Protecting Peaches from Spring Frost: Delaying Bloom with JA Inhibition

Introduction

Spring frost remains one of the most significant environmental threats to the productivity and economic viability of temperate fruit crops (Jahed et al. 2023). In recent years, shifting weather patterns and warm late-winter temperatures have substantially increased this issue by triggering early budbreak and early bloom (Tominaga et al. 2022; Saini et al. 2025). When peach trees bloom ahead of historical schedules, their cold-sensitive buds and flowers are exposed to late-spring freezing events for a prolonged period, often resulting in severe crop damage (Augspurger 2013), as in the complete crop loss during 2026 in Virginia and the surrounding region.

To protect orchards from frost damage, growers traditionally rely on a combination of passive and active management strategies (Jahed et al. 2023). Passive strategies involve long-term preventive measures integrated into orchard planning, such as selecting planting sites that avoid low-lying cold-air pockets or planting late-blooming, cold-hardy varieties (Liu and Sherif 2019; Jahed et al. 2023). Conversely, active strategies are emergency responses deployed just before or during a freeze to modify the orchard microclimate. These methods include operating heaters, using wind machines to pull down warmer air trapped in inversion layers, or running sprinklers that release small amounts of heat as water freezes (Pan et al. 2024).

Although active frost-protection methods can reduce injury under suitable conditions, their effectiveness depends strongly on the type and severity of the freeze. For instance, wind machines are most effective during radiation frosts, when a temperature inversion develops, and warmer air is present above the orchard canopy; their benefit is limited during advective freezes characterized by strong winds, extensive cold-air masses, and little or no inversion (Unterberger et al., 2018). Likewise, sprinkler-based protection can be effective when water is applied continuously at rates sufficient to compensate for evaporative and radiative heat losses, but inadequate application or interruption of irrigation during freezing conditions can increase tissue injury. These systems also require substantial infrastructure, energy, labor, and, in the case of sprinklers, large and reliable water supplies, which may restrict their use in water-limited regions (Balawejder et al., 2024). Heaters can provide additional protection but are generally constrained by high fuel requirements, operating costs, and limited efficiency under severe or windy freeze conditions. Together, these limitations emphasize the need for complementary, cost-effective frost-protection approaches that reduce dependence on favorable microclimatic conditions and intensive resource inputs.

One promising alternative is holding back the bloom itself, ensuring flowers open after the main threat of late frost has passed. Our research findings show that a natural plant hormone called jasmonic acid (JA) surges within peach buds right before budbreak, and appears to play an important role in triggering floral development (Jahed et al. 2025). Therefore, temporarily suppressing JA activity may provide a means of delaying bloom without adversely affecting tree growth or development. To evaluate this approach, we investigated the effects of two JA inhibitors: propyl gallate and antipyrine, on bloom progression over three consecutive growing seasons (2023–2025) utilizing two commercial peach cultivars, ‘Redhaven’ and ‘Sunhigh’.

Materials and Methods

Plant Materials, Treatments and Experimental Layout

This experiment was conducted at the Alson H. Smith Jr. Agricultural Research and Extension Center (AREC) in Winchester, VA over three consecutive growing seasons (2023–2025). We evaluated mature trees of two commercial peach varieties, ‘Redhaven’ and ‘Sunhigh,’ selecting three uniform, adjacent trees for each treatment group. Only the ‘Sunhigh’ cultivar was used in the 2025 season. Treatments consisted of foliar applications using two JA inhibitors: propyl gallate and antipyrine. Propyl gallate was applied at a rate of 2.65 pounds per acre, where antipyrine was applied at 7.8 pounds per acre in 2023, which was subsequently doubled to 15.7 pounds per acre for the 2024 and 2025 seasons. A nonionic surfactant (Regulaid) was added to all treatment mixtures as well as to the untreated control spray at a rate of 1 quart per 100 gallons of water (37.85 mL per 15 liters) to ensure thorough coverage and adherence to the buds. Applications were administered based on phenological progression, starting at the ‘Swollen Bud’ stage in 2023 (n=4 applications). In response to elevated early-spring temperatures that resulted in accelerated phenological advancement in subsequent years, applications were initiated at the ‘Calyx Green’ stage in 2024 and 2025, and the number of applications was reduced to three.

Tracking Bloom Progression

To track how the treatments affected flowering time, three branches, each containing at least 50 floral buds, were selected and tagged on each tree at the time of the first application. These buds were counted and recorded as the “initial bud count”. The number of open flowers was then recorded daily, beginning at the ‘Calyx Red’ stage. Bloom progression was assessed as the phenological transition of floral buds from the ‘swollen bud’ stage to anthesis (flower opening) in response to JA inhibitor treatments. Blooming rate was calculated as the ratio of the number of open flowers to the number of initial bud count on the tagged branches. Full bloom was defined as the stage when 50% (F50) of the counted floral buds on a branch had reached anthesis.

Evaluating Fruit Set and Quality

To assess the effects of treatments on fruit set (%) and fruit quality attributes, fruit set was recorded by counting the number of developing fruitlets on the tagged branches at two, four, and eight weeks after petal fall (WAPF), and at harvest. Fruit set was expressed as the percentage of initial floral buds, calculated according to (Jahed et al. 2025). To assess the impact of JA inhibitors on fruit quality, key attributes were evaluated at harvest, including fruit fresh weight, diameter, firmness, and total soluble solids (TSS), according to methods described by (Jahed et al. 2025).

