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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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12/1/2000

FLOWER BUD INDUCTION ADVISORY #1 for 2000-2001-12/1/00

As in the last two winters, we will attempt to relate the cool temperature accumulation and warming spells to the flower induction levels as the winter progresses. Weekly advisory reports on the accumulation of cool temperatures, generated from the FAWN weather system, and recommended spray timing for winter sprays will continue until mid or late January, after which it is less likely that any attempts to alter flower bud induction will succeed. Advisories will appear on the Citrus Research and Education Center Web Page. The website is at http://www.crec.ifas.ufl.edu (look under Grower Aids). This information will also be forwarded to County Extension offices and other publishing sources. It should be noted that waiting for these written announcements might cause delays in management practices, which may result in missing an important timing window.

The 2000-2001 winter

Winters in the 1990's have been the warmest on record, with the last three among the warmest. According to the National Weather Service, the 2000-2001 Fall-Winter weather in Florida is expected to be warmer than normal (1961-1990), but slightly cooler than the last three years. This provides the potential for more adequate period of cool temperatures than what we have received during the last three years. A prolonged period of cool temperatures (below 70 o F) is necessary for good flower bud induction to occur. Warm periods can initiate bud differentiation (growth) and discontinue the induction process.

In contrast with other Gulf Coast States, Florida is not expected to receive more precipitation than usual. These are good news as, in addition to temperature stress, flower bud induction may be complemented with drought stress. Drought stress constitutes the other natural process for flower bud induction in citrus. Drought stress must be sufficient to stop tree growth and cause induction to proceed. If little rain occurs during a warm winter significant drought stress may develop without irrigation. A combination of both processes may be used to achieve adequate flower bud induction. Withholding irrigation for at least 30 to 40 days should be long enough to increase flower induction. The time to which trees should be withdrawn from irrigation, depends on soil type, depth, and whether the grove is bedded or not. Drought stress also can stop initiation of differentiation of potential flower buds if intermittent warm periods occur during the winter. Multiple blooms may be avoided by keeping the trees drought stressed until they are ready to initiate flower bud differentiation in mid-January.

But not everything is good news, despite the fact that forecast probabilities favor warmer than normal (1961-1990) conditions in much of the southeast this winter, the National Weather Service expects more days with minimum daily temperature below freezing compared to the past 3 winters. This means that the changes of a freeze are increased. Precautions should be taken to face the likelihood of a Florida freeze.

So far this winter…

The Fall of 2000 has been cool and excellent induction temperatures have occurred. From October 1st until December 7th, we have had 850 hours below 70F, which is 300 more hours than last year (1999-2000). These are also more than 500 hours from the previous year (1998-1999) for the same period the year before. You may remember that in the spring of 1999 we had an extended flowering period that lasted for over three months. The experienced low temperatures should be more than enough to stop vegetative growth and get the flower bud induction process underway.

Although these are good news, it also means that bloom management should start earlier than the last two years. The occurrence of a warm spell may trigger unwanted growth at this stage. If a warm spell forces growth, this will be mostly vegetative. The few flowers forming would encounter lower temperatures and therefore low fruit set.

Bloom management.

Flowering may be managed in a variety of ways.

Drought stress. Cold temperature stress may be coupled with irrigation withdrawal (drought stress) to achieve increased stress and therefore greater induction levels. Drought stress can also be beneficial, as it will prevent the onset of growth during warm spells. If growth is not prevented prior to the full induction level, there will likely be an extended flowering period, which is not desirable.

Drought Stress on Bedded groves. Moderate drought stress may be maintained on flatwood bedded groves with the irrigation system shut down. By maintaining the water-table just below the bottom of the water furrows, daily stress can occur with some recovery each night from moisture taken up by the tree roots that extend to near the bottom of the water furrow. If the water table is one or more feet below the water furrow, severe drought stress will occur if irrigating is stopped and there is no rain. Also, if severe root pruning occurred due to high water levels during the previous two years, then a water table at the bottom of the furrow may still result in a severe stress level. Visual observation of temporary leaf wilting can be a good indicator of proper stress. Wilting by 10 or 11 am. and recovery overnight is ideal.

