How to Protect Fruit Trees From Late Spring Frost

How to Protect Trees From Late Spring Frost: The Night Routine

To protect a fruit tree from a late spring frost, water the soil under it in the morning, cover the whole tree with cloth in the afternoon so the fabric reaches the ground, hang an incandescent light under the cover, and leave everything in place until late the next morning. Act when the tree is in bloom and the forecast low is near 28°F or lower. That routine comes from Utah State University’s frost advisories for backyard growers, and it works because the cover traps heat that the soil gives off overnight.

1. Morning. Irrigate the ground under the tree so the wet soil absorbs heat during the day.
2. Afternoon. Apply the cloth before temperatures cool, to trap existing warmth. Use a light sheet, burlap or a frost blanket. Do not use a plastic tarp, since plastic is a poor insulator and tissue touching it may still freeze.
3. Check the hem. The covering must go to the ground on every side. Hold it off the flowers with canes or stakes where you can.
4. Add heat. Under the cover, hang incandescent lighting rated for outdoor use, such as a 100-watt bulb or a light string with 7-watt bulbs. New Mexico State University specifies incandescent bulbs for this job, not LEDs.
5. Next day. Do not remove the cloth until late in the morning. Then take it off or roll it up so pollinators can reach the blossoms.

This suits trees you can reach over: dwarf trees, young trees, and trees trained on a wall or fence. For wall-trained fruit the Royal Horticultural Society recommends two or three layers of horticultural fleece, hessian or shade netting, rolled up during the day. Larger trees are covered further down.

How Cold Is Too Cold for Buds and Blossoms

A dormant fruit tree shrugs off winter cold. The danger comes later, because flower buds lose hardiness with every stage of opening. For apples, the temperature that kills 10 percent of buds rises from 15°F at silver tip to 18°F at green tip, 23°F at half-inch green, 27°F at tight cluster and 28°F from first pink through full bloom. The 90 percent kill temperature rises alongside it, from 2°F at silver tip to 10°F, 15°F, 21°F, 24°F at first pink and 25°F at full bloom.

By full bloom, every common tree fruit sits in the same narrow band. The table gives critical temperatures from New Mexico State University’s Circular 701.

Fruit	10% kill, swollen bud	10% kill, full bloom	90% kill, full bloom
Apple	15°F (silver tip)	28°F	25°F
Pear	15°F	28°F	24°F
Peach and nectarine	18°F	27°F	24°F
Apricot	15°F (first swell)	27°F	22°F
Sweet cherry	17°F	28°F	25°F
Tart cherry	15°F	28°F	24°F
Plum	14°F	28°F	23°F

Two things follow. First, the margin is thin: only 3 to 5 degrees separate light losses from a wiped-out crop at full bloom. Utah State puts it as 28°F for just 30 minutes for a 10 percent loss and 25°F for 30 minutes for a 90 percent loss. Second, the risk does not end at petal fall. Young fruit is no hardier than open flowers, and the RHS advises keeping protection ready for two weeks after flowering if severe frosts threaten.

Early bloomers get caught most often. In New Mexico, apricots bloom in March, peaches from late March into April, and apples in early to mid-April, so an apricot can be in full flower and vulnerable at 27°F while an apple nearby is still at green tip and losing only 10 percent of its buds at 18°F. Bloom timing also affects pollination partners, covered in self-pollinating vs. cross-pollinating fruit trees.

A 10 percent loss is rarely the end of the crop, since apples and peaches are thinned by hand in a normal year anyway. See thinning apples and peaches.

Read the Forecast: Two Kinds of Freeze

Frost nights come in two types, and only one can be fought at home.

A radiation freeze happens on a clear, calm night. Heat stored in the upper soil radiates back to the sky, the air near the ground cools, and cold air settles in low spots. Covers, wet soil and small heat sources all work on these nights because they hold on to that soil heat. Humidity matters too. Penn State Extension notes that the temperature can fall fairly rapidly until the air approaches the dew point.

An advective freeze arrives when a large mass of cold, dry air moves in with winds above 5 mph. Penn State’s verdict is that little can be done to protect trees against it, other than possibly covering the crop.

