By Judy Ferring
Patrick County Master Gardeners
The time is ripe for planting new perennials in your garden. That includes fruit trees, even if you’re still haunted by visions of trees coaxed into premature bloom by a balmy mid-winter, then axed by three successive killing frosts.
“Virginia climatic conditions are such that good results can be obtained regardless of whether the trees are planted in fall or early spring,” says the Virginia Cooperative Extension Service. “Planting about a month after the first killing frost in the fall or about a month before bloom in the spring is generally recommended.
“The important things to remember are that trees should be dormant, and the soil should have proper moisture content.”
In an established fruit orchard, about the time harvests are completed, many fruit trees begin to form next year’s fruit buds, develop new wood and store carbohydrates for winter, explains Steve Renquist of the Oregon State University Extension Service. (Climate maps for central and western Oregon are zoned 7 a and 7b, similar to Virginia’s northwest piedmont, only in a dappled camo pattern rather than long swoops.)
Interrupted dormancy was the problem last spring. Warmer-than-usual mid-winter temperatures lured trees into breaking dormancy too soon. The truth is cold is an important component of a fruit tree’s annual regimen. “Temperate fruit trees need a specific range of winter chilling to break down the trees’ internal growth inhibitors and enable the blooms and leaves to emerge normally in the spring,” explain specialists at the Texas A&M state extension service.
As the term “chill hours” suggests, it’s not so much the how cold the tree gets as how many hours it is at the optimal temperature. Most scientists count a chill hour as any hour that is under 45 degrees Farenheit. The hours are not necessarily consecutive: some fluctuation above that red line is discounted so long as temperatures do not go above 60 degrees F. During its dormant chill time, the tree is protecting its vascular system and fruit buds from freezing – some call this a period of vegetative growth. When dormancy is broken by warm temperatures, the tree turns to reproductive growth – flowering and setting fruit. Without he proper amount of chilling time, blooms may not appear at all; appear too early and be damaged by frost or freeze; appear too late and have increased susceptibility to disease and pests; or produce crops of reduced quantity or quality.
The number of chill hours needed by a fruit tree depends on which cultivar it is – and there are thousands of cultivars. For instance, there are currently more than 8,000 apple varieties worldwide. Most need 500-1,000 chill hours but Dorsett Golden, found in the Bahamas, requires less than 100. “Apple varieties selected for a particular growing area should have a chilling requirement within 150 hours of the average winter chilling,” say Cooperative Extension fruit specialists at Texas A& M.
There’s an easy way out of making chill hour calculations when choosing the varieties to plant. Consult the Virginia Extension Service publication titled “Tree Fruit in the Home Garden.” It includes a detailed table of recommended fruit tree varieties, based on a number of factors, including period of dormancy. “They must be adapted to your soil and climatic conditions,” writes author Rongcai Yuan, assistant professor of horticulture at Virginia Tech. “If possible, without sacrificing too much yield or quality, select varieties with the fewest insect and disease problems.”
Yuan’s highly detailed publication begins with site selection and planting and continues through maintenance and pruning. It includes a table of recommended fruit tree varieties – apples, cherries, pears, plums, nectarines and peaches. “Apricots are not recommended,” says Yuan. “The buds of currently available varieties respond to the first warm days of early spring and are usually killed by frost or low temperatures common to most areas.”
The varieties that are listed are arranged in order of ripening and annotated as to their optimal usefulness for cooking, desserts and freezing. There is also a section on heritage fruit varieties in Virginia.
The risk of frost – and approaching dormancy – in Patrick County begins around September 29, increasing weekly until, by October 12, it’s almost certain, according to NOAH’s National Climactic Data Center. From then, temperatures will trending steadily downward until, around October 29, Patrick County is likely to see its first freeze. By November 21, freeze chances are 50-50; by December 15, almost certain.
Bottom line: new fruit trees should be planted by Halloween to give them time to settle in and begin their winter snooze.
And what if there is another killing frost next spring? First, keep mulch, ground covers and weeds away from the ground around the trees as much as possible – the bare, moist soil radiates more heat than mulched soil. Second, stash some old sheets or blankets in a handy spot where you can grab them when frost is predicted. Use them to cover the trees during the day in order to collect enough heat to keep them warm through the night. (It can also help to water trees before a potential freeze because water gives off heat and helps protect the trees but if a severe freeze is expected, make sure the soil has good drainage.)