Drought stress on deep-sandy soils. Drought stress is much more difficult to develop on deep sandy soils due to the deeper rooting zone. Two or three weeks may be required on deep sands in order to reach an adequate level of stress to begin flower bud induction. Drought stress can be used on Valencia trees, trees already harvested or Hamlin trees that will be harvested during December. Trees with fruit intended for fresh market should not be subjected to severe drought stress. Valencias hold their fruit very tightly, but Hamlins for later harvest or Pineapple oranges are likely to experience excessive drop of the current crop if subjected to even moderate drought stress.

If one or more winter-time rain fronts negate the attempt to maintain water stress, the associated cool weather behind the front (a typical occurrence) may provide some cool temperature induction. The issue will be whether enough induction has occurred.

Winter sprays of stress-boosting chemicals. Even in cool years of good induction conditions, winter sprays of stress-boosting chemicals can provide benefits by increasing flower bud induction and subsequently yields of at least Valencia trees on flatwood soils. Four years of tests on Valencia oranges on bedded grove sites have shown a consistent enhancement of total yield and pounds solids per acre from urea or potassium phosphite (PO3) sprays (Albrigo, Proceedings Florida State Hort. Soc., 1999, in press). Some grower tests using these materials have provided similar increased flowering and yield results on other cultivars as well as Valencia trees.

After some cool temperature induction or 30 days of drought stress, foliar sprays of either 50 to 60 lbs of spray grade urea per acre or 2.6 quarts of 28 % P as potassium phosphite per acre can enhance flower bud induction. Foliar sprays of urea or phosphorous acid products presumably work only if some induction has taken place from natural cold or drought stress. Lovatt, et al. (1988) demonstrated this for foliar sprays of low biuret urea. In our previous tests, the timing of this spray was between Christmas and January 5th. In years of strong El Nino or La Nina events, suitable timing may be earlier or later. In any case, the timing must be before significant high temperatures occur (3 or 4 days of maximums exceeding perhaps 85o F) which will start the flower bud differentiation process. It is currently still difficult to make precisely timed recommendations, and we cannot guarantee benefits.

If you are interested in applying either urea or phosphorous acid this winter as a test or on substantial acreage, you need to be prepared. The product must be on hand and the spray equipment available on short notice. Low biuret spray grade urea is needed (< 0.5 % biuret on a dry weight basis) or a suitable clean source of buffered phosphorous acid. If you plan to use an aerial applicator, they must use a micronair or similar distribution spray system and they must be booked in advance. This of course complicates timing. Ground sprayers can be used at low volume and fast speeds. Air blast sprayers will work at 50 to 80 gals per acre. Ultra low volume sprayers give good distribution in the outer canopy at 25 to 30 gals/acre. The amount of acreage you can cover will determine your priorities. Healthy trees that had (have) a good current crop should be a higher priority. Weaker trees may benefit, but have less capacity to carry a heavy crop. Navel and other low yielding cultivars (Ambersweet and some Rhode Red strains) that often over-flower, should not be treated to enhance flowering. We have some on-going experiments that suggest that GA3 at the same timing as the urea spray may be useful to reduce excessive flowering on these cultivars and create more mixed inflorescences with good leaf to flower ratios.

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


12/13/2000

FLOWER BUD INDUCTION ADVISORY #2 for 2000-2001-12/13/00

This is an early update to the first advisory because of the warm spell that has occurred this week. Lake Alfred, Immokalee and Tavares have experience 3 to 4 days above 80 degrees F while Ft. Pierce only one. There are an additional 3 to 6 days above 80 degrees predicted on the 8 day forecast.

