January 24, 2024
Flowering Bud Induction Advisory #5
Current Condition: Currently, all the citrus producing regions north, central, and southwest Florida are in very high flower induction levels which means that flower buds are fully induced, and the differentiation may have already begun. Indian river region is still in high induction level, warm conditions along with adequate soil moisture should be sufficient to sprout flowering buds. At this point no management practices can affect the flowering window or intensity, sprays to enhance or suppress flowering are useless at this point. It is a good time for growers to start preparing for pre and post flowering fertilization. Vegetative flushing should also be expected along with flowering as the weather starts warming up. It is anticipated that we will see the first wave of flowering in end of February in north Florida. Currently, the flowering model predicts two waves of flower for most the citrus producing regions, in Umatilla, February 21 and March 5; and one wave in Central Florida, March 3; two waves in Immokalee, March 2 and March 16; and one wave in Indian river region, March 8. It is expected that the flowering will be concise in most of the central Florida. In southwest Florida and east Florida, the flowering is expected to be slightly more prolonged. The weather for next two weeks is mixed with few warm days followed by cold days therefore, it is anticipated that flowering events should be pretty close to predicted dates. However, if temperatures stay above 75 F to long then flowering may happen sooner. Cool weather stops growth and then promotes induction of flower buds as more cool weather accumulates. After moderate induction, a warm spell when coincides with rainfall can initiate differentiation, which after sufficient days of warm temperatures will lead to bloom.
Some areas are already showing pockets of sporadic flowering however, these are the areas where crop load was low, and trees were injected. It is too early to speculate the effect/interaction of injections on flowering, but we continue to monitor.
Season Forecast: This is predicted to be a El Niño–Southern Oscillation (ENSO) winter, which means Florida will experience more rainfall pattern than normal. The daily temperatures are predicted to be average for this time of year however, due to higher than usual rainfall (associated overcast) slightly cooler temperatures can be predicted.
Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds, but excessive rainfall can interfere with bud induction as both low temperature and drought stress are needed for strong floral bud induction.
General Information: Cool weather stops growth and then promotes induction of flower buds as more cool weather accumulates. After moderate induction, a warm spell when coincides with rainfall can 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 (over 75 F) 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. Based on weather predictions, if you are concerned about early flowering in your region, a gibberellic acid (GA) application can prevent some early flowering.
Gibberellic Acid Application Consideration: Now it is too late to make Gibberellic Acid (GA) application with goal of suppressing flowering. DO NOT spray GA after first of January to manage flowering. GA can keep the fruit green, therefore can be a concern for fresh market fruit. The growers are highly recommended to not use GA in their groves from now until June Drop.
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.