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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 1998-1999-12/15/98

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 

So far this Fall-Winter, there has been very little cool temperature induction of citrus flower buds. Between the various citrus growing districts, there is probably very little difference in cool temperatures accumulated so far this year since daytime temperatures have been in the 80s. We are not close to 2 previous seasons (93-94 and 94-83) in which 1300 to 2800 hours below 75 degrees F, respectively, had occurred by the time 100 % of flower bud induction (FBI) had occurred. For Lake Alfred through Saturday, there was less than 600 hours below 75degrees F and even fewer hours in the more desirable range between 40 and 70 degrees F. Most induction to this point has probably been due to water stress. Even with micro sprinklers operated twice a week, growers are reporting daily wilting. The period of water stress induction may still be operative if your Sunday rainfall was less than 1/4 inch as reported for most FAWN stations in the major citrus areas. It would require more than ½ inch to have provided any stress release.

In two previous seasons, we have collected data on the amount of FBI as the Fall and Winter progressed. Insufficient data is available to develop a working model, but we can look at general trends as some aid in estimating our current status. The current cool weather accumulation this Fall-Winter is equal to that at which about 25 % of the maximum bloom induction occurred in 93-94 and 94-83, taking into consideration the least number of hours associated with 25 % bloom in those years. If the NOAA predictions are correct for this coming week, most of the temperatures should be in the effective FBI range. Even so, we will still have less hours of low temperatures accumulated than related to 50 % FBI in the 2 years of records. A second week of similar temperatures would bring us to about the 50 % level according to those previous years. If cool temperatures persist until then, we would be into the week after Christmas and have more probability of a response from foliar induction enhancement sprays. As long as cool temperatures continue, the trees should remain dormant and little bud differentiation is likely considering the low amount of inductive temperatures so far. However, any warm spell is likely to start flower bud differentiation. Of course many blocks of early cultivars like navels have flush and some bloom already. Two or three weeks of cool weather will bring on induction of a second bloom that may be small due to loss of potential buds to these Fall flushes.

FBI sprays need to be applied no later than the very beginning of a warm spell. This cool spell hopefully will last more than this week, spraying should be scheduled just before the next NOAA predicted warm spell of daytime highs in the high 70s or low 80s for 3 to 4 days. These are best guesses only. Unfortunately, we do not have sufficient data to predict more accurately either our exact FBI status nor just how much warm temperature will start the differentiation process. Once the differentiation process starts, a foliar spray cannot alter the determination of a buds status as vegetative or flowering.

It is likely that the action of either urea or phosphorous acid at this time is by enhancing stress induction of flower buds. The urea action may be by the rapid release of ammonia in the plant tissues, while PO3 in its various forms can be phytotoxic at high enough rates. The recommended rates of both are just below the threshold to cause tissue necrosis. The urea used should be low in biuret (less than 0.5 %) because leaf tissues do not recover from biuret toxicity and remain yellow. Suppliers should provide a written analysis of biuret content. Forms of PO3should be free of heavy metals or other contaminates. Good sources are essentially clear liquids, but an analysis of trace elements should be provided.

At this time it appears that any foliar sprays should be anticipated for 2 weeks from now or slightly later if cool temperatures were to 


FLOWER BUD INDUCTION ADVISORY #2 for 1998-1999-12/21/98

A best guess is that for non-water stressed trees, urea or phosphorous acid sprays should have been applied Sunday December 20 or Monday December 21 for enhancement of flower bud induction. Even if timing is optimum, induction temperatures have been low and the possibility of getting results is unknown. 

So far this Fall-Winter, cool temperature induction of citrus flower buds continues to be low. We have had only one week of ideal temperatures. The weather prediction for the next 4 days is for 80 degree F highs. The weather prediction for the next front, Christmas weekend, does not promise that cool weather will follow. January is predicted to be above normal. 

It is not likely that we will get a cool spell soon enough to continue flower bud induction. It is most likely that the trees will start flower bud differentiation of any available flower buds. Based on last weeks advisory, you should have sprayed to enhance flower bud induction on Sunday or today if you are not holding your trees back with water stress. 

