Amanda Green, Tree Fruit Specialist, OMAFRA
Buds are moving along quicker a week ahead compared to last year. With this earlier bloom, frost is a more pronounced concern. At first pink to post bloom, blossom buds are at their most vulnerable with 10% bud kill occurring at -2˚C and 90% bud kill occurring at -4˚C (Table 1). As we progress into bloom, you may not have the infrastructure in place to protect your crop or may not have coverage on all of your acres. Below I have listed the most effective tools for frost protection but these require the most infrastructure. In the event of a frost, you may be looking to methods to help prevent against frost damage, I have listed some alternative, last resort tools which may not be the most effective or cost-effective, but they are worth knowing about.
Table 1: Critical Spring Temperatures in ˚C for Apples
|Silver Tip||Green Tip||½ inch Green||Tight Cluster||First Pink||Full Pink||First Bloom||Full Bloom||Post Bloom|
*Adopted from Washington (WSU) Extension Bulletin EB0913: Critical temperatures for blossom buds, apples.
Most Effective Tools
Wind machines are very effective at providing protection when there is an inversion layer on still nights by mixing the warmer air on top with the colder air below. Temperature inversions can be up to a 5-10˚ C difference between ground level and 20 m. For more information on using wind machines please see the factsheet http://www.omafra.gov.on.ca/english/engineer/facts/10-045.htm
Overhead irrigation has been an effective frost protection method used on strawberries in Ontario and in apples in Washington State but is not commonly practiced in Ontario for apples. Overhead irrigation works to protect the crop by latent heat releasing when water changes state from liquid to solid. This works similarly to how steam can burn your hand as it condenses into a liquid. You have to be cautious with overhead irrigation as you need to make sure you can deliver enough water. The amount of water that needs to applied per hour per acre depends on the wind speed and temperature. You will also need to make sure you have enough water supply to keep irrigating until the ice begins to melt. For more information on overhead irrigation for frost protection please see the factsheet http://www.omafra.gov.on.ca/english/crops/facts/frosprot_straw.htm.
Another method to protecting your crop with irrigation is under the row irrigation. This would work simarly to overhead irrigation in that as the water changes state from liquid to solid, latent heat is released. It may not be as effective as overhead but there is less risk of freezing your crop if you run out of water.
Helicopters work similarly as wind machines with mixing the air when there is an inversion layer resulting in warming the air down by the crop. The set back to using helicopters is that they are expensive and cost hundreds of dollars per hour to rent.
Orchard heaters or fires can be used to try to raise the temperature in the area around the crop. This can be pretty inefficient as the heat will rise straight up and may have very little effect on raising the temperature.
There are sprays that claim to provide protection against frost like zinc and Pristine fungicide. There are many anecdotal claims to support the effectiveness of these products but limited evidence of the effectiveness these products when scientifically tested in a field situation. There are also anti-dessicant products that claim to provide frost control but frost damage occurs from internal dehydration of plant cells and is not related to transpiration which is what anti-dessicants target.
Pristine fungicide works by inhibiting the mitochondrial breathing chain at the level of the b/c1 complex for improving the tolerance of plants to low temperatures. When tested in the field by Dr. David Rosenberger et al. from Cornell, the seed count was higher in trees treated with pristine but there was no yield benefit to using Pristine. For more information on this trial please see: http://www.northeastipm.org/neipm/assets/File/TFWG-Rosenberger-Pristine-Frost.pdf. It is sold as a fungicide but may help with frost protection.
Zinc works by inhibiting ice-nucleating bacteria and can be applied the night before. Water melts at 0˚C but does not necessarily freeze at 0˚C. If water is homogenously pure it won’t freeze until very low temperatures. With foreign particles like ice-nucleating bacteria, ice can start forming at higher temperatures below zero. At temperatures above -5˚C, ice-nucleation bacteria causes ice formation on plant surfaces. The theory behind applying zinc is to inhibit the bacteria and will prevent ice from forming at temperatures just below zero. There is mixed and limited evidence on the effectiveness of using zinc
Promalin can be used as a frost rescue spray by promoting parthenocarpic (fruit with no seeds). The downside to parthenocarpic fruit is that they don’t size as well and will have a limited shelf life.
FAO, Chapter 6 Passive Protection Methods in Frost protection: fundamentals, practice and economics. http://www.fao.org/docrep/008/y7223e/y7223e0c.htm
Rademacher, W., Köhle, H., and Ulstad,V. Method for improving the tolerance of plants to chilling temperatures and/or frost. U.S. Patent Application No. 12/281,926.