As of 12/10/00 and based on FAWN data from October 1st, Ft. Pierce, Immokalee, Lake Alfred, and Tavares had accumulated 696, 735, 866 and 883 hours below 70 degrees F, respectively. This is sufficient that some buds are induced and the warm spell will have started them to differentiate if sufficient soil moisture is present. Some bud swell has been seen in Polk County this week.

Growers that managed to maintain a moderate water stress should have suppressed this first flower bud differentiation and will continue the induction process on these as well as other buds on the tree from the water stress and from the next cool weather period. We still want to get to Christmas or early January before stimulating differentiation by applying a flower bud enhancing spray such as urea and increasing irrigation rate.

Many growers have been reluctant to reduce irrigation due to small fruit size and the hope of increasing size by more frequent irrigation. By December, there is little likelihood of significantly increasing fruit size. Other people have not reduced irrigation due to fear of water use restrictions the rest of the winter and next spring. Trees that have been well irrigated will now have bud swell and these shoots will be very susceptible to a freeze if one should occur. Only Hamlin oranges are likely to experience significant drop later on from water stress now. Maintaining some drought stress during warm winter periods would appear to have more benefits than disadvantages.

Again, our goal is to accumulate more than 900 hours below 70 degrees without a warm spell stimulating growth of buds before we want to encourage the trees to begin differentiating flower buds.

Current Weather Status In Relationship To Citrus Flower Bud Induction

And Use Of Winter Foliar Sprays To Enhance Flower Bud Induction

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


12/18/2000

FLOWER BUD INDUCTION ADVISORY #3 for 2000-2001-12/18/00

As everyone has noticed today, we have passed through a typical December warm spell and now have a weather prediction from NOAA of temperatures below or near 70 degrees for most locations until Christmas. This should add about 170 hours to the current totals.

As of 12/17/00 and based on FAWN data from October 1st, Ft. Pierce, Immokalee, Lake Alfred, and Tavares had accumulated 734, 796, 888 and 970 hours below 70 degrees F, respectively. For some stations, there appears to be a data gap in October, which may total another 60 or 70 hours below 70 degrees F.

If trees were maintained in drought stress during last weeks warm spell, there will be sufficient flower bud induction by Christmas and urea sprays should increase bloom intensity. Minimum rates should be 25 lbs N as urea per acre (28 lbs is preferred). Such a spray will usually advance bloom date about one week. The best timing should be just before the next predicted warm spell.

The CREC WebSite (www.crec.ifas.ufl.edu) has a weather link, and this link includes the 8 day predictions by state (FL) and locations (see Ft. Myers, W. Palm Beach, Tampa & Orlando to bracket the citrus industry). We are in the process of building an induction monitoring system which will extrapolate these 8-day prediction values to each FAWN weather location in the state, but for now you must do your own evaluation of the four sites available.

If sufficient soil moisture was present last week, some bud swell should be evident this week. The bloom situation is much better if initiation and bud differentiation were avoided last week by maintaining a moderate level of drought stress. But if not, a gibberellin spray (20 oz per acre of 4 % GA) this week may minimize the bloom from this early bud break. The danger from doing this, however, is that there is no guarantee that sufficient cool hours will follow to provide an adequate bloom at a later date. A GA spray could stimulate shoot growth of the swelling buds if another warm spell comes soon. These buds or young shoots would then be much more susceptible to a freeze.

If some bud swell is occurring this week, it will probably take another 300 or 400 hours of cool temperatures to provide a second set of induced buds that can provide a reasonable set of fruit. If that much additional cool temperatures occurs, a urea spray just before the next warm spell may be beneficial, but we don't have much experience to base that possibility on.

Growers that managed to maintain a moderate water stress last week should have suppressed this first flower bud differentiation and can expect the induction process to continue on these as well as other buds on the tree. Both water stress and cool weather periods are building the induction level. Because of the extended warm period last week, it would be desirable to get to mid-January before stimulating differentiation either by applying a flower bud enhancing spray such as urea and/or increasing irrigation rate. If another warm spell comes after Christmas but before mid-January, then trees should be released from drought stress and allowed to begin flower bud initiation and differentiation. It is questionable if a urea spray will have an effect soon after last weeks warm spell.

