frost to find. The garden is not finished; it is resting.

How to see the last afternoon coming

1. From mid-September, glance at the overnight lows once a week. When they start reading in the low forties, you are in the window.
2. Watch for the warm, dry, still afternoon — or have it watched for. A forecast a week out is right about four times in five; three days out, better. (See how far ahead you can trust a forecast.)
3. When it comes, treat it as the last one whether or not it is. If another turns up, that is a gift, and the bulbs are already in.

Perfect Day does the watching: describe the afternoon once — the temperatures, the dryness, the hours — and you hear once, days ahead, when it is coming. The rest of the time it says nothing, which in October is most of the time. In spring the same idea runs forwards: the best weekend to plant with the kids.