We do not know if the action of either urea or phosphorous acid to enhance stress induction of flower buds can be effective once the flower bud differentiation process starts. If these sprays can be effective, it will be for only a very few days, probably 3 or 4 at most. We have seen a response one year when we may have been 3 or 4 days into the differentiation process. It is certainly an economic risk (wasted spray costs) to treat later than 1 or 2 days into this current warm spell. 
If you have water stressed your trees as suggested in earlier advisories, you can expect to continue the induction process if you did not receive substantial rain in this latest front. Most areas report less than ½ inch, but some areas did receive higher rainfall. To assure continued water stress induction, a higher level of stress is advisable this week to insure that the flower bud differentiation process cannot start even with high temperatures favorable for growth. Using water stress is similar to conditions in the tropics, where flower development does not start until enough rain or irrigation releases the trees from drought stress. A minimum of one more week of water stress is probably advisable and 2 weeks would be better. For processing oranges, this is probably a good idea to assure sufficient bloom for a reasonable crop. It is likely that this will be a year in which some June bloom will occur because of low Spring fruit sets. 

For early crops of fresh fruit (navels and grapefruit particularly), urea sprays may boost flowering and cause an earlier bloom which is favorable for higher prices even though crop load may be fairly low. Even with urea and some water stress, it is likely that the resulting bloom will still be light. If so, some enhancement of flower and fruitlet set will be in order in the Spring bloom period. 

If the weather predictions are wrong, we may get another induction period for more flower buds and a second bloom within the regular bloom period. This would be a repeat of last year, but with the first buds starting differentiation about 10 days later than last year. Last year we had very good induction later in January and early February. The weather predictions do not appear favorable for that. If January is warmer than normal, we can expect an early, light bloom. 
We (graduate student Juan Valiente and I) have developed a technique to excise defoliated shoots from the tree, treat them to prevent desiccation and decay and force the buds to grow out in an apparently normal manner within about 10+ days in a growth chamber. So far we have seen few flowers in the emerging inflorescences. On Friday, shoots were put in the growth chamber after the past week of cool temperatures. Unfortunately, it will be another 10 days before we can determine if a substantial increase in flower bud induction had taken place. This methodology, which is currently supported by citrus box tax money, will allow us to evaluate induction conditions in a timely manner so that we can better understand the process. Further, it can be used to evaluate if enhancement techniques are working at different time intervals during or shortly after the induction period. 

In summary, for the current situation, flower induction appears to be low based on cool temperature accumulation. Enhancement foliar sprays of urea or phosphorous acid are not likely to work after one or two more days, if for that long. If water stress has been established and it was not broken by the Sunday front, the stress should be continued for one or two more weeks. Irrigation should then follow and enhancement spray. This may provide a reasonable level of bloom within the normal bloom period. 

Next week we will further assess what is happening to our normal abnormal Florida weather, how it is impacting us and if it may be advisable to start a prolonged drought stress treatment on some blocks to induce a reasonable late bloom in April. Such a water stress period needs to be 35 to 40 days and approximately another 30 days are required for the flower bud differentiation process to reach full bloom. But first we need a better assessment of our current flowering potential status which we will try to provide next Monday.


FLOWER BUD INDUCTION ADVISORY #3 for 1998-1999-12/28/98

We are progressing toward cool temperature induction of citrus flower buds but that progress continues to be slow. The Weather Service has lowered their temperature predictions following the Christmas weekend front. We can expect some additional induction temperatures this week as the short-term prediction is now for normal temperatures. January may still be above normal.

Last year we had very good induction later in January and early February. The weather predictions do not appear favorable for that, but usually January is the coldest month, and we can hope for continued cool weather. As of December 26th, accumulation of cool temperatures had reached 680 hours below 70 degrees F since November 1st at Lake Alfred. In your area, accumulated hours of cool temperatures may be a little different if you access the FAWN weather system. Coastal and South Florida locations accumulate less cool temperature than Northern Ridge areas. The FAWN system also shows accumulated hours since October 1st rather than from November 1st .

This 680 hours at Lake Alfred is about equal to the hours in 1994-83 when 50 % of the eventual flower buds had been induced. If the current projected cool spell lasts until next weekend, we could have as much cool weather as when 75 % of the flower buds had been induced in 94-83. In the year previous to that, the accumulated cool temperatures were nearly one and one half times as much at the same stage of flower bud induction. At this stage of our studies, we do not understand why those 2 years were that different in hours of cool temperature at the same stage of flower bud induction. We have been estimating our current status on the least stringent year but further work is necessary to, hopefully, understand and develop a meaningful equation for the relationship of cool temperatures to flower bud induction. The amount of warm temperatures that intervene during the Winter as well as the amount of the previous or current crop load may be important as negative effects on induction for the coming crop.

