December 19, 2022
Flowering Bud Induction Advisory #3
Current Condition (as of December 19, 2022): Currently, around the ridge and north of the ridge, the citrus producing regions have accumulated a good amount of induction hours (IH) leading these regions into moderate to high flower induction (above 600 IH). This means that a warm spell can result in the differentiation of buds, thus resulting in a significant wave of flowering. If warm weather prevails in January-February (as expected as per weather predictions), the first wave of flowering will occur in late February in this region. The citrus producing regions south of the ridge and east coast are still in low to moderate flower bud induction (around 500 IH). The next 10 days will bring cooler temperature’s resulting in accumulation of about 150 IH or more in most of the citrus-producing area. More IH in the next couple of weeks will be good and will push the trees toward moderate-high flower bud induction period, possibly leading to the first wave of flowering around late February (warm-weather depending).
This year flowering can be unpredictable not only because of the upcoming weather conditions but also due to the effects of Hurricanes Ian and Nicole. In many areas, trees can be seen off-season flowering, however, these flowering events should not be of economic significance. Our trees lost a significant amount of leaves and fruit and endured high speed wind and rain, thus, increasing the stress level on trees. Generally, fruit presence on the tree interrupts flower bud induction, therefore, with fruit loss one can expect to see early flowering. However, the significant leaf loss that has happened in last month also plays a major role in upcoming flowering. Due to significant leaf loss most of the trees are flushing, therefore utilizing the carbohydrate resources thus reduction in carbohydrate level may reduce overall flowering. Under these circumstances and the current weather condition I expect to see 2-3 cohorts of flowering starting mid-February.
Gibberellic Acid Application Consideration Update: Gibberellic acid sprays can be used to suppress early spring flowering but the timing of application is critical for GA to be effective. GA (to suppress flowering) should be applied before warm temperatures (that is before differentiation begins). Previous research on HLB-affected trees 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.
GA induced suppression of flowering benefits HLB-affected trees by preserving carbohydrates that can be later used by growing fruit and tree. Therefore, growers who are considering GA application to suppress flowering should carefully choose time to spray. GA application should be targeted at moderate-high flower bud induction period to get most effect. A late GA application might not be useful either. As a general rule of thumb in most of the citrus producing regions of Florida moderate to high flower bud induction should be by 2-3rd week in December.
*As per current weather conditions and predictions (as of December 19, 2022), growers who are interested in suppressing early flowering and reducing the intensity of flowering, should consider a GA spraythe in next 7 days for central Florida and 7-10 days southwest Florida (when the trees are predicted to be in moderate-high flower bud induction). It is advised to regularly check the flower bud induction model to get a better idea of existing conditions for your region.*
DO NOT spray GA after the first of January to manage flowering. Once the trees ar in high induction level, the benefit from GA application will be marginal.
GA can keep the fruit green, therefore can be a concern for fresh market fruit.
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 it 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 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.
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 the 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.