This page covers frost recovery. For frost protection, go here.
Lynn's Frost Recovery presentation PDF here
A severe frost occurred on the early morning of April 12, 2022-some growers calling it the worst frost event they've ever seen. After an unseasonably warm spring, promoting early budbreak (as much as 2 weeks earlier than the typical April 1 date); some vineyards had primary shoots out several inches when the frost hit. Temperatures recorded on the foothill PMI stations ranged between 30°F (Gold Hill-El Dorado) to 26°F (Rombauer and Eagle-Amador).
Grapevine Secondary, Latent and Lateral Shoots May Produce (some) Fruit After Frost Injury to Primary Shoots.
Cluster arising on shoot from latent basal bud after 2 count buds were frost killed.
|Modified from photo taken by R. Smith||Photo: R. Smith|
In order to understand the effect of a freeze on grape yield, it is helpful to review grape botany. Recall that each fruiting bud (i.e. the 2 ‘count’ buds on a 2 bud spur) is a compound bud composed of a primary bud, a secondary and a tertiary bud. The primary bud is the most mature, and is the shoot ready to go (complete with leaves, tendril and clusters) when spring arrives.
If something bad (like frost) kills the primary bud, the secondary bud will usually produce a shoot- these clusters from the secondary bud will be smaller with fewer berries. Martinson, Lasko and Bates (Cornell University) estimate the secondary bud gives about 30% of the yield a primary bud would. Tertiary buds are rarely fruitful.
In addition to secondary buds, latent buds-those that are typically dormant in the grapevine spurs, arms and trunk-may also, in some circumstances, produce fruit; although most latent shoots are fruitless.
Finally, lateral shoots-those arising from an apical bud off the main shoot-can sometimes produce fruit. Fruit arising from laterals is termed “second crop” because it is much later to mature than the crop arising from count buds. Some varieties, such as 'Gamay', consistently produce a good second crop, making these varieties good choices for frost-prone sites. However, management (mildew sprays, ripening and harvest logistics) of second crop needs to be carefully considered since it is so much later than the primary crop.
Should You Remove Frosted Shoots? No!
The following UC studies showed that removing frost damaged shoots does not improve yield over “doing nothing”, and would not be the most cost-effective practice. “Doing nothing” is, for once, the best advice. In fact, it is possible that if you remove frost damaged shoots, you may inadvertently remove secondary or even latent buds that might give you a, albeit smaller, crop.
In 1964 James Lider tried to replicate the successful results reported by Winkler in 1933 when he stripped frosted shoots from 'Tokay' frosted vines. Lider, working at the UC Oakville Station in Napa after a severe frost event on April 24, conducted treatments on ‘Folle blanche’, ‘Cabernet sauvignon’, and ‘White Riesling’. The experiment on ‘Folle blanche’ was a completely randomized test with control and treatments including removal of frost injured shoots either by hand or with a pruning shears. The ‘Cab. sauv.’ and ‘White Riesling’ trials were smaller. Lider found no significant differences in yields from any treatment in any of the 3 varieties compared to an untreated control. In fact, no one has ever been able to reproduce the Winkler results.
In 1972, Kasimatis and Kissler conducted a similar experiment as Lider in the cultivars ‘Tokay’, ‘Carignane’, ‘Zinfandel’, ‘Chenin blanc’ and ‘Grenache’ when severe frost hit the San Joaquin Valley on March 31. Applying 3 treatments: breaking out all shoots, breaking out only frost damage shoots, or no treatment. They did a thorough evaluation of yield, separating fruit coming from the primary, secondary, basal, latent or auxillary (second crop) buds. Again, they found no difference in yields compared to untreated controls-where NOTHING was done (the exception was reduced yield as compared to the untreated control in ‘Tokay’ and ‘Carignane’ when all shoots were removed).
Williams, Dokoozlian and Wample discuss the effects of freezing on grape in The Handbook of Environmental Physiology of Fruit Crops Vol. 1-Temperate Crops (1994, Andersen, ed.) They also provide previously unpublished data from Wample showing yield response of ‘Cabernet sauvignon’, ‘White Riesling’, and ‘Chardonnay’ following frost injury and after treatments including removal of frosted shoots by hand, removal of frosted shoots with shears, and no treatment compared to severely frosted and non-frosted vines. Like Lider (1964) and Kasimatis and Kissler (1974), Wample’s study showed that shoot removal treatments did not affect yield in ‘Cab. sauv.’ or ‘White Riesling’, and for ‘Chardonnay’ shoot removal treatments actually reduced yield.
Personal communications with Rhonda Smith (Emeritus Viticulture Advisor Sonoma County) and Glenn McGourty (Emeritus Viticulture Advisor Mendocino/Lake County) revealed similar results when Glenn and Rhonda basically repeated the “removal of frosted shoots” experiment in pilot studies of their own.
This second leaf vine may not make it due to frost injury.
Young Vines Suffer the Most
Recently planted vineyards where vines have not yet fully developed structural parts like trunk and cordons where carbohydrates are stored can suffer severe frost damage. Since these vines are close to the ground (and groundcover) where temperatures are the lowest and ice nucleating bacteria are found, they experience the worse conditions. Rhonda Smith and our UC Viticulture Specialist Kaan Kurtural do not advise additional fertilization in this situation. Growers should apply their typical fertilizer program. Additional nitrogen is not expected to help when there are no active growing points to take it up or root to shoot connections are weakened. Additional nitrogen may be lost to volatilization or leaching. Growers with frosted young vineyards may want to wait before adding inputs to see if their vines recover. Replanting may be in order.
Canopy Management for Next Year's Fruit
Vines experiencing heavy frost damage will likely fill in with lateral shoot growth and growers will need to pay close attention to opening up the canopy with shoot thinning for dappled light. Remember, next year's fruit begins to differentiate this year, just after bloom. The amount of light the canes and spurs see should be enough to initate flower primordia.
updated 4-22-22 by L. R. Wunderlich