Frost and freeze protection for nursery crops
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EmailNursery growers should keep in mind methods to protect crops from record-breaking warm temperatures.
Current record warm temperatures this early in the season means that plants may be susceptible to frost or freeze damage if we experience extreme cold following this warm trend. During February, the Michigan State University Enviroweather station in West Olive, Michigan, recorded six days in the 50s and two days in the 60s degrees Fahrenheit (Figure 1). There is a record low amount of ice cover on Lake Michigan (Figure 2) this season, which usually moderates large temperature swings in inland locations during the spring and fall. While a few warm days will not be sufficient to break dormancy of some plants, the 14-day forecast has many days in the 50s and 60s. Historically, our last frost day is mid-May which is still very far away. Most farmers remember the damage to fruit crops in 2012 from the unseasonably warm temperatures. There’s been substantial winter injury to nursery crops during other years’ early warm temperatures.
Growers can protect plants as temperatures return to normal by covering with blanket-type materials or using wind machines. Heaters are effective, but often cost prohibitive. Finally, irrigation can be used to provide protection from frosts depending on the irrigation system. The effectiveness of any of these techniques depends on the severity, duration and type of event for which protection is being attempted.
Frost vs. freeze
When listening to weather reports, you will hear the terms frost and freeze. Distinguishing between the two types of events is important when implementing cold protection. Freezes occur when a windborne (wind speeds usually above 5 mph) cold air mass moves into an area bringing temperatures below 32 degrees Fahrenheit. The thickness of the cold air mass is usually between 500 to 5,000 feet. Frosts occur when the sky is clear and winds are calm, creating a temperature inversion where temperatures near the ground surface drop below freezing while higher air is warmer.
Winter blankets
Cold protection is difficult for freezes because of higher wind speeds and the thickness of the cold air mass. Covering plants with lightweight fabric helps to trap radiant heat from the ground around the plant canopy and help prevent air temperatures from dropping below freezing. Some key considerations for using frost fabrics: use light-colored, woven or non-woven materials that can breathe; avoid dark-colors and impermeable materials; and remove frost blankets as soon as temperatures rise above freezing because heat can build quickly under covers once the sun comes out. Snow is also still a possibility and snow on top of blankets can cause breakage to plants. Make sure the fabric is held securely in place in case the wind should pick up. Plant size and spacing limit the practicality of blankets, therefore smaller, shorter, tightly-spaced plants can more effectively be covered than large plants on wide spacings.
Hoophouses
Hoophouses are very effective for freezes since they eliminate the effect of wind speed and allow use of more techniques than unprotected plantings. Properly ventilated houses will help reduce plant susceptibility to cold temperatures. If plants are coming out of dormancy too early, growers can close vents and covering plants inside hoophouses with an additional layer of a blanket material will provide additional cold protection. However, in the daytime as temperatures increase, especially on sunny days, removing blankets and opening doors and vents will be necessary. Heaters can be used to maintain hoophouses above freezing if available, although cost is a factor. Heating houses with higher value crops might make sense. Irrigation can also be used inside hoophouses to provide cold protection and can be very effective since wind speed is practically zero in hoophouses. However, diseases and other pests can become a problem due to poor drainage, low transpiration and high humidity. If plants are elevated from the ground, drainage will be less of an issue.
Irrigation for frost protection
Irrigation can be used for frost (not freeze) protection in many horticultural settings. The irrigation application rate and wind speed are critical in the effectiveness of irrigation for frost protection. Irrigation can protect plants from frost damage since a small amount of heat is released as water changes from liquid to solid; however, if the application rate is not fast enough or wind speeds are too high, more damage can be done to the plants than if protection was not attempted. Water removes much more heat from the air as it evaporates due to evaporative cooling than it provides when it freezes. If evaporation occurs as you are applying water, you will be dropping temperatures around the plants rather than protecting them. For this reason, it is essential that irrigation be run consistently throughout the frost event to provide protection so that new ice is constantly formed.
Irrigation should be discontinued once temperatures rise back above freezing. Generally, irrigation is not effective for wind speeds higher than 5 mph for taller plants and 10 mph for plants growing close to the ground. When using irrigation in hoophouses, once temperatures outside rise above freezing, ventilate houses to reduce humidity, increase air flow and help minimize disease problems. Ice buildup and damage must be considered as well as saturating the soil to prevent flooding damage and disease problems. For an excellent discussion of irrigation for frost protection, see Chapter 12 of Irrigation of Forest Tree Nurseries in the Forest Nursery Manual.
Wind machines in frost situations
Wind machines are also practical for certain frost situations. Under frost conditions, air further from the ground surface is warmer so wind machines redistribute the warmer air to crop level. The temperature differential between the surface, the effective mixing height of the wind machine, and the effective radius of mixing (motor horsepower/air flow rate) determines the amount of protection that can be provided. Wind machines are not effective for freeze situations.
Unfortunately, most of these techniques require preexisting installation. Winter blankets are the main exception. These techniques also require appropriate management and implementation under proper conditions. Using techniques that only work for frosts will not be effective, and possibly counterproductive, under freezes. Make sure that protection is provided until temperatures rise above freezing. Also, remember that these techniques may have consequences for production decisions as we move into spring. Biotic and abiotic stresses can result from blankets and irrigation. Irrigation, especially if soils become saturated, should be considered when implementing fertilizer programs. Saturated soils can also affect timing of cultural practices by delaying access to fields.
The MSU Extension fruit team also has some excellent articles on late freeze events such as, What can fruit growers do if a freeze is coming?
