by Sanne Kure-Jensen
Terence Bradshaw, assistant director of the University of Vermont’s Horticulture Research Center, led a workshop on Grapevine Cold Hardiness at the New England Vegetable & Fruit Growers Conference.
“Healthy vines will be more cold-hardy than weak vines,” said Bradshaw, who is a tree fruit and viticulture research specialist at the university. “The best way to protect vines from cold injury is to select the right variety for site conditions.” Careful vineyard management will also help maintain strong plants and maximize their cold tolerance and production quality.
A vine’s ability to tolerate cold depends on its physiological state and its genetics. A fully dormant plant will be much more tolerant than one deacclimating during a spring thaw. Similarly, before reaching full dormancy, vines may be injured by a sudden fall frost. Breeders select grape varieties for their tolerance of certain temperature ranges. If a vine sustains significant cold injury, careful pruning can retain some production and yield while limiting the risk of further vine stress. Monitor the vines regularly, and promptly prune out diseased and dead material. Bradshaw suggests 4 to 6 vineyard spray treatments per season may be sufficient with cold-hardy hybrid cultivars. The most critical treatments are before and just after bloom.
Fall and Spring Cold Injury
Changing day length triggers plants to slow their vine growth. This is when plants concentrate sugars in their ripening fruit. Like antifreeze, sugars lower the liquid’s freezing point, making vines less likely to suffer cold injury in low temperatures.
Vineyard managers remove fruit early for 1- to 3-year-old vines. This allows vines to store their sugars in plant tissues, especially in its roots, protecting young, shallow roots from winter cold.
Grapes will not ripen below 50 degrees Fahrenheit. Harvest grapes at their proper ripeness, as they will not continue ripening after harvest. Grapes will not ripen if their leaves are killed by frost.
Weed competition reduces energy available to plants in the fall when they need to store sugars and shut down. Pests, diseases or storms can damage or reduce leaf canopy and plant energy levels, causing delayed dormancy and/or reduced cold tolerance.
During a mild fall, a sudden frosty night can damage non-dormant buds. Mild springs with several days over 50 degrees Fahrenheit will deaclimate plants and trigger an increase in their biological processes. Subsequent cold periods — even for a few hours — can cause cold injury as can winter thaw/cold cycles followed by cold periods.
Vineyard managers can assess cold-injury levels by randomly collecting 100 buds (20 five-bud cane sections) across a particular variety and plot in spring just prior to pruning. Bring buds indoors to thaw. Later, use a razor to cut cross sections in the compound buds. Brown or black tissue inside the bud center indicates damage.
Compound grape buds borne at nodes on canes each contain three-bud primardia. The primary buds are the largest, and they produce fruit in the coming season. Secondary buds produce about one-third less fruit that primary buds. Tertiary buds do not typically produce fruit at all.
Pruning appropriately for the level of bud damage can lead to a reasonable crop in many varieties. If many primary buds are damaged, leave enough secondary buds to offer fruiting potential as well as stem and leaf growth to sustain the vines. When primary buds are destroyed and secondary buds are viable, many grape cultivars will still produce up to 65 percent of their typical yield.
Bradshaw recommended managers do multiple bud assessments for each grape cultivar and site condition. Each site variable like slope direction, soil type and moisture level will influence plant’s cold tolerance. Wetter soils will lower vines’ cold tolerance.
General Pruning Guidelines
Vineyard managers should use care when pruning, but not be frightened of the task. There are many accepted techniques, and grapevines are very vigorous. They will grow back after pruning, giving managers a chance to try again.
Leonard Perry, University of Vermont Extension Horticulturist, recommended vineyard managers prune during late winter on dormant vines.
• In the first year, establish upright shoots from 2 to 3 buds. Use bamboo stakes and string or growing tubes. These will be future trunks
• In the second year, leave 1 to 2 upright shoots. Establish bilateral canes (cordons) along trellis/wires, and remove side shoots and stem above top wire. Train two shoots/canes, one in each direction, along top wire and secure with plastic ties. Remove other first-year shoots and flower buds early. Allow other side shoots during season.
• In the third year and beyond, remove previous side shoots/fruiting canes leaving 4 to 6 bud spurs. Leave one side shoot/spur every 6 inches along cane. Cut back to 2 to 3 buds in spring after growth starts and danger of frost past. Remove suckers from lower trunk. Leave fruiting canes from nearest cordon/wire. In mid-simmer, thin the clusters when forming. Remove some or most leaves than shade fruit clusters.
• During growing season, thin fruit clusters, remove some leaves over clusters to expose them to sunlight.
Grapevine cold hardiness and strategies to handle winter cold
by Sanne Kure-Jensen