The buds that were induced prior to the high temperatures last week most likely started flower bud differentiation. Based on previous years, the induction process may continue in buds that require more cool temperatures to reach a satisfactory state for differentiation to begin. Last year is a good example of this phenomenon which will result in 2 sets of flower buds with different average bloom dates. The question is how much additional cool temperatures are needed to get another significant bloom.

If you had water stressed your trees before the warm spell, as suggested in earlier advisories, you can expect the induction process continued if you did not receive substantial rain. The new cool front should bring the trees to a reasonable level of combined stresses by the end of this coming week. A spray of either urea (25 to 28 lbs of N/acre) or phosphorous acid (2.6 qts/acre of 30 % product) to enhance stress induction of flower buds can be effective. The best guess for spray scheduling is early in the week of January 3rd. Continued cool temperatures or maintenance of water stress could allow a further delay, but general flower bud differentiation usually occurs by January 10 to 15 or earlier.

In summary, flower induction still appears to be low based on cool temperature accumulation, but hours are increasing. Enhancement foliar sprays of urea or phosphorous acid are not likely to work on non-water stressed trees until after some additional cool temperatures accumulate. If water stress has been established and was not broken by the recent fronts, the stress should be continued for at least one more week. If an enhancement spray is used, a through irrigation can then follow to push flower bud differentiation and early bloom. However, if cool weather is expected to continue, bringing the trees completely out of water stress is probably not advisable until after January 15th because of potential of a freeze.

A final advisory will be provided next Monday, January 4th. This would normally be the end of the recommended period for flower bud induction enhancement sprays. Later at bloom, we will recap our studies of flower bud induction for this year.


FLOWER BUD INDUCTION ADVISORY #4 for 1998-1999-1/4/99

The progress of Florida citrus trees toward induction of citrus flower buds from cool temperatures has been favorable this past week with only part of one day warmer than desired. This week should continue to be very favorable and apparently without any freezing temperatures. The projection for Lake Alfred through Saturday 8 January is to have accumulated about 875 hours below 70 degrees F and 1125 hours below 75 degrees F. This will exceed the hours related to the 75 % induction stage for the 94-83 season. If next week is again cool due to another projected storm system moving East, we could accumulate as many induction hours as in 94-83. However, in the 93-94 season we accumulated an additional 900 hours below 70 degrees F compared to the 94-83 season or what we project for this year. 

It would appear that it is still likely to be a low flower induction year. Enhancement sprays on certain types of situations appear still in order. Timing should be the end of this week or early next week, if weather projections are for continued cold weather into next week. Delays beyond that are not advised as earlier studies in Gainesville showed that flower bud differentiation started, even that far North, by January 20th. 

Enhancement foliar sprays of urea or phosphorous acid may be useful since induction temperatures are still low. Priority situations for enhancement foliar sprays are blocks with high yields in this current crop, particularly Valencias since the current crop is still on the trees. Hamlins and other cultivars with a good crop this past year, particularly if not yet harvested, are likely to benefit also. Low bearing cultivars such as some Rhode Red Valencia strains, Ambersweet and navels usually bloom heavier than needed. This year might provide less flower competition because of the low amount of induction. Some tests have shown increased yields when flowering was purposely reduced or when gibberellic acid was applied at petal fall to increase fruit set in these cultivars. 

Several groves throughout the State have bloom or young fruit in post bloom stages. In these groves, about 1/3 rd of the tree surface had an early light to moderate flowering. These trees probably responded in tree canopy areas with very easy to induce buds to a combination of drought stress and the little cool weather in October and early November. The second set of flowers, also from easy to induce buds probably started differentiation about 20-22 December during the 4 to 5 day period with high temperatures above 80 degrees F. The last set of flowers will come from buds induced during the current cold period. Their differentiation may start with less warm temperature than the previous set but still should require some high 70 degree weather. 
In summary, induction by cool temperatures is on the low side and enhancement sprays may still be beneficial for the last wave of flower bud induction. We should expect distinct blooms or an apparent protracted bloom this year. Foliar nutrition sprays during the bloom period may be difficult to time because of a protracted bloom period. It may be best to apply a single spray timed when the fruitlets from the earliest flowering are well into the post bloom period and the later flower buds are still in an early bloom stage. 

This is the final advisory for this Winter. I hope they have proved useful and my best guesses are not too far wrong. I would appreciate email responses this Spring as to your observations of bloom intensity and spread, your enhancement sprays experiences, and if applied, did they appeared to help you bloom situation. We would be interested in how well the very early, December, bloom set for those who had such blocks.