Having some snow on trees and ground, even if it is not that heavy like the one we had last night, is not bad at all. Having a layer of dry snow on the ground should provide good insulation for soil and roots against the expected low temperatures we will face tonight and tomorrow morning. Also, large limbs that are still loaded with snow should get some protection. Using sprinklers will worsen the situation as wet snow works as a good thermal conductor and will transfer these freezing temperatures to roots much faster than dry snow.
The simple and direct answer is YES, BUT you have to consider the right time/conditions.
When you use sprinklers to reduce frost damage, you are simply using the internal (latent) energy of water molecules as a source of heat to your trees. Simply, water exists in three forms (liquid, solid (ice) and gas (vapor), and the transition among these forms can produce energy (exothermic) or consume energy (endothermic) as appears in the diagram below. So, when the air temperature is below freezing points (< 32 oF), and you use sprinklers, you indeed encourage the transition of water from its liquid phase into ice in order to release the latent energy into sensible energy (heat) that plant tissues can use to warm up. Great! Where is the problem?
There are three things you have to understand and consider in order to get this positive effect of sprinklers and not to harm your plants. First, you have to watch the wind. The Wind encourages the transition of the water from the liquid phase into the gaseous phase (vapor) and while this happens, heat in the air and around your plants will be consumed in an endothermic reaction. So, it’s always recommended that you don’t use sprinklers if the wind is above (10 mph). The table below (Table 1) should help you decide the amount of water needed (inches/h) under different wind conditions (mph) and under different temperatures.
Table (1): Amount of water (inches/h) that should be provided for frost protection under different wind speeds (mph). (U of Florida Ext. Circ. 287)
Second is the dew point: without putting complication to the topic, low dew points implies low humidity and if the dew point is too low, the water you add through sprinklers will be soon evaporated to compensate for the low air humidity and consequently cools down air around your plants. So, under moderate dew points, a part of water you add through sprinklers will turn into vapor and consumes heat and another part will turn into ice and produces heat, but AFTER SOME TIME, the net energy will be positive and you will get the benefit of using sprinklers. However, if the dew point is too low, it means that the air is too dry and it will take MUCH TIME to be saturated with vapor. During this time, the air temperature might reach to the critical temperature at which your buds/flowers will be damaged. In this case, sprinklers will cause more damage to your plants than without sprinklers. To help you decide whether sprinklers will be useful for your case or not, use the following table (Table 2) to determine the temperature at which you should turn on/off your sprinklers. If the dew point and the critical damage temperatures are not in the range shown in the table, don’t use sprinklers.
Table (2) shows the minimum temperature at which sprinklers should be turned on/off (UC-Davis, FP005 Quick Answers).
Simply, select the critical temperature for frost damage (the columns) and the dew-point temperature (the rows). The temperature where the row value crosses with the column value is the minimum air temperature at which you can turn on/off your sprinklers.
– you can use this tool to determine the dew point (https://www.weather.gov/epz/wxcalc_rh).
Or, you can get the dew points from Intellicase (use your zip code) http://www.intellicast.com/Local/Weather.aspx?location=USVA0837
The third thing to consider is the amount of water that your sprinklers can provide. If you are not sure that your sprinklers can provide sufficient amount of water, don’t use them at all. Table (1) should help you determine the amount of water required under different conditions.
Two things-the developmental stage of your buds and the critical temperature for frost damage
– For deciduous like pome and stone fruits, dormant buds are protected enough and not too much affected by the frost and freeze conditions. However, once the buds swell and become ready to burst, low temperatures are a major threat for the developing floral organs and the fresh vegetative tissues. For the northern parts of Virginia, and from what I see in our apple orchard at AHS Jr. AREC, I would not raise any red flags regarding the frost that might occur tonight (March 10) or over the weekend. Why? because most apple varieties are still between the silver tip and the green tip stage and we would need some degrees between 5-10 F or below to worry about bud damage, which is not the case based on the weather forecasting for this period.
Here is the table that has been developed by WSU for the critical temperatures that might cause damage to apple and other fruit trees.
To download the whole table, use the link https://extension.usu.edu/files/publications/factsheet/pub__5191779.pdf
– Another source that may be useful for some of you is the apple freeze risk tool developed by Cornell University. Link:
In this tool, you can select the location (using ZIP code), the variety (only three apple varieties), and whether you would like to see the full season trend (choose full season) or the current (this is the default setting). The tool takes into consideration the minimum temp., the developmental stage, and the temp. where 50% buds could be killed due to frost. If the minimum temp line (the blue line), crosses with the critical temp (the orange line), then the expectation is 50% bud damage.
If you look at the chart above, you can easily realize why I am stating that it might be useful for SOME (not all), because we at Winchester area are not even close to the tight cluster in Red Delicious (or any other variety) as this tool suggests; and hence the prediction of the 50% damage during the weekend (March 11, 12) is unrealistic. However, if this tool hits the right stage for you, I would say use it as a reliable source for frost prediction. Otherwise, use the above-mentioned table to assess the damage, if any.
Orchard management and profitable production of tree fruits has become driven by robust science and thorough research. In light of this fact, this multidisciplinary questionnaire has been put together by a group of AHS Jr.-AREC scientists to help them identify and set research questions that reflect the interests of Virginia tree fruit growers and stakeholders. A similar questionnaire was distributed 6 years ago and has served to guide AHS Jr.-AREC scientists designing their field-blocks and shaping their research directions. Based on this survey, many experiments in pomology, plant pathology and entomology have been initiated. This includes, but is not limited to, experiments on crop load management practices, orchard tree planting density, variety and rootstock evaluation, physiological disorders, fungicides and insecticide efficacy trials, mating disruption trials and organic practices. This year’s questionnaire is meant to investigate the new trends and interests of Virginia fruit producers that may ignite new ideas or further promote researching the existing ones.
We are expecting responses from tree fruit growers, private agricultural consultants, agribusiness field representatives, VCE specialists and VCE extension educators in Virginia. An online version of the 2017 questionnaire can be accessed here (https://virginiatech.qualtrics.com/SE/?SID=SV_ezHdupTmKNcdO05). Please complete before March 15th.
A paper version of this questionnaire will be distributed during the upcoming fruit school meetings (Feb 13-17).