November 21, 2022
Flowering Bud Induction Advisory #1
Season Forecast: This is going to be a La Niña winter, which means Florida will experience temperatures warmer than normal. Rainfall pattern, though, is not very clear but it is expected to be below average for at least until the end of the year. Basically, it is going to warmer and drier for the next few weeks (at least)! The precipitation pattern for February-March is still unclear!
Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds but intermediate warm periods during the winter can lead to multiple flower cohorts and a prolonged bloom.
Current Condition: Currently, a majority of the citrus producing regions have low flower bud induction. So far Ummatila has accumulated about 400 IH and the rest of the areas surrounding Lake Alfred, Immokalee, and Indian River are still a 300 IH which means low bud induction. It is still early in the induction. The next 10 days will be intermediate for cool temperature accumulation with about 100 to 170 IH in most of the citrus producing area. More IH in the next couple of weeks will be good and will push the trees toward low-moderate flower bud induction period, which is good and needed.
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. 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 do not expect to see a significant early flowering to happen though I expect to see 2-3 cohorts of flowering. The situation will become clearer in the next couple of weeks as we head into cooler temperatures.
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.
Gibberellic Acid Application Consideration: 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. For example, this year so far we are have had low bud induction. Even within the next 10-14 days we are expected to be marginally in a moderate flower bud induction period, therefore, under such situation growers should wait for a week or so to apply GA (in order to suppress flowering). Nevertheless, multiple GA sprays in Fall improves the chance of influencing 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.
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.