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Flower Bud Induction Overview and Advisory

NOTICE FOR CITRUS EXTENSION AGENTS & SPECIALISTS AND GROWER NEWSLETTERS
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

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11/16/2001

FLOWER BUD INDUCTION ADVISORY #1 for 2001-2002-11/16/01

NOTICE FOR CITRUS EXTENSION AGENTS & SPECIALISTS AND GROWER NEWSLETTERS 
The following information has been developed as part of the Decision Information System for Citrus. 
L. Gene Albrigo, Horticulturist
Citrus Research & Education Center, Lake Alfred, FL 

It is the time of year to start following citrus flower bud induction conditions for the coming year's bloom. 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 leads to bloom. The meteorologists say that this winter in Florida will be cooler than normal, an El Nino year. Potentially sufficient cool temperatures should accumulate, below 68 degrees F., to induce adequate flower buds for an economic crop. Sufficient flower bud induction under Florida conditions is achieved when total accumulated cool hours of 850 to 1000 hours below 68 degrees F. occurs without interruption before a warm spell triggers growth, i.e.., 7 to 12 days with max. temperatures > 70 to 75 degrees F. So far this year sufficient cool weather has occurred to slow down or stop vegetative growth on mature trees, 160 to 190.hr < 65 degrees F. in southern districts and 230 to 250 hr < 65 degree F. in northern districts. This information is available on the Florida Automated Weather System (fawn.ifas.ufl.edu).

The major concern for the next 45 days is the possibility of an early warm spell that will initiate differentiation of easily induced flower buds or push vegetative buds to grow. Some flower buds will be induced in the range of 300 to 600 accumulated hr < 68 degrees F. These early warm events therefore result in many buds remaining that can be induced by later cool spells and multiple blooms will occur. If winter bud break is not prevented in Florida, multiple blooms occur in about half of the years. The early time period in which some bud growth can occur and lead to multiple blooms is roughly Thanksgiving to Christmas. Presently, the only management tool available to eliminate or reduce the chance of multiple blooms is to allow water stress to develop during this time period.

If no rains interrupt a mild stress condition of the citrus tree, buds will not grow in response to warm temperatures. These warm spells in the late fall-early winter usually last less than two weeks. A warm spell is predicted for the Thanksgiving week ( www.crec.ifas.ufl.edu, click on weather links and 8 day forecast). After a warm spell has passed, trees again can be watered to minimize fruit water stress. Although no weather prediction is guaranteed even by the meteorologist, rains in the winter usually come on the fronts of cool spells. Therefore, the chances of applying water stress to prevent an early flower bud differentiation is reasonably good for some warm spells.

With the shallow soils in bedded groves, it is relatively easy to reach sufficient stress to suppress growth by withholding irrigation for a few days. In deeper sandy soils, 2 or more weeks may be required. To minimize the time to initiate water stress, the soil should be allowed to dry out in late fall so that trees show wilt by mid-day. For bedded groves, minimum irrigation can then be applied as needed until a weather prediction indicates a warm spell is expected. At this time irrigation should be shut down. For deep sands, the soil needs to be dried out and left that way until at least Christmas so that no growth can occur. This may be risky for Hamlin or other early maturing cultivars that tend to drop fruit near harvest.

Much of what has been stated above has now been incorporated into a ‘Flowering Expert System for Florida Citrus'. This system will be available for testing with a limited number of growers this winter. A discussion of this system will occur in a seminar at the Citrus Research and Education Center at 1:30 on Monday, November 19th. The background data and system will be presented as an exit seminar by PhD student Juan Valiente.

Future advisories will update accumulating weather effects on flower bud induction and methods for enhancing or reducing flowering intensity as conditions and cultivars dictate.