Results and Discussion

Overall, applying JA inhibitors successfully delayed bloom in both peach varieties, though the exact magnitude of the delay depended on the specific chemical and the year’s weather. Antipyrine appears to be the most reliable treatment, consistently holding back bloom across both cultivars and years. Propyl gallate also achieved delays, but its effectiveness fluctuated more unpredictably from year to year (Figure 1).

The physical impact of these treatments is clearly visible on branches collected from the orchard. Buds treated with JA inhibitors, particularly antipyrine, remained tight and closed, whereas untreated control branches exhibited advanced floral development with prominent pink petals emerging (Figure 2).

By slowing bud development and delaying the transition to open bloom, these treatments may help reduce the period during which peach flowers are most vulnerable to spring frost. The year-to-year differences in treatment success were closely tied to early spring temperatures, which strongly influence the timing and duration of bloom. For instance, unusually warm weather in 2024 forced an early and shortened bloom. In ‘Redhaven’, untreated trees reached 50% full bloom on March 25 in 2023, but a full week earlier on March 18 in 2024. Despite this accelerated timeline, antipyrine still delayed ‘Redhaven’ flowering by four days in 2023 and three days in 2024, while propyl gallate delayed it by one and three days, respectively (Figure 1). ‘Sunhigh’, which has showy flowers (Figure 2), naturally progresses toward bloom more slowly. Across the three years, untreated ‘Sunhigh’ trees reached 50% bloom between late March and early April. Antipyrine delayed bloom by five days in 2023, two days in 2024, and three days in 2025. Propyl gallate, by contrast, produced smaller delays of two days in 2023 and one day in both 2024 and 2025 (Figure 1). While delaying bloom by two to five days may seem brief on a calendar, this window is highly significant for practical orchard management. Spring freeze events are often short and highly dependent on brief overnight temperature drops (Jahed et al. 2025, 2026). By using compounds like antipyrine to keep protective bud tissues closed for a few more days, growers may be able to shift bloom away from a damaging frost event. This short delay can potentially make the difference between substantial crop injury and a successful harvest.

To ensure that the treatments did not negatively affect crop production, fruit set was monitored throughout the season without routine fruit thinning. Overall, neither treatment had a significant negative effect on fruit set, and in some cases, fruit set was improved. In ‘Redhaven’, for example, antipyrine increased final fruit set in 2023 (Table 1), while in 2024 it performed similarly to the untreated control (Table 2). Propyl gallate increased the number of developing fruitlets early in the season in both years, but by harvest, final fruit set was similar to that of untreated trees.

In ‘Sunhigh’, early-season fruit set in 2023 was lower in treated trees than in the control, although all treatments ultimately had similarly low final crop loads. It is important to note that ‘Sunhigh’ experienced severe orchard-wide fruit drop in 2023, with nearly the entire crop lost across both treated and untreated trees. This loss was primarily associated with a late frost shortly after bloom that severely damaged newly formed fruitlets (Table 1). In 2024, however, propyl gallate increased fruit set in ‘Sunhigh’ compared with both antipyrine-treated and untreated trees (Table 2).

Table 1. Fruit set (%) at two weeks after petal fall (WAPF), 4 WAPF, 8 WAPF and harvest of ‘Redhaven’ and ‘Sunhigh’ in 2023.

Table 2. Fruit set (%) at two weeks after petal fall (WAPF), 4 WAPF, 8 WAPF and harvest of ‘Redhaven’ and ‘Sunhigh’ in 2024.

Beyond crop yield, the harvested peaches were evaluated for firmness, fresh weight, size, and sweetness to ensure the bloom-delay sprays did not harm overall fruit quality. In ‘Redhaven’, neither treatment altered the fruit’s firmness or sweetness in either year. In ‘Sunhigh’, firmness and sweetness were also unchanged in 2023, while antipyrine slightly increased fruit firmness in 2024. For fruit size and weight, propyl gallate produced larger peaches in both varieties during the 2023 season (Table 3). In 2024, antipyrine stood out for size, resulting in notably larger ‘Redhaven’ peaches compared to the untreated trees (Table 4). Overall, these results suggest that the bloom-delay treatments did not negatively affect fruit quality and, in some cases, increased fruit size or firmness at harvest.

Table 1. Effects of bloom delay agents on fruit quality attributes of ‘Redhaven’ and ‘Sunhigh’ peaches in 2023.

Table 1. Effects of bloom delay agents on fruit quality attributes of ‘Redhaven’ and ‘Sunhigh’ peaches in 2024.

Conclusion

• Applying JA inhibitors, particularly antipyrine, delayed peach bloom by approximately two to five days by slowing floral bud development.
• The extent of bloom delay varied with cultivar and seasonal weather conditions, with warmer spring temperatures accelerating bud development.
• Neither treatment negatively affected final fruit set or crop load under the conditions evaluated.
• Fruit quality was also maintained, with no consistent negative effects on firmness or sweetness. In some years, treated trees produced larger fruit than untreated controls.
• Overall, delaying bloom with JA inhibitors shows promise as a resource-efficient strategy for reducing the risk of late-spring frost injury and may complement existing frost-protection practices.

Acknowledgements

The authors gratefully acknowledge Drs. Md Tabibul Islam, Jianyang Liu, Amolpreet Kaur Saini, and Mohammad M.H. Tipu for their critical contributions to sample collection, preparation, and preservation. We also extend our sincere thanks to Mr. Gerald Michaels III and Ms. Katherine Furcho for their valuable support with field data collection, sample preparation, and tissue grinding for molecular and biochemical analyses.

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