Frost damage

What you are looking at

Growth that was fine yesterday is black, limp and hanging today. Magnolia and camellia flowers have gone brown at the edges and turned papery. Potato foliage that was 20 cm high is flat on the ground and blackened.

The damage is patchy in a particular way. Whatever was exposed on the night is hit and whatever was sheltered by a wall, a canopy or a neighbouring plant is untouched, sometimes on the same branch. That patchiness, and the fact that it arrived in a single night and has not spread since, is the diagnosis.

Frost damage does not progress. If the affected area is bigger a week later, something else is going on.

Late frosts do the harm, not deep ones

A hardy plant survives minus ten in January without noticing. The same plant loses a season’s growth to minus two in May.

The difference is what state the tissue is in. Dormant wood is full of sugars and largely empty of free water, and it tolerates cold that would destroy the same cells in spring. New growth is soft, full of water, and has no protection at all.

So the dangerous nights in a British garden are the clear still ones in April and May, after everything has broken into leaf, and again in early autumn before growth has hardened. A mild March that brings everything on early makes a subsequent frost far more damaging, which is why some of the worst years follow a warm spring.

Two other things follow from this. Cold air is dense and flows downhill, so it collects at the bottom of a slope, in a dip, and on the uphill side of a solid fence, which can be several degrees colder than ground twenty metres away. And frozen tissue is destroyed mainly during a rapid thaw, which is why an east-facing camellia catching the early sun loses its flowers while a west-facing one keeps them.

Leave the damage on

The instinct after a frost is to cut the blackened growth off, and it is the wrong one.

Dead material insulates the live buds behind it, and cutting back immediately exposes those buds and stimulates a fresh flush of soft growth into a season that usually has more frosts in it. The second hit is what kills plants that would have recovered from the first.

Wait until the danger has passed, which in most of Britain means the end of May, then cut back to a live bud and feed and water the plant so it can rebuild.

What to do about it

Leave the damaged growth in place until the last frosts have gone.

Cut back to live wood in late spring, then mulch, water and feed.

Learn where the cold sits in your garden and keep vulnerable plants out of it.

Move camellias and magnolias out of the early morning sun.

Fleece blossom, new growth and potato shoots on clear still nights and uncover them by day.

Earth potatoes up over emerging shoots when frost is forecast.

Plant tender annuals after your local last frost date rather than on the first warm weekend.

Stop feeding after midsummer so growth hardens before autumn.

Group pots against a wall and wrap them, since roots in a container freeze where roots in the ground do not.

What it gets mistaken for

Leaf scorch browns margins and tips through a hot dry spell, building over days rather than appearing overnight.

Wind damage browns the windward side of an evergreen over a cold dry period and looks very similar. The two commonly happen together.

Potato blight spreads through a crop day by day in warm humid weather. Frost takes the whole exposed canopy in one night and stops.

Blossom wilt browns flowers and the spur behind them and keeps spreading into the wood.

Waterlogging kills over the winter on wet ground, and the cold gets the blame. Look at the roots before deciding.