What temperature kills plants?

There is no single temperature that kills all plants. Tender plants such as tomatoes and basil are damaged or killed by an air frost at 0°C, and some show chill damage on cold nights below about 5°C even without any frost. Half-hardy plants tolerate a light, brief frost. Fully hardy plants take temperatures well below freezing, with the toughest rated by the RHS to minus 20°C and beyond. The stage of growth matters as much as the number: a seedling of a hardy plant is far more vulnerable than its parent. And because frost forms at ground level before the air itself freezes, your plants can be frosted on a night the forecast said stayed above zero.

Tender, half-hardy and hardy: the three answers to one question

Gardeners sort plants into hardiness groups because "what temperature kills plants?" has a different answer for each group. The RHS hardiness ratings put numbers on those groups, and they are worth knowing in rough form.

The other variable is stage of growth. Seedlings and anything newly planted out have no reserves, so treat every seedling as tender until it is established, whatever the label says about the parent plant. The RHS also notes that plants build frost resistance during a spell of cold weather, which is why a sudden early autumn frost or a late spring one does more damage than the same temperature in midwinter.

Ground frost vs air frost: why plants freeze on "frost-free" nights

The Met Office records an air frost when the air temperature falls below 0°C, measured at least 1 metre above the ground. A ground frost happens when the ground surface itself reaches 0°C, and it can occur while the air above is still a few degrees warmer, because the ground loses heat to a clear sky faster than the air does.

That gap matters in a garden. Low-growing plants and seedlings live in the coldest layer of air, at ground level. A forecast minimum of 2°C or 3°C describes the air at screen height, not the surface your lettuces sit on. The Met Office's own rule of thumb is that frost may be possible when 0°C to 4°C is forecast on a night with little or no cloud and light winds. If you grow anything tender, treat those forecasts as frost nights and cover up. Our frost protection guide covers what to use, starting with things you already own.

Why wind and wet change the answer

Still, clear nights are frost nights

Frost forms when the ground radiates its heat away to a clear sky and the air sits still enough for a cold layer to settle. A breeze mixes the air and often keeps frost away on a night that would otherwise freeze. That is why the classic frost warning is a clear evening sky with no wind, and why frost gathers in sheltered hollows and valley bottoms where cold air collects.

Cold wind is its own kind of damage

Wind does not cause frost, but cold, dry winter winds strip moisture from evergreen leaves faster than frozen roots can replace it. The browned, scorched foliage that shows up on evergreens after a cold snap is often wind damage rather than freezing. Windbreaks and sheltered positions do as much for borderline plants as any cover.

Cold and wet together kill more plants than cold alone

The RHS points out that many winter losses come from the combination of cold and wet, not the temperature reading by itself. Roots sitting in cold, waterlogged soil rot, and plants from dry climates (lavender and most silver-leaved plants, for instance) can survive a freeze in free-draining soil yet die in the same freeze with wet feet. Good drainage and raising pots off the ground are frost protection, even though neither changes the temperature.

How long the cold lasts

A single brief dip to freezing around dawn is a different event from a day-and-night freeze that lasts through the week. Duration takes a borderline plant from scorched to dead, because the cold reaches the roots and the plant never gets a daytime thaw to recover in. This is another reason a typical January is survivable for a half-hardy plant in a mild garden while a rare cold snap is not.

Frosted plants are not always dead plants

Frost damage shows up as blackened, limp or scorched growth, and it looks worse than it often is. The RHS advice is to leave the damaged growth in place until the risk of frost has passed, because it shields the buds and stems below from the next frost, and then cut back to healthy growth in spring. Hardy and half-hardy plants regularly regrow from the base after a frosting that ruined their top growth.

Tender plants are the exception. A tomato or basil plant blackened right through by frost is finished, which is why the calendar matters more than the rescue. Know your dates, and the whole question gets easier: look up your first and last frost dates by postcode, then work through the frost protection guide in the weeks before the autumn date.

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