If you have any questions, please contact me (albrigo@ufl.edu


11/24/2001

FLOWER BUD INDUCTION ADVISORY #2 for 2001-2002-11/24/01

NOTICE FOR CITRUS EXTENSION AGENTS & SPECIALISTS AND GROWER NEWSLETTERS 

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

See the previous advisory for background information. 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 leads to bloom. In the first advisory, I mistakenly stated that this winter in Florida would be cooler than normal, an El Nino year. It is in fact supposed to be a neutral year. By January sufficient cool temperatures may accumulate to induce adequate flower buds, but so far temperatures have been moderately warm. We are a long way from a total accumulation of 850 to 1000 hours below 68 degrees F. so that sufficient bloom will occur to result in an economic crop. So far this year cool weather hours have only reached 230 to 270 hr < 65 degrees F. in southern districts and 315 to 330 hr < 65 degree F. in northern districts. These cool temperature hr accumulations are at least 120 hr less than last year by the same calendar date. This information can be accessed on the Florida Automated Weather System (fawn.ifas.ufl.edu). Of more concern is the continued accumulation of temperatures with daily maxima of near to > 80 degrees F. With continued irrigation these temperatures may stimulate a fall flush of growth. Any buds that sprout now are lost as potential flower buds for induction later in the winter.

The major concern for the next 40 to 45 days continues to be the possibility of this early warm spell initiating differentiation of easily induced flower buds or pushing vegetative buds to grow. Some flower buds will be induced after 300 to 600 hr < 68 degrees F accumulate. A warm spell can then result in a weak flowering event leaving many buds that can be induced by later cool spells and multiple blooms will occur. In Florida, without winter growth control, multiple blooms occur in about half of the years. The critical time period in which bud growth starts too early is Thanksgiving to Christmas. The only practical way to reduce the chance of multiple blooms is to allow water stress to develop during this time period.

Citrus buds will not grow in response to warm temperatures if sufficient water stress is present. Warm spells in the late fall-early winter usually last less than two weeks. However, the current warm spell is predicted to last for another 5 to 6 days (( www.crec.ifas.ufl.edu), to see predicted weather click on weather links and 8 day forecast). Mature, well watered trees with light to moderate crops may push a fall flush of growth. Younger trees are likely pushing a vegetative flush of growth.

With the shallow soils in bedded groves, it is relatively easy to reach sufficient stress to suppress growth by withholding irrigation for a few days. In deeper sandy soils, 2 or more weeks may be required. To minimize the time to initiate water stress, the soil should be allowed to dry out to the wilting point in late fall. For bedded groves, minimum irrigation then should be applied until the weather is predicted to turn warm. At this time irrigation should be shut down giving 6 to 7 days for water stress to develop. For deep sands, the soil needs to be dried out and left that way until at least Christmas so that no growth can occur. This may be risky for Hamlin or other cultivars that easily drop fruit near harvest. After a warm spell has passed, trees again can be lightly irrigated to minimize fruit water stress and the chance of fruit drop. Until the current warm spell ends, the best procedure to increase flowering is to hold the trees in a low level of water stress. Water stress is not favored as a flower induction procedure for Florida since more than 40 days of stress are needed to reach a moderate level of flowering. Short periods of water stress to prevent early bud sprout do appear feasible however and some growers have used this procedure successfully in previous years.

Since this winter may have relatively low accumulated hours below 68 degrees F, foliar urea applied just before a Christmas to New Year warm spell may be beneficial to enhance flower bud induction. We will discuss this treatment and update flower bud induction levels next week.

Much of what has been stated above has now been incorporated into a 'Flowering Expert System for Florida Citrus'. This system will be available for testing with a limited number of growers this winter. We still need some cooperators for testing this system. Contact Gene Albrigo (albrigo@lal.ifas.ufl.edu) if you are interested. A training session will occur in one or two weeks

If you have any questions, please contact me (albrigo@ufl.edu).


