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Flower Bud Induction Overview and Advisory

The following information has been developed as part of the Decision Information System for Citrus. (http://disc.ifas.ufl.edu/bloom)

Dr. Tripti Vashisth, Horticulturist
Citrus Research & Education Center, Lake Alfred, FL

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FLOWER BUD INDUCTION ADVISORY #1 for 2014-2015-11/5/15

The following information has been developed as part of the Decision Information System for Citrus. 
L. Gene Albrigo, Horticulturist Emeritus
Citrus Research & Education Center, Lake Alfred, FL 

This is a service to our citrus growers posted on the CREC website. The indicated Expert System on intensity and time of bloom can be accessed at the designated Web Site: http://disc.ifas.ufl.edu/bloom. If you are not familiar with the website and flower bud induction in citrus you should read the overview section below the current status paragraph.

Flowering related to the current 2014 -15 Crop – There was a light but consistent bloom centered in January that must have been a water stress bloom initiated in November due to the effect of HLB on root systems.  There was a flowering cohort toward the end of February after nearly 900 hours of inductive temperature induction (< 68 degrees F).  Remnants of the January bloom overlapped with the beginning of this cohort.  A second regular cohort occurred toward the end of March after more than 1200 hours of induction had occurred, resulting in an extended bloom.  The January bloom with associated rains may have been the inoculum source for extensive PFD seen in the regular bloom since little PFD has been seen the past few years.  In spite of PFD, NASS predicted a larger crop for this harvest season than the previous year supporting the effect of the high induction levels.

Flower bud induction status 2014-15 - his is supposed to be a weak ENSO winter with more cool temperature accumulation and rainfall than last year.  Currently, citrus locations have accumulated low, inductive temperatures, < 68 degrees F, of nearly 300 to 400 hours from southern to northern areas, respectively. The next 7 days will be intermediate for cool temperature accumulation with about 70 to 110 hours, south to north. Continued accumulation of cool temperatures and prevention of growth during a winter warm spell is very important for good 2015-16 citrus production.  Another 4+ weeks with 80 to 100 hours/week od inductive temperatures will give the trees a low economic level of flowering.
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 (see later section on use of drought stress).  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. The next advisory will be after December 2nd unless a warm period is predicted or other unusual events occur.   Remember drought stress adds to flower bud induction, but it also increases drop of the current crop.

Overview of flower bud induction in Florida – Citrus flower bud induction starts in the fall and usually is completed by early January. Low temperatures first stop growth and then promote induction of flower buds as more hours of low temperatures accumulate (below 68 degrees F, 19 0C). Periods of high temperatures in winter can then initiate bud differentiation which after sufficient days of warm winter-springtime temperatures leads to bloom. The meteorologists predict that this winter in Florida will be a weak ENSO year, below average temperatures and higher than average rainfall. Under these conditions, enough hours of low temperatures below 68 degrees F. will usually accumulate to induce an economic level of flower buds, but intermediate warm periods during the winter can lead to multiple flower cohorts and a very prolonged bloom.  Other conditions that can interfere with good flower bud induction include: 1) exceptionally high previous crop or 2) excessive leaf loss from hurricanes, freezes or other causes (canker, HLB). Excessive leaf loss leads to low carbohydrate levels in developing buds which reduces their ability to become flower buds. None of the adverse climatic conditions appear to be in play for the coming season’s flower bud induction. The biggest concern may be reduced available carbohydrates because of HLB.  

