How do I protect plants from a late frost after growth has already started?

A late frost is a different problem from a winter one. In winter the plant is dormant and hardened; in late spring the same plant is carrying soft, unlignified, water-filled growth that freezes at a temperature the dormant plant would have shrugged off. That is why a mild winter followed by a late frost does more damage than a cold winter did.

Step by step

1. Decide tonight's risk from the sky and the wind, not the number.Clear, calm and a forecast minimum within a few degrees of freezing is a frost night. If cloud is thickening or the wind is holding up, the risk drops substantially.
2. Triage in order of value and vulnerability.First, open blossom on fruit — one night decides the whole year's crop. Second, just-emerged potato shoots and newly planted tender crops. Third, soft new growth on tender perennials and shrubs. Established hardy plants generally need nothing.
3. Water dry soil in the afternoon.Moist soil stores more heat than dry soil and releases it slowly overnight. On a marginal night this is a real, free improvement, and it matters most in a dry spring.
4. Cover in the early evening.Get the cover on while the ground is still giving off the day's heat. Fleece over hoops or canes, clear of the foliage, taken to soil level all round and weighted continuously.
5. Earth up potatoes rather than covering them.Drawing soil over emerged shoots is quicker than fleecing a whole row and works well. The shoots grow back through it, and if the top growth is lost the plant regrows from the tubers anyway — later, and with a smaller crop.
6. Move what can be moved.Pots and trays go against the house wall, under an overhang, into a shed, a garage, a porch or a greenhouse. This is more effective than any cover and takes less time.
7. In the morning, shade anything frosted from direct sun.Frozen tissue is damaged further by thawing fast. If frosted growth is about to catch early sunshine, throw fleece over it or spray it gently with cold water to thaw it slowly. Then leave the plant alone.
8. Wait before cutting anything.Blackened growth looks finished and often is not. Wait until new growth appears so you can see what is alive, and cut back to that. Damaged tissue also gives some protection to the buds beneath it if another frost comes.

Tips

• Blossom that has already set tiny fruit is more vulnerable than blossom still in bud. If you can only cover some of a tree, cover the lowest branches — that is where the cold air pools.
• A tree too big to cover can sometimes be helped by covering the ground beneath it: bare, moist, firm soil radiates more stored heat upward at night than a mulched or grassed surface does. Pull mulch back on frost nights in the blossom period.
• Do not feed with nitrogen in a frost-risk period. It pushes soft growth exactly when soft growth is the liability.
• If a late frost catches a plant every year in the same spot, the answer is the position rather than the covering. That is a frost pocket telling you something.

Common mistakes

• Covering at bedtime — By then the ground has already given up most of its stored heat to a clear sky. The cover then traps cold air rather than warm, and gives a fraction of the benefit.
• Cutting off frosted growth immediately — You cannot tell what is dead for several days, the damaged material shelters the buds underneath from any further frost, and cutting stimulates new soft growth straight into the remaining risk period.
• Removing the cover as soon as the sun comes up on a frosted plant — Fast thawing in direct sun causes more cell damage than the freeze itself. Leave the cover on until the frost has gone off gently.

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