12/2/2001

FLOWER BUD INDUCTION ADVISORY #3 for 2001-2002-12/2/01

The continued warm winter is adversely affecting flower bud induction (FBI) for next years crop. Little cool temperatures for FBI have occurred and warm temperatures are predicted for the next week also. Even the little accumulated cool temperatures (about 400 hr) may be of little value since daily maximum temperatures have been above 80o F almost every day. In many groves where irrigation has continued at application rates recommended to prevent water stress, buds are pushing for an early Winter flush. This situation further reduces potential for next year's crop as those buds could have become flower buds later in the Winter. Some grower's have shut down irrigation. Water stress can prevent growth during a warm spell and sufficient drought stress can induce flower buds. This process, however, appears to require 35 to 45 days of water stress to provide a reasonable level of flowering. For Hamlin trees with fruit still on the tree, allowing excessive water stress to develop is risky as excessive fruit drop may occur and photosynthesis is reduced which limits sugars to increase fruit Brix. If and when cool temperatures do occur, most daily temperatures below 69o F, flower bud induction can proceed from cold induction conditions adding to any water stress effects on FBI. Irrigating during cold periods will not stimulate bud-break, but it may be advisable to limit irrigation so that water stress can easily be reintroduced. Trees may require water stress again after some cool weather occurs if the cool period is insufficient or the warm period occurs too early and will result in flower bud differentiation (growth) leading to an early bloom of processing oranges.

Conditions are not favorable for applying a winter urea or phosphorous acid spray to enhance FBI. These sprays are not effective to enhance FBI until some natural induction has occurred. Our best results have occurred when foliar sprays of these materials were applied after at least 800 -900 hours below 69o F had accumulated. As stated earlier, accumulation of these lower temperatures up to now may need to be discounted due to high daytime temperatures.

An early bloom in trees with fruit intended for processing may lead to early maturation, with low Brix and low acidity as occurred this season. A possible way to avoid this may be to apply water stress during any early January warm spell in order to delay initiation of differentiation until late January. This idea merits examination if conditions warrant doing so this season. Continued lack of low cool temperatures for induction may dictate using water stress in January anyway if rainfall does not counter this procedure.

Monitoring flower bud induction and bloom date along with some strategies for altering both are now incorporated in a ‘Florida Citrus Flowering Expert System' which will be introduced to some grower cooperators next Friday. Contact Dr. L. Gene Albrigo at albrigo@lal.ifas.ufl.edu if you are not in that group and wish to be. A limited number of additional cooperators may be added.

If you have any suggestions or questions, please contact me (albrigo@ufl.edu)


12/12/2001

FLOWER BUD INDUCTION ADVISORY #4 for 2001-2002-12/12/01

The continued warm winter is still adversely affecting flower bud induction (FBI) for next years crop. Little cool temperatures for FBI have occurred and warm temperatures are predicted through next Monday. On top of the continued daily maximum temperatures above 80o F, most citrus areas received about a quarter inch or more rain. In many groves where irrigation was discontinued or reduced to promote water stress, rainfall in excess of this amount may have negated the attempt to prevent bud break. Certainly rainfall in excess of one half inch will promote bud break on most soils. If you have soil moisture probes and your soil moisture levels at the 8 to 12 inch depths were not raised above a 60 % available moisture level, you may have maintained sufficient stress to prevent growth.

All bud break now is reducing potential for flower buds for the spring bloom. From some recent studies done under our Decision Information Systems for Citrus (DISC), we have determined that at least 20 % of the flowering potential is lost for each of the buds that is lost to a late fall or early winter vegetative flush. An example would be that if 1/3rd of last year's Spring and Summer flush had two buds (terminal end) flushing, you would have 1/3 of flush x 2 buds (20 %) = 13 % less flowering potential for next year. We still have potential for flower buds because January is our coolest month and hopefully temperatures will cool down without going to freeze levels.

Until we get some sustained FBI from cool temperatures or maintenance of water stress, there is no benefit from making a Winter urea or PO3 spray. We will revisit the situation next week to see if any cool weather is in our forecast.

The automated flower bud induction and bloom date monitoring system, ‘Florida Citrus Flowering Decision Support System' was introduced to some grower cooperators last Friday. Contact Dr. L. Gene Albrigo at albrigo@lal.ifas.ufl.edu if you were not in that group or were not contacted later and still wish to be included in testing the system the rest of this winter. A limited number of additional cooperators will be added later this week when the software is distributed.

