Late summer and fall are critical periods in Florida blueberry fields. To the casual observer, there doesn’t appear to be a lot happening this time of year. However, at the cellular level, it’s an important time for flower bud initiation and development, which may begin as early as August or September and continue throughout the fall and early winter. 

Summer growth following post-harvest hedging and topping produces the new wood where most of next year’s berry crop will be produced. As plants enter late summer and fall, healthy leaves on the summer growth are needed for flower bud initiation and development to occur. Axillary buds, located at the base of the leaf petiole, begin their existence as vegetative buds but have the potential to transition into flower buds depending on plant and environmental conditions. 

Shortening day length and cooler night temperatures are the environmental conditions that promote the formation of flower buds. The process may begin as early as August on some cultivars such as “Emerald,” or later in the fall on other cultivars such as “Jewel.” The presence of healthy leaves on the fruiting wood during the flower bud initiation and development period is very important. The contrast between the timing of the beginning of flower bud initiation between “Emerald” and “Jewel” was observed at very early stages microscopically (Table 1) at the Plant Research and Education Center in Citra. The bud position is determined by counting back from the shoot terminal. For example, bud position 4 would be located at the 4th node back from the shoot terminal. Flower bud initiation begins on the most terminal buds and gradually progresses down the shoot. By early September most buds at positions 1 and 2 show some evidence of floral development for “Emerald” but not for “Jewel.” Flower bud initiation in “Jewel” began later at some time between early September and early October and rapidly continued throughout October and early November. 

Table 1. The percent of buds containing floral structures at different node positions and dates for “Emerald” and “Jewel” southern highbush blueberry in north-central Florida.  

From Kovaleski et al. (2015) J. Am. Soc. Hort. Sci. 140:38-44. 

In a field study, leaves were removed from shoots at intervals throughout the fall. The removal of leaves in early September and early October reduced flower bud number and leaf removal in early September, early October, and early November reduced the number of fruit (Figure 1 and Table 2). Defoliation in early September and early October also reduced the number of fruit produced per flower bud (data not shown). The study demonstrated that the effects of leaf removal on flower bud development were local. Defoliation on a given shoot had a negative effect on flower bud initiation and development on that shoot, but not necessarily on adjacent shoots that had full complements of leaves. While this study was done with “Misty,” which is now an obsolete cultivar in Florida, the defoliation response described has also been shown in rabbiteye cultivars and is believed to be a common response across most blueberry cultivars.  

Figure 1. Leaves removed from a blueberry shoot in September (left) versus leaves not removed from a shoot (right) show the importance of leaves on blueberry flower bud development in the fall. 

Table 2. Fall defoliation affects flower and fruit number of SHB. 

From Williamson and Miller. 2002. HortTechnology. 12:214-216. 

The effects of daylength and temperature, and how daylength and temperature interact to affect flower bud initiation in blueberry was studied using growth chambers where the photoperiod and temperature could be controlled independently of one another. The findings showed that the best combination for flower bud initiation was short photoperiods combined with cool temperatures (Table 3). When plants were exposed to high temperatures (~82 F), blueberry flower bud initiation was impaired, even under short day conditions. In this study short days were 8 hours of light followed by 16 hours of darkness. For the purposes of this experiment, the short day + night interrupt treatment can be thought of as long day conditions. Although there is little that can be done to reduce temperatures in a blueberry field, this relationship between photoperiod and temperature is nonetheless interesting. 

Table 3. Effect of temperature and photoperiod on flower bud number per plant 

From Spann et al., 2003. HortScience 38:192-195. 

Leaf retention throughout the summer and during flower bud initiation (and beyond) is even more critical in the evergreen production system that is commonly used in central Florida and exclusively used in south-central Florida. An intact and healthy canopy of foliage is essential not only for good floral bud set, but also to help support an early harvest, which is important with Florida’s short market window. The impact of this was seen recently in the aftermath of Hurricane Ian in south-central Florida, which significantly defoliated much of the blueberry acreage in September, just as floral bud initiation was getting ready to take place in many cultivars. Between the defoliation and other hurricane-related damage, the yield in that area was reduced by as much as 80% over the prior year. 

To summarize, healthy leaves are very important for flower bud initiation and development needed for next year’s crop. In the fall, leaves play a critical role in determining flower bud initiation and development which can have a direct effect on berry yield. When growing under the evergreen production system, clean healthy foliage is also critical to support the early developing crop. 

Literature Cited 

Kovaleski, A. P., J.G. Williamson, J.W. Olmstead, and R.L. Darnell. (2015) Am. Soc. Hort. Sci. 38-44. 2015. Inflorescence bud initiation, development and bloom in two southern highbush blueberry cultivars. J. Am. Soc. Hort. Sci. 140: 38-44. 

Spann, T.M., J. G. Williamson, and R.L. Darnell. 2003. Photoperiodic effects on vegetative and reproductive growth of Vaccinium darrowii and V. corymbosum interspecific hybrids.  HortScience 38:192-195. 

Williamson, J.G. and E.P. Miller. 2002. Early and mid-fall defoliation reduces flower bud number and yield of southern highbush blueberry. HortTechnology. 12:214-216. 

CREDITS:

JEFF WILLIAMSON, Professor, Horticultural Sciences Department, UF/IFAS
& DOUG PHILLIPS, Blueberry Extension Coordinator, Horticultural Sciences Department, UF/IFAS