Once buds show color, check the hourly forecast for the low, the wind and the sky.

Protecting Trees That Are Too Big to Cover

A cloth cover is the best option for backyard trees that are not too tall. For a full-size tree, Utah State lists three fallbacks: hang incandescent lights in the canopy, water the root zone on the mornings before the cold nights, or run a barbecue grill near the tree on its low setting overnight once the temperature in the yard approaches 27°F. Keep any flame well clear of branches and covers.

Keep the ground working for you. Wet soil absorbs more heat by day and releases it at night. The RHS adds that grass around trees should be mown short in the flowering season, because long grass prevents heat being radiated from the soil.

Do not try to copy the overhead sprinklers used in commercial orchards. They gain 4 to 6 degrees, but Penn State describes systems that apply 0.10 to 0.15 inch of water per hour, protect only down to 24°F and do not work in high winds. That is an engineered system, and a garden sprinkler is no substitute.

Then accept some loss on a big tree. Iowa State University Extension goes as far as saying there are no practical ways for home gardeners to prevent freeze damage on fruit trees, and that the best approach is to wait, see how the tree responds and give it consistent care afterward.

Site and Variety Choices That Reduce Frost Risk

The cheapest protection is decided at planting. Cold air drains downhill and pools in hollows, so avoid frost pockets. New Mexico State’s guidance for orchards is a site at least 50 feet above the valley floor on a 6 to 8 degree slope for good air drainage. In a yard, that translates to the upper part of any slope. How much space fruit trees need covers the rest of the siting decision.

Aspect cuts both ways. The RHS favors a sunny, sheltered position such as a south-facing wall for warmth. University of Nebraska Extension points out that north and east facing slopes or sides of buildings warm up more slowly in spring, which helps delay budbreak until after frost season. In a region with regular April frosts, the cooler side is the safer place for an apricot or peach.

Variety matters as much as position. Late-flowering apples such as ‘Edward VII’ and ‘Crawley Beauty’ are among the late-flowering cultivars the RHS names. If you are choosing a tree now, see bare-root vs. potted fruit trees, and consider dwarf fruit trees for containers, which can be wheeled into a garage for the night.

How to Treat Trees With Frost Damage in Spring

Frost damage is easy to recognize on open growth. Freezing can destroy open flowers, turning them brown, limp and mushy. Leaves hit by temperatures below 32°F shrivel, turn brown or black, go limp and eventually drop. Buds are harder to judge from outside. New Mexico State’s method is to check buds or flowers for frost damage with a razor blade and a magnifying glass.

Treatment is mostly patience:

• Wait before pruning. Most trees and shrubs rebound within weeks by producing new growth. Prune damaged branch tips only after they are confirmed dry and brittle. Stems that fail to recover can be pruned out in late spring.
• Do not fertilize. Fertilizer is not recommended for recovery from cold damage.
• Water in dry spells. Deep, infrequent watering that keeps the soil moist helps, particularly for trees planted within the last 3 to 5 years.
• Mulch the root zone. A 2 to 4 inch layer of shredded wood, leaves or straw, spread well past the edge of the canopy, steadies soil moisture while the tree recovers.

Fruit that survives a frost may carry scars. Apple and pear fruitlets damaged by cold can develop heavy russeting and cracked skin. A light crop after frost says nothing about the health of the tree itself. For the other common reason young fruit falls, see early fruit drop.

Quick recap

• Open blossoms are the weak point: at full bloom about 10 percent die at 27°F to 28°F and about 90 percent at 22°F to 25°F, depending on the fruit.
• On a frost night, water the soil in the morning, cover the tree to the ground with cloth in the afternoon, add a 100-watt incandescent bulb or 7-watt string, and uncover late the next morning.
• Skip plastic covers and home sprinkler systems.
• Plant on a slope out of frost pockets, and choose late-flowering varieties where April frosts are common.
• After damage, wait for regrowth, hold the fertilizer, water in dry spells, and prune only dead, brittle wood in late spring.