Pruning cold-injured grapes
Grapevines are resilient and can recover quickly from cold injury if there is adequate soil moisture.
Dr. Markus Keller received a "viticulture paper of the year" award for his pruning study on cold-injured Merlot grapevines.
The next time a major freeze damages wine grape vineyards in Washington State, growers would be better off to save labor costs and not prune at all, says a university scientist. Studies of cold-injured vines found no difference in the vine's recovery regardless of the number of buds left after pruning.
Eastern Washington vineyards are prone to cold injury from arctic temperatures in winter as well as sudden freezes in the fall that occur before grapevines have acquired cold hardiness. In 2002, temperatures dropped to about 11°F on Halloween, and caused extensive trunk injury in some Washington vineyards. A similar, though less severe, freeze occurred the following Halloween, with temperatures around 16°F.
The back-to-back freezes enabled Washington State University's Dr. Markus Keller to conduct a pruning experiment in an injured vineyard to study the effect of pruning time and bud number on vine recovery and performance. The study was conducted in a Merlot vineyard planted on own-rooted vines in 1999 at WSU's Prosser research station.
"There was tremendous phloem damage from the 2002 Halloween freeze," said Keller, adding that some vines had 100 percent phloem injury. "The textbooks tell us that as the vine goes through bud break, the buds release the hormone auxin that induces the vine to heal and recover. So, if you leave more buds on the plant, you should have more auxin hormone and have faster recovery."
Pruning treatments were imposed on the vines following the fall freezes, in the springs of 2003 and 2004. Keller varied the number of buds left on the damaged vines from 0 to about 200 and pruned at two different times: before bud break in early March and after bud break in mid-May.
"With the different number of buds left on the vine, we thought we'd get a whole range of recovery," Keller said. "But sure enough, they all recovered. There was absolutely no difference between the vines."
Even disbudded vines grew up to 25 vigorous shoots from basal buds on the canes and latent buds on the cordon, and many of these shoots were fruitful, he added.
"People sometimes say that you have to prune severely after a freeze because if the injured vine is allowed to grow a large leaf area, it can collapse later in the season," he said. "But we didn't find that. Very vigorous vines and lightly pruned vines all did well, provided we supplied enough water. And that means making sure there's enough water even at bud break, which isn't always the case. If you don't see sap flow (bleeding) before bud break, that's a sure sign that soil moisture is insufficient and you need to turn the water on."
The nonpruned vines had the highest yields without compromising fruit composition, Keller said. He also found that he could return to spur pruning after two seasons without any significant carryover effects.
The study suggests that pruning time and bud number may not influence vine survival and recovery following cold injury to the phloem, at least as long as a few latent buds survive, he stated. "As long as sufficient soil moisture is available in the spring, vines seem to be able to rapidly recover from extensive bud damage and repair severe phloem injury."
Armed with the new information, Keller now recommends that growers avoid pruning or do minimal pruning after cold injury. Minimal or light mechanical pruning may be a viable, inexpensive, and temporary strategy to achieve acceptable yields in seasons following extensive bud damage and phloem injury, he said.
Growers could save on labor costs by not pruning and still have more crop than if they pruned more heavily, but he emphasized that there has to be enough soil moisture early in the season to prevent canopy collapse. "Don't let these vines be water stressed until after fruit set," he warns.
"Vines are very good at self regulation," Keller said. "It's hard to overcrop, but if you do, you could still come in later and thin the crop."
Keller's pruning study on cold-injured Merlot grapes was published in the American Journal of Enology and Viticulture in 2007, and was named "viticulture paper of the year."