The below-freezing temperatures on Tuesday (4/8) and Wednesday (8/9) will likely invite frost damage to peach orchards in New Jersey. In several parts of New Jersey, peach flower buds are in the bud swell to the petal fall stage.

Figure 1. Peach bud development stages and corresponding critical temperatures. Adapted from MSU Fruit Extension. Photos by H. Gohil

Bloom progression in southern New Jersey indicates that except for very few delayed blooming varieties such as ‘Gloria’ and “Scarlet Rose’ most varieties will be in the bloom or post-bloom, next week. These are the most vulnerable stages to frost damage (Figure 1). Often, the cause is a few hours below the injury threshold temperature for the developing buds. Monitoring the growth stage and weather conditions will help in determining the actual threat of frost damage and whether to start a frost protection system.

Monitor the orchard temperature. Frost can occur when the ambient temperature dips below freezing (32°F). The hourly falling rate and the lowest forecasted temperature should also be monitored.  Because the threshold temperature for each bud development stage is different, and a slight temperature difference can make a difference between frost damage and no damage, it is essential to know the precise temperature. In most cases, the orchard temperature will not be the same as forecasted or reported by the weather channels because the weather station could be farther away than the orchard location. Remember that the nighttime temperatures often dip below the predicted temperatures. It is crucial to have a thermometer in the orchard. Even the data from the nearby weather station could be unreliable. Also, frost alarm systems can give precise temperatures in the orchard and send warning messages to your mobile phone at a set temperature. Some systems can also report wet-bulb temperature, essential when using overhead irrigation for frost protection (Figure 2).

Monitor the bud development stage for each variety block, starting from bud swell. Critical temperatures have been researched and calculated for each bud development stage. For example, at 25°F, a peach orchard in full bloom can expect up to 90% crop loss, but at the first pink stage, it will suffer only marginal loss (Figure 1).

The critical temperature for damage at a particular bud stage may vary by 4-5°F depending on temperatures during the previous few days before the cold event.

Figure 2. Weather Station with in-built frost alarm system in Peach Orchard, Medford, Southern NJ.

Therefore, growers should use caution when using these published critical temperatures for active frost protection methods. Also, knowing the bud development stage for all varieties in the orchard will allow you to prioritize varieties or blocks that need frost protection.

Cloud coverage:  Clear skies enhance radiative heat loss into the sky, which results in lower temperatures than when there is cloud cover, which can trap radiant energy from the ground.

Wind Speed: Wind increases the rate of heat loss from the ground and plant tissues. Low speed allows the formation of the inversion layer. Most frost protection methods will have reduced efficiency at wind speeds above 10 mph. Fire or heat application-based methods are less effective at wind speeds below five mph.

The frost protection method should only be employed after determining the actual risk of frost and considering the cost-benefit analysis. The articles below have information on passive and active frost protection methods.

• Active Frost Protection Methods in the Orchard. https://njaes.rutgers.edu/e363/
• Preparing Orchard against Frost – Low Tech Strategies. https://plant-pest-advisory.rutgers.edu/preparing-orchards-against-the-frost-low-tech-strategies-2/