January 31, 2025
Flowering Bud Induction Advisory #4
Season Forecast: For the past several weeks we have been experiencing cold weather. The National Oceanic and Atmospheric Administration published its latest weather temperature for January, air temperatures are expected to be below normal during this period. Even though we have not observed La Niña conditions yet, these La Niña conditions are expected to persist through February to April 2025, which means Florida will experience temperatures warmer than normal and rainfall lower than normal. If the dry weather prevails during the bloom period, there could potentially be lower incidence of postbloom fruit drop.
Current Condition: Currently, all the citrus producing regions of Florida are in very high flower induction levels which means that sufficient inductive conditions have occurred to cause flower bud differentiation of economic importance. Warm conditions along with adequate soil moisture should be sufficient to sprout flowering buds. We do expect to start having warm weather in the next few days in most of the regions of Florida. So flowering is anticipated to start happening in the next 3-4 weeks, significant bloom (major wave) may not happen until first week of March. It seems that that the flowering is going to be more synchronized than what we have seen in the last couple of years. We expect to see the first wave of flowering in the first week of March in north Florida and central Florida. Currently, the flowering model predicts two waves of flower in Umatilla, March 8 and 17; two waves in Immokalee, March 14 and 26; two waves in St. Lucie area, March 14 and 25; and one wave in Central Florida, March 7. We expect concise flowering for about 4-5 weeks.
Cool weather stops growth and then promotes induction of flower buds as more cool weather accumulates. A warm spell can then initiate differentiation, which after sufficient days of warm temperatures will lead to bloom. Trees will be very vulnerable to growth stimulation by a warm period after they accumulate 300-400 hours of cool temps if soil moisture is adequate. Keep track of induction hours in your area and watch for projected warm periods from the weather services. Normal healthy trees could have their induction boosted by applying some drought stress. Unfortunately, with vulnerable root systems associated with HLB you shouldn’t risk heavier preharvest fruit drop of the current crop by using water stress to prevent unwanted early vegetative growth and enhance induction of flowers. Management of flower bud induction is limited at this time.
Now is a good time to start planning for spring fertilization, ideally, fertilize in the next couple of weeks before the trees start to flush.
Flowering related management considerations for HLB-affected trees:
- DO NOT drought stress HLB-affected trees even though drought stress promotes flower induction and suppress vegetative growth, you should not risk current crop due to additional drought stress. Drought stress can exacerbate fruit drop. Daily, lower volume irrigations to minimize fall drought stress is suggested, especially when the weather is warm.
- Flowering enhancing fertilizer to increase the number of flowers are NOT suggested for severely HLB-affected trees as they are very less likely to benefit because of 2 reasons: (1) HLB-affected trees have more dead wood therefore, there are fewer buds available to become flower, interestingly a good branch of severe HLB trees has same flowering potential as mild HLB trees. So additional flowering promoting fertilizer is not needed. (2) High twig dieback and low fruitlet retention is the main concern with severe HLB trees in regards to fruit set. Only 2% of the total flowers turn into harvestable crop therefore, pushing tree to flower more is not advisable as that is likely to waste trees’ energy and resources in extra flowers.
- Gibberellic acid (GA) sprays can be used to suppress early spring flowering but the timing of application is critical for GA to be effective. GA should be applied before warm temperatures (that is before differentiation begins). Previous research on HLB-affected trees in 2017-2018 (a La Niña winter) has shown that when GA applied monthly in fall, early flowering was suppressed. Therefore, if you have a weak crop load and are forecasted to have warm spells, GA application can be considered to suppress off season flowering.
DO NOT SPRAY GA AT THIS TIME