If you have any questions, please contact me (albrigo@ufl.edu)


1/7/2002

FLOWER BUD INDUCTION ADVISORY #7 for 2001-2002-1/7/02

Cool temperatures for FBI have continued and are predicted to continue for the next week. For all groves that were not placed in water stress conditions in late November-December, the earliest date that trees should be allowed to start flower bud differentiation is still in late January (27th). After this date, trees can be allowed to respond to a warm spell by providing adequate soil moisture for growth (resume full irrigation). A urea or phosphorous acid spray should be applied the week before the warm spell occurs in which the trees are allowed to begin growth. Since many of the terminal end buds on last year's spring and summer flush have started to swell, any additional induction that can be achieved will increase the likelihood that buds further down the shoot will be flower buds. For some locations, our new ‘Citrus Flowering Decision Support System' predicts that the differentiation of a bloom started in late December. I think that this flush will be mostly vegetative, but time will tell.

For groves in which water stress was applied in November and December, the trees may have additional flowering induction from the combination of water stress and some cool temperature accumulation. Unfortunately, the level of water stress required to accomplish that may not have been achieved by most of us that were trying to regulate our irrigation. In a block of 6 year old Hamlin and Flame trees on a deep sand where we reduced the amount of water applied per irrigation and allowed trees to show wilt by 10 am before reapplying water, we have about 60 % of the first 4 buds from the terminal end swelling with a few buds in early flush stages. If none of those buds produce flowers, I estimate that we have lost about 50 % of our flowering potential. It is not easy to determine if buds are swelling in comparison to the bud remaining quiescent. The bud scales will be swollen and a lighter (lemon green) color than the non-growing buds. On grapefruit trees, swelling buds will be pubescent. Those that are pushing new growth are easy to evaluate. Based on our experience, I advocate holding all trees back from any further bud growth until February if it is possible.

Unless cool temperatures continue well into February, urea sprays are advised just before a projected warm spell and should be applied at 25 to 28 lbs N per acre in 10-15 (aerial with good spray distributors), 25-30 (ultra low volume), or 50-125 (speed sprayer) gal/ac of water. Speed sprays can be calibrated for as little as 50 or 60 gal/ac at 4-5 mph if large acreage needs to be covered. High speeds do increase risk of operator driving error. Phosphorous acid should be applied at the equivalent of 2.6 quarts of 26-28 % P product in similar gal/ac as recommended for urea sprays.

If accumulated hours get above the 800 level counting from 19 December 2001 seedless cultivars and mandarins coming into the on-year may have sufficient flowering without enhancement sprays. We will continue to monitor cool weather into February since the potential levels of flowering and needs for flower bud induction altering sprays will have to be reevaluated if cool weather persists until then.

Contact Dr. L. Gene Albrigo at albrigo@lal.ifas.ufl.edu if you have questions or observations about trying to maintain light stress on citrus trees under Florida conditions.

If you have any questions, please contact me (albrigo@ufl.edu)


1/11/2002

FLOWER BUD INDUCTION ADVISORY #8 for 2001-2002-1/11/02

Cool temperatures for FBI may continue the next week in northern districts, but sufficient warm temperatures are predicted for flatwoods areas and probably the southern ridge (south of Avon Park) that initiation of another set of flower buds should occur this coming week. For these areas, now is the time for urea or PO3 sprays. For Northern areas, trees may slide through the marginal temperatures and accumulate more cool temperatures before bud growth starts. Sprays to enhance flower buds could be applied now in these areas, but if daytime highs stay in the low 70s and turn cooler in about 4 days as predicted, growers can wait for the next warm spell before spraying in these northern areas.

Urea sprays should be applied at 25 to 28 lbs N per acre in 10-15 (aerial with good spray distributors), 25-30 (ultra low volume), or 50-125 (speed sprayer) gal/ac of water. Speed sprays can be calibrated for as little as 50 or 60 gal/ac at 4-5 mph if large acreage needs to be covered. Phosphorous acid should be applied at the equivalent of 2.6 quarts of 26-28 % P product in similar gal/ac as recommended for urea sprays.

Contact Dr. L. Gene Albrigo at albrigo@lal.ifas.ufl.edu if you have questions about this advisory or citrus tree flowering under Florida conditions.

