The trigger
Cover when the forecast low is 4 °C (39 °F) or below and the night is clear and calm.
That threshold is above freezing on purpose. Air temperature is measured about 1.5 metres up, and on a clear still night the ground and the foliage radiate heat to the sky and end up two to four degrees colder than the air. A reported 4 °C can put a leaf surface at freezing.
Two adjustments to the trigger:
- Cloudy and breezy? You have more margin. Cloud returns heat to the ground and wind stops cold concentrating at the surface, so 2 °C overcast is often less dangerous than 4 °C clear.
- In a hollow or the low corner of a slope? Subtract two or three degrees from whatever the forecast says, because cold air drains downhill and pools there.
The mechanism behind both is in at what temperature frost forms.
Which plants actually need it
Covering everything is a lot of work for no benefit. The ones that need it:
Cover, every time: tomatoes, peppers, aubergines, basil, courgettes and squash, beans, cucumbers, dahlias in leaf, and anything you have just planted out.
Cover on a hard frost: newly planted perennials in their first autumn, tender bulbs still in the ground, fruit blossom in spring, and hydrangea buds in a late frost.
Do not bother: kale, cabbage, sprouts, leeks, parsnips, carrots, chard and spinach. Most are unharmed, and several — parsnips and kale especially — genuinely taste better after a frost, because the plant converts starch to sugar as a defence.
Bring inside instead: containers, which are far more vulnerable than the same plant in the ground because the roots have no soil mass buffering them.
Timing, which is where it goes wrong
A cover does not generate heat. It slows the escape of heat already stored in the soil. Everything about timing follows from that.
Put covers on before dusk. While the ground is still warm and there is heat to trap. A cover thrown over a plant at 11 pm is enclosing air that has already cooled, and does almost nothing.
Take them off in the morning, once the temperature is safely above freezing. Under sun a fleece can overheat quickly, and permanently covered foliage stays damp and gets diseased.
Take covers to the ground. This matters more than the material. A sheet draped over the top of a plant with the sides open lets the warm air out and is worth very little. Pegged, weighted or tucked down to soil level, the same sheet traps a pocket of warm air and works.
What each material is worth
| Cover | Protection | Notes |
|---|---|---|
| Horticultural fleece, single | 1–2 °C | Breathable, can stay on a cold overcast day |
| Horticultural fleece, doubled | 2–4 °C | The best simple option for a hard night |
| Old bed sheet or towel | 1–2 °C | Fine for one night; remove in the morning |
| Cardboard box | 2–3 °C | Excellent for a single plant. Weight it down |
| Cloche or tunnel | 2–4 °C | Ventilate as soon as the sun is on it |
| Plastic sheet on a frame | 2–3 °C | Only if it is held clear of the foliage |
| Plastic directly on leaves | Can make it worse | Conducts cold in; condensation freezes on the leaf |
| Mulch over roots | Protects roots only | Does nothing for the foliage above it |
Notice the ceiling. Even a good cover buys you a few degrees, so a −6 °C night will get through anything you drape over a tomato. Past a certain point the answer is not a better cover, it is harvesting or moving the plant.
The afternoon routine
For a night that has crossed the trigger:
- Water the soil in the afternoon. Moist soil stores and releases more heat overnight. Water the ground, not the leaves.
- Harvest anything ready. Tomatoes, peppers, beans and squash will not improve after a frost.
- Move containers against a house wall, under a porch, or into a shed or garage. Pots are the easiest thing to save and the most likely to be lost.
- Cover before dusk, taking each cover down to the ground and weighting the edges.
- Uncover in the morning once you are back above freezing.
A south or west-facing house wall absorbs sun all day and radiates it back overnight, and it blocks the sky the plant would otherwise be radiating to. A bed or a group of pots against that wall can survive nights that kill the same plants ten metres away in the open, with no cover at all.
When to stop covering and accept the season
There is a point where covering becomes a way of postponing the inevitable by a week at real cost in effort.
For tender annuals, that point is the first hard freeze — around −4 °C (25 °F) — or three or four nights of frost in a row. The plants have stopped producing anything useful, the fruit has stopped ripening, and the covers are earning nothing.
The plants worth continuing to protect are the ones with a future: perennials in their first year, tender bulbs still lifting, and anything you intend to bring through the winter. Those are covered in overwintering plants.