Under normal Florida weather conditions but with a moderate to heavy previous crop, sufficient flower bud induction should be achieved when total accumulated hours of low temperatures exceed 800 hours below 68 degrees F. If the crop load is light, sufficient flower bud induction may occur after 700-750 hours of accumulated low temperatures. A warm period of 7 to 12 days, with maximum temperatures from 80 to 85 degrees F., can trigger growth (bud swelling) if a minimum total hours of low temperatures have accumulated (300-400 hours below 68 degrees F). Later in the winter when the accumulated cool temperature induction hours are higher, fewer days and lower daytime highs (75 degrees F.) are required in a warm period to stimulate growth of buds. Weather information relative to Florida citrus flower bud development for the current and several previous years (back to 1998) can be obtained from the Florida Automated Weather System (fawn.ifas.ufl.edu) for locations near you. An 7 day forecast from the National Weather Service predicts Florida weather for several sites around the citrus belt for the next week. Find this information at: http://www.nws.noaa.gov/mdl/forecast/text/state/FL.MRF.htm. This is an easy way to see if a warm period, which could trigger flower bud growth, is predicted for your specific area in Florida.

Some flower buds will be induced in the range of 300 to 450 accumulated hours < 68 degrees F. Warm events after these levels of induction result in weak flowering intensity, and therefore many buds remain that can be induced by later cool periods, or these buds may sprout as vegetative shoots if warm weather continues and the trees are well irrigated. The first situation results in multiple cohorts of flower buds developing to different bloom dates. The second condition leads to low flowering-fruit set and excessive early spring vegetative growth. During the years from 1963 to 2003, multiple blooms occurred in over half of the years. Historically, the time period in which an early warm period (7-12 day) can lead to an initial low number of buds growing and flowering is roughly mid-November to early-December. Then after more cool temperatures additional flower buds are induced and a later warm period starts their growth. This pattern can repeat itself, often leading to 3 blooms in the same season. 

Presently, the only management tools available to eliminate or reduce the chance of multiple blooms are sufficient drought stress to stop growth or a timely gibberellin (GA) spray just before initiation of the first wave of flower bud growth. Water stress may be provided by stopping irrigation well before these predicted warm periods occur. If the warm periods(s) are of the typical 7 to 10 day duration, a coincident short period of drought stress will have little impact on current crop development or fruit quality in healthy trees. Sufficient drought stress may be interpreted as leaf wilt observed by 10 or 11 am, but leaves recovering by early the next morning. If no rains interrupt a drought stress condition in citrus trees, buds will not grow in response to high temperatures. If a warm period has passed, trees again can be irrigated to minimize current crop stress. Although no weather prediction is guaranteed, rains in the winter usually come on the fronts for cool periods. Sufficiently cool temperatures after a cold front rain will usually prevent growth even though soil moisture is adequate for growth. Since winter rains usually occur just before cool temperatures, the chances that drought stress will prevent an early flower bud differentiation event are reasonably good for many warm periods. Even so, growers in some growing districts have often found it difficult to maintain winter drought stress. 

In the shallow soils of bedded groves, it is relatively easy to create sufficient water stress to suppress growth by withholding irrigation for a few days if no rains occur. In deeper, sandy soils, 2 or more weeks without irrigation or rainfall may be required. To minimize the time required for soil to dry sufficiently to initiate water stress, the soil should be allowed to dry out by mid-November so that trees show wilt by mid-day. For bedded groves, minimum irrigation can then be applied at low rates as needed until a weather prediction indicates a warm period is expected. At this time, irrigation should be shut down. For deep sands, the soil needs to be dried out and kept nearly dry below 6 to 8 inches of depth until at least Christmas so that no growth can occur. Minimum irrigations that re-wet perhaps the top 6 to 8 inches of the root zone may minimize excessive drought, while allowing quick return to a water stress condition if a high temperature period is forecast. Soil moisture monitoring can help to achieve these goals. Prolonged late-fall, early-winter drought may be risky for ‘Hamlin’ or other early maturing cultivars not yet harvested that tend to drop fruit near harvest. In recent studies, Valencia trees in Central Florida have had good flowering and no apparent impact on current crop when irrigation was stopped in early December and resumed in the spring. Now in the face of HLB and related preharvest drop, it may not be a good idea to allow trees to become drought stressed.

