Frost is common in the north-eastern US, however, frost events during bloom, specifically in orchard crops can be economically devastating. Often the cause is a few hours of temperatures below the injury threshold temperature for the developing buds as cooler nights result in the rapid loss of thermal energy. Fortunately, there are several passive methods or low-tech strategies, that can help maintain a slightly higher orchard floor temperature, has the potential to prevent substantial crop loss.
Site selection is the most effective frost protection method. When planning a new orchard, selecting a site with proper cold air drainage is the most valuable decision that a grower can make and enjoy the rewards for many years. Cold air drainage is important, avoid buying a site or planting trees on the site that accumulates cold air. Also, identify areas where the cold air moves in and out. When possible plant early blooming varieties in the least frost-prone areas.
Do not Invite the Frost with excess vegetation and ground cover. The management of weeds along the row and sod between the rows will influence orchard temperatures. The bare ground will absorb more heat during the day and release more heat at night than soil covered in vegetation. During the day 45-50% of solar radiation gets reflected or absorbed by the atmosphere and the remaining 50-55% is absorbed by the earth. Vegetation reduces this heat transfer to the soil because it reflects light before it can reach the soil surface. This can be particularly important during spring bloom when just a few more degrees can make the difference between killing the flowers or not!
Maintaining sod between the rows at a minimum height by frequently mowing will help with frost protection. Similarly, keeping the rows weed-free by using postemergence herbicides to kill emerged weeds and removing weed residues from the previous season will maximize the warming of the soil by solar energy during the day. Ideally, maintaining weed-free rows should also be considered by applying residual herbicides in the fall. Depending on herbicides applied in the fall, this will prevent or reduce weed emergence in early spring and give you more flexibility for applying your residual and postemergence herbicides in spring. Removing tall weeds and windbreaks along the edges of orchards will also help improving airflow and to limit the accumulation of cold air pockets in some areas of the orchard.
Wet soil stores a much greater amount of heat than dry soil, as moisture, fills the air space between particles. If soil is irrigated to field capacity (maximum water holding capacity of that soil), it can then store as much heat in the orchard soil as possible during the day and thus release a greater amount of heat through the night. Sandier soils store less energy than loamy soils because more air space between soil particles compared to loamy soils. It may require more than 8–10 hours of irrigation for heavier soils to reach field capacity. Make sure wind speed is under 10 mph. If the wind is above this rate then evaporative cooling of air above the surface can be counterproductive.
Table 1. Relative temperature differences as influenced by orchard floor conditions (Courtesy, Robert Crassweller, PSU).
| Bare, firm, moist ground | Warmest |
| Shredded cover crop, moist ground | ½°F colder |
| Low-growing cover crop | 1°-3°F colder |
| Dry, firm ground. | 2°F colder |
| Freshly disked, fluffy ground | 2°F colder |
| High cover crop | 2°-4°Fcolder |
| Where cover crop restricts air drainage | 6°-8°F colder |
Delayed pruning delays bud development and that generally helps avoid damage from early spring frosts. Years of observation in commercial orchards in the northeast indicate that more buds survive after a frost event, specifically in peaches, if the pruning is delayed. Often large farm operations begin pruning early in the dormant season due to constraints of labor availability and time. In such cases, avoid pruning upper branches until the risk of a late freeze has passed. Also, avoid hard blossom thinning on peaches on lower branches or blossom thinning altogether. Instead, focus on only fruit thinning which can serve as insurance against crop loss. Remember that in frost years, any available crop will likely command a good price.
It is also important to keep trees healthy with proper nutrient and pest management. Healthy trees may still lose a crop, but they are more likely to survive the next season, compared to poor vigor and diseased trees.
The success of Frost Protection will depend on an understanding of the working principles of each method and the combination of environmental factors necessitating its use. The bulletin https://njaes.rutgers.edu/e363/ explains what to monitor before a frost event and active frost protection methods such as irrigation, heat application, and mixing of air.
