By Emma Alstott and Olivia Meyer et.al
Spring frost damage in strawberries and apples can have significant consequences for yield and fruit quality. In strawberries, late spring frosts can kill open blossoms, turning their centers black and preventing fruit formation. Developing fruit may also suffer from deformities or reduced size if exposed to freezing temperatures. Apple trees are particularly vulnerable when buds begin to swell and progress through bloom; frost can damage flower buds, leading to reduced fruit set, or cause russeting and deformities on developing fruit. Severity depends on factors such as temperature, duration of exposure, and bud development stage, with early-opening varieties often at greater risk. Protective measures like row covers, wind machines, and irrigation systems are commonly used to mitigate damage and ensure a successful harvest.
Protective Measure Options
Irrigation Systems
Using irrigation systems for frost protection in strawberries and apples can be highly effective but requires careful risk assessment to avoid unintended damage. Overhead irrigation works by releasing latent heat as water freezes on plant surfaces, maintaining tissue temperatures near 32°F (0°C). However, its success depends on precise water application rates, temperature thresholds, and continuous operation until thawing conditions return.
The main risks with overhead irrigation include excessive ice buildup that can break branches in apples or crush strawberry plants, evaporative cooling if water application is insufficient or stops prematurely, and excessive water accumulation leading to root rots or soil saturation (AKA flooding). Growers must assess weather forecasts before deciding to use this mitigation technique. An example of two factors that can cause the irrigation method to actually do more damage than the frost would be if wind speeds above are 10 mph or if humidity levels are low. These factors increase the cooling effects of the irrigation water, causing internal ice crystals in the plant. Water availability is also critical; irrigation must be sustained throughout the frost event, requiring a reliable water source that won't freeze before application (e.g., a well or municipal source). Monitoring real-time weather conditions and calibrating irrigation rates accordingly is essential to maximize protection while minimizing risks, which is very taxing mentally and emotionally on growers.
Row Covers
Row covers are a more passive-active hybrid of frost protection method. They act by reducing heat loss and preventing cold air penetration that reach the plants and cause cold temperature damage. Effectiveness is dependent on heavy-weight row cover material (often polyethylene or polypropylene – heavier) which can offer better insulation properties. However, frost damage may still occur in areas where the row cover is in direct contact with blossoms. This method doesn't prevent flowers and fruit from freezing, but rather slows the fall in temperatures to buy more time. This is more commonly used in strawberries, though work is being done in other states on implementing row covers in high density apples.
Wind Machines
Wind technology or airflow disturbance can be an integrative approach to be combined with irrigation to help conserve water use. By generating air flow during thermal inversions, the wind machines can force warmer air from the inversion layer downwards to mix with the cold air near the ground to mitigate the stable stratification of air and warm the air temperature that surround the plants to reduce frost risk, especially in orchards. Airflow disturbance technology is more effective during periods of radiative freezes vs. advective freezes where cold air is continuously moving into the area and is accompanied by wind and cloud cover (i.e no inversion present).
Considering Temperature Thresholds
When decided what mitigation efforts are optimal, it is important to consider critical temperatures. While each of these methods can be helpful, there are risks and added inputs involved. The damage a certain freeze may cause will vary between fruit species, variety/cultivar, and growth stage. These critical temperatures allow a grower to have an idea of how much of their crop they will be risking if they decide to not implement protective measures. This ensures growers are equipped to make the most cost-effective decision for their operation. Most open flower buds suffer some level of damage when temperatures dip below 28oF.
For backyard growers, frost coverings are the easiest and best bet for mitigating frost damage. Sprinkler systems can be used for overhead irrigation, but may not be cost effective depending on the water source.
Source : iastate.edu