By Tripti Vashisth and Mercy Olmstead
Figure 1: Chill-unit accumulation in Florida (below 45 F through Feb. 10) with low-chill peach/nectarine (N) cultivar options.
Credit: Courtesy of the Florida State Historical Society and Mercy Olmstead, UF.
Interest in Florida peach production remains steady, with approximately 2,000 acres in the state. Florida peach growers have a number of advantages:
1) Early flowering and fruit set result in the ability to harvest fruit earlier in the domestic market window, yielding higher economic returns.
2) Recent surveys show that consumers prefer local produce, making early Florida peaches desirable in comparison to peaches imported from Latin America.
3) Peach breeding programs led by University of Florida and Texas A&M have resulted in a number of low-chill peach varieties which are suitable to be grown in the southeastern United States.
CHILL-UNIT ACCUMULATION
Peach trees are deciduous and enter dormancy. This means the trees shed their leaves during the late fall and early winter. During this dormant stage, a certain amount of cold weather (measured by accumulation of chill units) is needed to resume normal growth in the spring.
Low-chill peach varieties require fewer chill units compared to varieties grown in more northern states, making them suitable for mild winter regions of the Southeast. Therefore, selecting the right peach cultivar is very important. Peach varieties that do not accumulate the required chill-unit accumulation can result in poor or uneven budbreak, sporadic flowering, delayed leaf emergence and poor fruit set. Figure 1 shows the chill accumulation range in Florida and potential peach variety options for those regions. Fawn.ifas.ufl.edu and Agroclimate.org are the most common websites to monitor chill-unit accumulation in Florida.
Figure 2: Chill-unit accumulation for the current season, last season and the historic average in Polk County, Florida. In 2016 (shown by turquoise line), accumulated chill hours were less than 50 for the entire season. The figure is directly adapted from http://agroclimate.org/tools/Chill-Hours-Calculator.
There are several models used to calculate the accumulated chill units. The most common model, Weinberger (1950), adds the total number of hours below 45 F during the fall and winter months. The range of time used for chill-unit accumulation calculation takes into account the time period from defoliation to first bud swell, which for Florida is typically Oct. 1 to Feb. 10.
Even with several low-chill varieties available to Florida growers, mild winter temperatures and climate variability challenge Florida peach production. In 2016, Central Florida accumulated less than 50 chill units by the end of January (Figure 2), which is less than half of the historic average at any given time. Mild winters interrupt the onset of dormancy and chill-unit accumulation, causing extended bloom periods, non-uniform flowering and leaf budbreak. Both extended bloom and non-uniform budbreak can cause loss of marketable fruit and non-uniform ripening, creating labor and economic challenges.
In growing areas with low chill-unit accumulation, hydrogen cyanamide (HCN) has been used to aid in the process of overcoming chill-unit requirements. Several reports on grape, blueberry, kiwi, apple and peach indicate that when inadequate chilling is received during the dormant season, the use of HCN is effective in releasing dormancy and enhancing uniform budbreak. HCN works best when a significant amount of chill units have already accumulated. However, if applied after bud swell, phytotoxicity can occur, damaging the flower buds.
TRIAL SEES SUCCESS
In order to ensure that HCN works well for low-chill peach cultivars under Florida conditions, we set up a trial at the University of Florida Plant Science Research and Education Unit in Citra (Marion County) with the peach cultivar TropicBeauty. We applied HCN at a rate of 1.2 percent active ingredient and monitored the trees for floral and vegetative growth.
Trees treated with HCN broke bud approximately one-month earlier than the control, and floral budbreak was uniform and compressed (Figure 3). The fruit from HCN-treated trees ripened uniformly and were of marketable quality. The overall yield was higher from untreated (control) trees as there was a prolonged bloom with fruit set throughout the spring; however, these fruits were of low quality and unmarketable. Overall, this trial suggests that HCN can be successfully used in Florida for uniform budbreak and to compensate for insufficient chill units.
Figure 3: Floral budbreak in hydrogen cyanamide-treated and untreated control trees over 40 days after hydrogen cyanamide application in TropicBeauty peach.
GETTING APPLICATION TIMING RIGHT
When using HCN as a management tool, it is critical to apply it at the right time, as some amount of chill units are required for good efficacy. Late application may result in phytotoxicity and can cause flower and vegetative bud damage.
Pollen grain color has been initially suggested as an indicator of when to apply HCN (aces.edu/dept/peaches/chillcom20jan.html). Therefore, in our trial, we monitored flower bud pollen grain color. We found pollen grains ranging in color from translucent white to bright opaque yellow on the tree at any given time in late December.
Figure 3: Vegetative budbreak in hydrogen cyanamide-treated and untreated control trees over 40 days after hydrogen cyanamide application in TropicBeauty peach.
The application was made when the majority of flower buds contained translucent pollen grains. When pollen grains are translucent and not opaque, HCN has been successfully applied with positive effects on budbreak and uniform blooming. However, as the pollen grain color changes to yellow, the application of HCN should be avoided as buds may be too advanced in maturity.
HCN is highly toxic with severe side effects if proper protection and caution is not exercised. Read and follow the label carefully.
Tripti Vashisth is an assistant professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred. Mercy Olmstead is a former University of Florida associate professor and Extension specialist.
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