If you have any questions, please contact me (albrigo@ufl.edu)


 

1/18/2002

FLOWER BUD INDUCTION ADVISORY #9 for 2001-2002-1/18/02

Sufficient warm temperatures to initiate flower bud differentiation are occurring or predicted for all citrus areas. For all areas were urea or PO3 sprays were planned, they should be applied soon if they have not already been done. For Northern areas, sprays may be effective if sprayed well into next week.. For Southern areas, sprays to enhance flower buds need be applied by early next week to have a reasonable chance of being effective.

In most groves examined some buds are pushing from differentiation and bud swell that started by 20 December. There are a few flowers in that flush, usually not more than one per inflorescence if present at all. Most of the flowers will be in the flush being initiated now. I predict that due to the low accumulation of cool temperatures the inflorescences will have many leaves and few flowers. Therefore a flower enhancing spray is warranted for most cultivars if you can get it on soon. The good side of low flowering with many leafy inflorescences is that set should be high. Seedless cultivars and mandarins that had a light crop last year may have about the correct number of flowers due to the low induction levels. It is my best guess that in these cases neither flowering enhancement nor reduction (gibberellic acid) sprays should be applied.

Where urea is used, sprays should be applied at 25 to 28 lbs N per acre in 10-15 (aerial with good spray distribution (powered spray distributors)), 25-30 (ultra low volume), or 50-125 (speed sprayer) gal/ac of water. Speed sprays can be calibrated for as little as 50 or 60 gal/ac at 4-5 mph if large acreage needs to be covered. Phosphorous acid should be applied at the equivalent of 2.6 quarts of 26-28 % P product in similar gal/ac as recommended for urea sprays.

Currently, bloom dates are predicted to be about 5-10 March and 25 March to 1 April. This means the blooms will overlap. Periodically, there will be updates of the projected bloom date from our new flowering decision support program. Also, observations of flowering levels will be presented when flowers are visible. I would appreciate email observations of bloom characteristics from growers in all districts.

Contact Dr. L. Gene Albrigo at albrigo@lal.ifas.ufl.edu if you have questions about this advisory or citrus tree flowering under Florida conditions.

If you have any questions, please contact me (albrigo@ufl.edu)


1/29/2002

FLOWER BUD INDUCTION ADVISORY #10 for 2001-2002-1/29/02

Sufficient warm temperatures occurred prior to December 18th and again from January 18-20 to 25-27 January to initiate 2 flower bud differentiation sets in all citrus areas. Currently, if unseasonably warm weather continues, bloom dates for these 2 sets of floral buds could be about February 17th and March 20th. It is not likely that additional flowering flushes will occur this season.

Because the daily maxima were above 80 degrees F almost every day from November 17 until 18 December, the initiation process of that set of buds may have started several days earlier than 18 December. Daily minima were near or above 60 degrees C from November 17th also. This could result in an even earlier bloom for those buds than mid-February, occurring mostly in the terminal 2 or 3 bud positions on last summers shoots.

Many growers report seeing flush and report that some of it has one or two pin-heads flowers visible. This is better than our worst prediction, that no flowers might occur in the first flush. Still it appears that there will be few flowers and mostly leaves in the first bud sprouting.

Most of the flowers will be in the second flush that started to differentiate a week ago. Due to the low accumulation of cool temperatures between the two flushes, the inflorescences probably will have many leaves and few flowers. Although more flowers would be desirable, better set occurs when there are several leaves associated with the flowers in each inflorescence.

Since the differentiation process is now well underway, flower enhancing sprays of urea or phosphorous acid are not likely to work.

In summary, peak bloom dates are now predicted to be about mid- to late February and 20 to 25 March. This means the blooms will overlap with some flowers appearing throughout the period from mid-February to late March. The first bloom should be a very light flowering with 1 or 2 flowers per inflorescence when flowers are present. Periodically, there will be updates of the projected bloom date from our new flowering decision support program. Also, additional state-wide observations of flowering levels will be presented as the inflorescences develop. I would appreciate email observations of bloom characteristics from growers in all districts.