Chinooks, late freezes, and fruit buds
Winter warm spells and spring frosts kill fruit crops in different months. How to site a tree and which fruits lose most often.
A fruit tree can be perfectly hardy and still give you nothing for five years. In this region the tree and the crop fail on different nights. One failure is midwinter. A chinook or any long warm spell pushes buds and trunks out of deep dormancy.
The next drop to zero, or well below, kills tissue that would have survived if it had stayed hardened. The other failure is spring. Flowers open, a clear night drops to the upper 20s, and the crop is gone while the tree looks fine by June.
People call both events “a late freeze.” They are not the same event, and a cultivar choice that helps with one can do nothing about the other.
Apricots are the clearest example of the spring failure. They bloom early, often weeks before the last 32°F date in the NOAA tables.
A Front Range median last frost near the end of April or the first days of May, which is what the Stapleton, Boulder, and Fort Collins rows show, arrives after many apricot flowers have already opened.
Peaches do the same a little later. Sweet cherries join them.
Sour cherries, many European plums, and apples bloom later and miss more of those nights. They still lose crops. They lose fewer years.
Where the cold sits
Utah State University’s note on a fickle climate makes the siting rule from the orchard business. Cold air drains downhill and pools where it cannot leave. Commercial orchards sit on benches. A home tree in a creek bottom or in a hollow behind a solid fence is collecting the frost of a colder place than the map elevation suggests.
Put fruit on the slope, not in the bowl. A slight north or east aspect can delay bloom, which is useful where spring frost is the main thief, at the cost of a cooler summer and slower ripening.
A south wall does the reverse. University of Idaho’s bulletin on woody plants in harsh climates says south- and west-facing masonry can add on the order of five degrees of frost protection for nearby plantings, and that the same heat swings can pull woody plants out of acclimation and dry them out.
The wall is a good place for a tomato and a risky place for an evergreen.
For a fruit tree it is a trade: earlier bloom, more spring-frost risk, more winter sunscald on the trunk. Whitewash or a trunk guard on the south side of a young tree is ordinary protection. Take fabric wraps off in summer.
Grand Junction shows what a different freeze calendar does. Walker Field’s median last 32°F spring date in the NOAA Colorado table is April 25. The Western Slope fruit industry is that climate plus irrigation and a site that sheds cold air, not a secret cultivar.
Denver Stapleton’s median is April 30, only a few days later on paper, and Front Range gardeners still lose peach crops because bloom does not read the median.
Laramie’s median last 32°F date in the University of Wyoming table is June 7. A peach in Laramie is not a plan. Match the crop to the calendar for your station before you match the variety.
What to plant if you want a harvest
If you have lost the crop twice, stop buying early-blooming trees for a year. Plant small fruit. Currants, gooseberries, and serviceberry are the shrubs mountain towns already harvest, covered on the small-fruit page.
Sour cherries crop more reliably than sweet cherries in late-frost yards. Apples with a cold-climate record, on a rootstock you know, are the tree to try once the site is chosen.
The apple page and the stone-fruit page hold those names. None of them repeal a hollow.
Pollination and pest pressure still apply in a warm valley. A tree that survives the frost can lose the fruit to codling moth or to hail.
Sanitation and a hail cover are part of expecting a crop, not extras. Fire blight likes lush growth. Do not push nitrogen on apples and pears. A modest, slightly hungry tree winters better and blights less.
Reading the year you are in
A warm week in January is not a favor. If buds have swollen and a hard cold is forecast, there is little you can do for a large tree except hope the hardening was not fully lost.
For a small tree you can sometimes wrap or cover the whole plant for a night or two. That is realistic at six feet and ridiculous at twenty. This is an argument for keeping fruit trees small, on a rootstock and a pruning plan that stays coverable, rather than an argument for heroic tarps.
A warm week in April is the classic Front Range trap. The median last frost has not arrived, the late date is still ahead, and the garden center is full of blooming peaches. Buying them that week is how the tree goes in the ground and the blossoms die on Thursday.
Plant the tree for the decade, not for the blossoms that are already open, and choose a species that blooms later if your late date is routinely after peach bloom. The decision rule is in reading a spring frost.
Keep a two-column notebook: the night the flower buds died, and the night the wood died. After a few years you will know which problem you actually have. A site with wood death needs hardier cultivars and less south-wall heat.
A site with blossom death needs later bloom, a higher position on the slope, or a switch to small fruit. Solving the wrong one is why the next tree dies the same way.
The night a freeze is forecast
When the forecast crosses your crop’s limit during bloom, act in the afternoon, not at midnight. A small tree can be draped with a frost blanket that reaches the ground, so the heat stored in the soil is what you are trapping.
A blanket thrown over the top of a tree and tied to the trunk does very little, because the cold air is still sitting on the blossoms and the soil heat is outside the tent. Hoops or a simple frame keep the fabric off the flowers.
Wet soil holds more heat than dry soil, so water the root zone in the morning before a radiational night if the soil has been dry. Do not spray the open blossoms with a hose at dusk in a wind. You will chill them.
Christmas lights, the old incandescent kind, used to be a small-orchard trick under a cover. Most houses no longer have those bulbs, and LED strings do not warm the air. Do not count on a string of decorative lights.
A cover and a small tree are the tools that still work. A large, unpruned apricot in a hollow will lose the crop, and the honest response is a smaller, later-blooming tree on a slope next time.
After the night, leave the cover on until the sun is up and the temperature under it is above freezing. Pulling a frozen blanket off at dawn can finish the damage the night started.
Then write down what happened. A blossom kill with live wood is a siting and species problem. Blackened twigs are a hardiness and dehardening problem. The next purchase should answer the column you just filled in.
Checklist
- Separate winter dehardening from spring blossom freezes in your notes.
- Plant on a slope or bench, not in a frost pocket.
- Guard young trunks from south-west winter sun.
- Prefer later-blooming fruit if your late 32°F date falls after apricot and peach bloom.
- Keep trees small enough to cover on a single bad night if the crop matters.
- Do not push nitrogen.
- After two lost crops of the same species, switch species or switch to shrubs.
- Use your station’s median and late dates. Do not borrow Grand Junction’s calendar or Laramie’s.
Sources
- USU Extension, Fickle Climate Keeps Gardeners Guessing.
- University of Idaho Extension, BUL 859, on exposures, frost protection near masonry, and woody-plant acclimation.
- NOAA NCEI Colorado freeze-occurrence table, Denver Stapleton and Grand Junction Walker Field medians.
- University of Wyoming Extension spring freeze probability table, Laramie median.
- CSU Extension, Iron Chlorosis of Woody Plants, because fruit trees are also on the chlorosis list and a yellow tree is a third, separate problem.