Application of a GA spray is an alternative.  GA will reverse induction and knock out a weak first flower initiation, but it has to be applied just before or as the warm period starts.  If induction level is above 600 hours the spray will not completely stop all of the flowering, but a more concentrated flowering should occur after the second warm period.  The GA application will work better if the warm period occurs after only 400 to 450 hours of cool temperatures.
Much of what has been stated above has now been incorporated into a ‘Flowering Expert System for Florida Citrus’. Figure 1 represents the different aspects of flower induction as depicted by the software program. The program gives an average bloom situation represented by the shades of green to white, vegetative to heavy flowering, respectively. If the current crop is very heavy, then more cool induction is needed to compensate for the crop load effect. If the current crop is lighter or tree condition better, then fewer total cool temperature hours are needed for an equal level of flowering. Recommendations (bottom text) do consider the current crop level in assessing when action should be taken to try to prevent or to promote initiation of the flower bud growth process. The recommended actions are for healthy trees and may poorly apply to HLB affected trees, today’s typical Florida citrus tree. The system is available on-line: http://disc.ifas.ufl.edu/bloom . The on-line version is in black and white and does not list the accumulated hours of cool temperatures or the predicted bloom date.  

You must interpret those from the intersections with the graph axis. An improved on-line version is being developed this season.

Additional advisories will follow this preliminary one, roughly bi-weekly, and update the reader on accumulating hours of related cool or warm temperatures and other weather effects on flower bud induction. Methods for enhancing (urea or PO3 sprays) or reducing (GA3 sprays) flowering intensity as conditions and cultivars dictate will be discussed in later advisories. Read the archived advisories from previous years (link at top of this page) for more background.

See a previous background introduction for previous important yield responses to cool temperatures: FLOWER BUD INDUCTION ADVISORY #1 for 2012-2013. 


FLOWER BUD INDUCTION ADVISORY #2 for 2014-2015-12/03/14

The following information has been developed as part of the Decision Information System for Citrus. 
Tripti Vashisth, Horticulturist Emeritus
Citrus Research & Education Center, Lake Alfred, FL

This is a service to our citrus growers posted on the CREC website.  The internet Expert System on intensity and time of bloom can be accessed anytime: http://orb.at.ufl.edu/DISC/bloom  This website was temporarily down but is running now.

Current Status: The projection is for a weak ENSO winter with slightly above average cool temperature accumulation.  The accumulated hours below the threshold for induction, <68° F, through December 1 were <530 to 730 from southern to northern citrus areas.  Another 35 to 75 hours are predicted for the next week. The minimum hours in southern areas will be about 250 hours less than the desired 850 after next week. So far, the Indian River has a few more inductive hours than Immokalee this year.

In order to improve the induction level beyond a minimum, trees need to remain at rest at least through Christmas.  Cool temperature accumulation or drought stress can increase induction levels. Three more weeks of cool temperature induction may add another 250-300 hours, which would bring the East Coast growing areas to near 850 hours, an acceptable level of flowering for an economic crop.  A level of 900 hours or more should be reached in most growing areas north of Sebring in three more weeks if cooler temperatures persist. 

In Central to southern growing areas daytime high temperatures are projected to be in the low-80 degree range for the next 5 days, If temperatures are higher than this or high temperatures continue for more days, shoot terminal buds may initiate growth at the current induction levels. Induction levels are now high enough that a warm period will easily initiate bud growth in the 1st and 2nd terminal buds.   At a minimum level of bud induction when bud growth starts, the flower buds will have few flowers and more leaves.

Because of preharvest fruit drop associated with drought stress, an alternative is to enhance flower bud induction with a stress inducing spray of urea or PO3 at the beginning of a winter warm period after more than 600 hours of cool temperatures accumulate.

