How do I protect fruit blossom from a late frost?

Frost damage to fruit is decided in a handful of nights each spring, on a plant that gives no warning. Cold air is heavier than warm air, so it drains downhill overnight and collects wherever it cannot escape — behind a solid fence, in a walled corner, at the bottom of a slope. On a clear still night, plants also radiate heat to the sky and can drop several degrees below the air temperature the forecast gives you.

Safety

What you'll need

• Horticultural fleece
• Clothes pegs or clips
• Canes to hold fleece off the flowers (optional)
• A min-max thermometer (optional)

Step by step

1. Know which stage the blossom is at.Tight buds tolerate a few degrees of frost. As the flowers open they become far more vulnerable, and open blossom and newly set fruitlets are killed at around -2°C. The riskiest fortnight is when the tree is in full flower.
2. Watch the forecast for clear, still nights rather than cold ones.A cloudy, breezy night at 1°C is usually harmless. A clear, still night forecast at 2°C can put ground frost on the blossom, because the plant radiates heat to a clear sky and the air does not mix.
3. Cover small, trained and potted trees before dusk.Horticultural fleece, one or two layers, draped over the whole tree and reaching the ground so it traps the warmth rising from the soil. Peg it so it cannot blow off. Cover before sunset — putting fleece on at 10pm covers a tree that has already cooled.
4. Take the fleece off in the morning.Not optional. Blossom left covered cannot be pollinated, and fleece left on in sunshine cooks the flowers. Off in the morning, back on at dusk, for as many nights as the risk lasts.
5. Move pots against a house wall or under cover.The easiest protection there is. A wall radiates stored heat overnight and a porch or open shed keeps the sky off entirely.
6. Do not fleece a tree in flower and leave it there for a week.Blossom needs insects. A tree wrapped through its whole flowering period sets nothing, exactly as if it had been frosted.
7. Fix the siting problem permanently where you can.Replacing a solid fence panel with something a wind can pass through lets cold air drain away instead of pooling. Removing the bottom of a hedge does the same. This is the only measure that helps a full-size tree.
8. Choose late-flowering varieties in a cold or frost-prone garden.Late-flowering apples such as Crawley Beauty and Edward VII, later pears such as Concorde, and late plums avoid a good proportion of frost years. In a frost pocket, this decision matters more than anything you do on the night.
9. Check the damage a couple of days later.Open a flower and look at the centre. A dead one has a blackened or brown centre with the petals still white. If most centres are black, that crop has gone and no amount of feeding or watering brings it back.

Tips

• Watering the ground in the afternoon before a frosty night genuinely helps a little: wet soil stores and releases more heat than dry soil.
• Bare firmed soil under a tree radiates more warmth overnight than long grass or a thick dry mulch does.
• Plastic sheeting is not fleece. Where it touches the flowers it conducts cold straight to them and does more harm than nothing at all.
• A tree with a long flowering season, or one that flowers over a fortnight, often escapes with a partial crop where a short-flowering one loses everything.
• Commercial growers use overhead irrigation through the frost, which works by releasing latent heat as the water freezes. It is not practical in a garden and half-doing it makes things worse.

Common mistakes

• Covering with polythene laid straight onto the flowers — Plastic conducts cold at every point of contact and traps condensation. Fleece is porous and insulates; polythene needs a frame holding it well clear if it is used at all.
• Leaving the cover on all through flowering — No insect access means no pollination. You end up with an unfrosted crop failure instead of a frosted one.
• Assuming the forecast temperature is what the blossom feels — Forecasts are for air at screen height in the open. A tree in a hollow on a clear still night can be several degrees colder.
• Cutting off frosted blossom — It does nothing. Leave the tree alone, feed and water normally, and let it put the season's energy into next year's fruit buds.

Questions people ask