Clive Kaiser, Extension Tree Fruit Specialist
Oregon State University, Umatilla County

On July 21, 2015, a dedicated group of growers from Oregon and Washington gathered in the Walla Walla Valley to listen to renowned experts about how to minimize cold damage to grapevines and fruit trees.

Dr. Glenn McGourty, viticulture and plant science advisor, University of California Cooperative Extension in Mendocino and Lake counties, outlined the basics of frost protection, offering practical examples of passive and active frost protection techniques and the role of ice nucleating bacteria and how to control them. He also discussed remediation of frost-damaged vines and differentiated between winter injury, when temperatures drop below -12°F and frost events when temperatures drop below 32°F.  During the latter, ice crystals form between the cells and disrupt the cell membranes. With cell membrane integrity gone, the cell contents desiccate and the cells fail. The foliage turns black in spring and brown in fall. Glenn also distinguished between radiation frosts- when inversions occur- versus advective frosts, when a large, cold air mass is usually accompanied by wind and low humidity.

Measures to avoid frost exposure include:

  1. Careful site selection; uplands are best if you have the choice. Glenn cited the age old Latin saying “Bacchus amat colle” which translates to “Bacchus loves the hillock” and further suggested south and west facing slopes tend to be the warmest – one upside to other aspects is that bud break may be delayed in spring, thus helping to avoid early spring frosts.
  2. Manage brush, trees and other air dams that trap cold air in the vineyard.
  3. Soil water management: bare, packed soils offer the most protection but present risk of erosion, loss of soil organic matter, destruction of soil structure, and poor footing for early spring spraying.
  4. Maintain the soil moisture near field capacity: wet the top 12 inches of the soil surface 2-3 days in advance of a predicted frost event. It is not necessary to wet the entire profile as the top layer provides insulation and protects soil lower in the profile from losing heat. Glenn also compared different types of vineyard floor management systems ranging from bare, firm, moist ground (being the warmest) to tall cover crops with restricted air drainage, which can be6-8°F cooler.
  5. Delay pruning where practical: consider double pruning by leaving long spurs (6-10 buds), which can be cut back to two spurs later in spring. This will result in delayed bud break and can avoid frost damage by gaining a week of delay.
  6. Mow cover crop as close to the ground as possible in late winter before bud break and the first spring frost.
  7. If available, utilize overhead sprinklers as frost protection, especially where cover crops are present.

Glenn discussed ice nucleating bacteria, which are ubiquitous in nature. Common examples include Pseudomonas syringae, Pseudomonas flourescens, Psuedomonas viridflava, Erwinia herbicola and Xanthamonas campestris var. vesicatoria. The presence of these bacteria will lead to increased freezing. For example, snow-blowing freeze-dried Pseudomonas syringae, which will produce snow at 27°F versus water at 15°F, is an obvious effect of these pathogens, therefore controlling the bacteria will help prevent ice nucleation. Applying up to three copper hydroxide sprays one week apart in spring, after bud break but before a frost event, will have a beneficial effect of protecting foliage against frost damage as long as temperatures do not drop below 25°F.

Glenn also discussed his regeneration pruning work on damaged vines. This research indicated that comparing no pruning to breaking damaged shoots, cutting out spurs, or cutting damaged shoots, yields mixed results and is cultivar dependent. Positive impacts of pruning were seen in Chardonnay but not Cabernet Sauvignon. Pruning did not impact fruit quality on either cultivar, however, significant differences occurred in shoot emergence of spurs which may result in lower pruning costs in future years.

Dr. Imed Dami, research and extension viticulturist at Ohio State University discussed his research using vegetable oil to delay bud break.  He discovered that an application of eight percent vegetable oil (v/v) with an emulsifier applied mid-winter can delay bud break by seven to ten days without affecting yield or fruit quality.  He also discussed other practices such as “hilling up” (burying graft unions with soil) the vines to protect the stems against cold damage. When temperatures dropped below 15°F in Ohio, the best pruning technique as found to be give-bud hedging. When regrowth from the ground took place, training and pulling the new shoots immediately resulted in the best recovery over time. Bull canes (vigorous canes greater than pencil thickness) must be removed since they often do not harden off properly resulting in more cold damage the next winter.

Dr. Mark Battany, University of California Cooperative Extension farm advisory in San Luis Obispo County and a self-confessed gizmologist, presented his findings on active versus passive control of frost. He highlighted the importance of managing cover crops and delaying bud break in spring, as well as the importance of knowing the temperature at both five and 35 feet above ground level. Wind machines only work when an inversion exists due to radiant conditions. In fact, when an advective freeze (polar express) occurs you can cause more damage by running a wind machine, as the air temperature at 35 feet can be substantially colder than at five feet. He also explained that for a wind machine to work well the temperature difference between the two different heights needs to be at least 8°F for the wind machine to have an effect. Mixing the air from the warmer layer will result in averaging of the two temperatures and a 4°F gain temperatures can be expected.  He also presented work showing that wind machines which blow warm air down are far more effective that those that blow cold air up.  He suggested using black float ball modified sensors rather than standard shielded air temperature readings as the latter are not accurate measures of plant tissue temperatures at night because objects exposed to the sky are subject to radiation heat losses. Consequently, the leaf temperatures are actually colder that those in the standard shielded sensors.

Finally, Dr. Kevin Kerr of Brock University provided an overview of the cold management techniques utilized around the Niagara peninsula in Canada. The focus of Kevin’s work is to understand and predict when freezing temperatures are going cause issues. His team has developed an extensive network of differential thermal analysis (DTA) using programmable freezers and measure cold hardiness of buds throughout the region. This is presented to participants through a novel VineAlert system. This includes both rates of acclimation and de-acclimation of bud. This information is being used by the growers to determine precisely when the wind machines will be required to help protect buds and as a result has saved their industry more than $13.8 million in lost grape revenue as well as saving another $1 million in operating costs of the wind machines. Lessons learned include: – early shutting down of vines in fall result in hardier buds; a wet harvest season results in slower vine acclimation; excessive crop load result in less hardy vines; too little crop leads to excess vigor which results in less hardy vines; de-acclimation begins in late January with a very rapid rate of de-acclimation during March; once the vine de-acclimates, there is no going back to maximum hardiness vines need 72 hours of sub-32°F re-exposure to stop de-acclimation from continuing.; maximum hardiness is achieved by prolonged exposure at 32°F or colder but does NOT require extremely cold temperatures (<20°F) to reach maximum cold hardiness.

All in all, the volume of information imparted by the speakers was vast. Both theoretical knowledge and practical applications were abundant and useful to all the participants, who expressed their sincere thanks to all the speakers for making the trek to the Walla Walla Valley.

If you have any additional questions about this workshop, please contact me at: clive.kaiser@oregonstate.edu.