This can boost flowering levels as if additional cool temperatures had occurred.  A flower bud induction enhancement spray of urea or a phosphorous acid product sprayed during the early part of the  warm period probably will be effective. Growers can consider applying either 53 to 60 lbs of foliar urea/acre or a PO3 product at 3 pints to 2 quarts per acre depending on which product is used (60 % P (3pts) or if 26 % P (2 qts)).  The chosen material should be applied in 80 to 125 gal of water early in a warm period. These products apparently increase the stress level and enhance the amount of flowering induced by the cool temperatures. We have not tested these sprays on HLB affected trees, but these trees, if not severely declined, may also respond.  A downside of this is the additional cost with productions costs already very high due to HLB related treatments.

If cool temperatures continue for 3 weeks, flower enhancing sprays may not be needed even in southern areas.  Trees with a weak root systems (all HLB trees) may express water stress which would enhance flowering.  On the other hand weak root systems from HLB could also be a problem. Poor production of cytokinins by roots in the spring minimizes final flower development.  Unfortunately our knowledge of how HLB alters tree physiology is very limited and we can only guess at much of the response.

Moderate drought stress increases cold hardiness on healthy trees, also increases flower bud induction and prevents bud growth in warm weather. Flowers are visible on some limbs in trees indicating that at least some parts of trees are stressed.  This is not as heavy as last year’s January bloom. The HLB associated drought stress is likely to increase preharvest fruit drop now, particularly in Hamlins and early mandarins with HLB, as long as rain frequency is not steady.

Don’t forget that winter freezes occur most often between Christmas and 15 January. However, an El Nino year is less likely to have freeze events.  Watch the weather for warm periods and freeze potential.


FLOWER BUD INDUCTION ADVISORY #3 for 2014-2015-12/17/14

This is a service to our citrus growers posted on the CREC website. The indicated Expert System on intensity and time of bloom can be accessed at the designated Web Site: http://disc.ifas.ufl.edu/bloom

Current Status:  Within 4 days cool temperature induction will have reached 820 to 950 hours below 68° F for southern to central citrus producing regions in Florida.

With the higher induction levels that are now present, the buds will begin to grow if daytime high temperatures reach 80°F for just a few continuous days. Such conditions are predicted for South Florida citrus areas.  The Indian River and  Central Florida will be border-line for growth initiation temperatures.

Since cool temperatures have reached these levels, flower enhancing sprays may not be needed, but one of the chemical sales companies correctly pointed out that enhancing flowering intensity will reduce the possibility of multiple blooms.  The advantage of this is to shorten the time when harsher psyllid sprays can’t be applied and reduce the chance of another PFD event this season and the number of sprays that are needed if PFD does occur.  

See the last advisory for flower enhancement spray information.  If you choose to apply either urea and/or PO3 you should do so immediately.  The recommendations are for one or the other.  We have not data on combining the two products at some reduced rate.


 FLOWER BUD INDUCTION ADVISORY #4 for 2014-2015-1/14/15

This is a service to our citrus growers posted on the CREC website.  The internet Expert System on intensity and time of bloom can be accessed anytime: http://disc.ifas.ufl.edu/bloom.   

If you are not familiar with the website and flower bud induction in citrus you should read the overview section in the first advisory this year.

Current Status: All Florida Citrus Districts have initiation of bud growth from cool temperature induction plus sufficient growth temperatures.  The projected full bloom date for the cool temperature induced buds is February 15 to 20.

However, most trees in Central Florida already have flower buds with pin-head flowers and some at popcorn stage.  Vegetative buds are also breaking. The most advanced trees are weaker Hamlin that have already been harvested. Valencia trees and stronger trees have less bud break.

These buds are too advanced to be the projected mid-February bloom.  It is most likely that this bud break is primarily the stress bloom associated with drought stress that HLB trees are under.  On the weaker, harvested Hamlin trees this cohort of buds that is now developing on some trees is quite extensive.  Similar development has been reported from the Arcadia area.  I believe it is the general state of the trees throughout Florida and similar to last year’s early January stress bloom, but 2 or 3 weeks later, perhaps related to the extended rainy period this past fall. 

If we have a late January and a mid-February flowering cohort, another extended bloom period with greater potential for PFD development should be watched carefully.  Have you checked your flower bud status?