Again, our goal is to accumulate more than 900 hours below 70 degrees without a warm spell stimulating growth of buds. If we have reached late December or early January, we can then encourage the trees to begin differentiating flower buds by using a urea spray and increasing irrigation just before the warm spell begins. If a prolonged cool spell now occurs, seedless cultivars like navels and Ambersweet may benefit from a 20 oz per acre spray of GA just before the predicted warm growth temperatures.

Trees that did have adequate moisture last week for growth should have at least two flushes of bloom in the spring.

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


12/24/2000

FLOWER BUD INDUCTION ADVISORY #4 for 2000-2001-12/24/00

The progress of Florida citrus trees toward induction of citrus flower buds from cool temperatures was favorable last week. The cool temperature accumulation below 70 degrees F at Lake Alfred through yesterday is now 1224 hours through Saturday 23 December compared to less than 900 hours below 70 degrees F through January 8th two years ago. This exceeds the hours related to the 75 % induction stage for the 94-83 season. Fort Pierce and Immokalee are 252 and 152 cool inductive hours, respectively, behind Lake Alfred. As usual, Tavares has been cooler than Lake Alfred and has about 40 more hours below 70 degrees F through yesterday. These relative positions are fairly normal.

Presently, it is a mid-moderate flower induction year. The forecast of temperatures for the next 8 day indicates that inductive temperatures will continue for at least another week. The WebSite for these temperature predictions (www.crec.ifas.ufl.edu , then connect to Weather and 8 day forecast for Florida locations near you) should be examined later this week for the temperature predictions for the week of New Year's day. Enhancement sprays should be effective and can be applied by next weekend. The best time will be just before the next warm spell (3 or 4 days with 80+ degree F maximum). There still may be some benefit if sprays are applied the first 2 or 3 days of a warm spell. We of course still have the possibility of a continued cold spell into mid-January or even early February. Since we have had one warm spell to break continuous flower bud induction, it would be better to wait at least another week if cool weather continues.

In summary, induction by cool temperatures is at the mid-moderate level and enhancement sprays may still be beneficial for flower bud induction if applied before the next warm spell in spite of one warm spell having already occurred. We should expect a fairly concentrated or slightly protracted bloom this year if sufficient water stress was maintained in early December's warm spell. For other blocks, two blooms or a protracted bloom should be expected. Foliar nutrition or PFD control sprays during the bloom period may be more difficult to time because of a protracted bloom period in many blocks that had good soil moisture in early December..

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


12/29/2000

FLOWER BUD INDUCTION ADVISORY #5 for 2000-2001-12/29/00

The progress of Florida citrus trees toward induction of citrus flower buds from cool temperatures was favorable this week and will continue next week. The cool temperature accumulation below 70 degrees F at Lake Alfred through yesterday is now 1330 hours through Thursday 28 December compared to less than 900 hours below 70 degrees F through January 8th two years ago. This exceeds the hours related to the 75 % induction stage for the 94-83 season. Fort Pierce and Immokalee have 1042 and 1126 cool inductive hours, respectively, while Tavares has 1376 inductive hours.

Presently, it is a upper-moderate flower induction year, especially considering the predicted cool weather for next week.. The forecast of temperatures for the next 8 days indicates that inductive temperatures will continue with some temperatures being lower than ideal. The WebSite for these temperature predictions ( www.crec.ifas.ufl.edu , then connect to Weather and 8 day forecast for Florida locations near you) should be examined for the Sunday freeze forecast and further temperature predictions for the week after New Year's day. Enhancement sprays should be effective by next week if daytime temperatures warm to above 70 degrees F. Lower temperatures will reduce uptake of the applied chemical. The best spray timing will be just before the next warm spell (3 or 4 days with 80+ degree F maximum). There still may be some benefit if sprays are applied the first 1 or 2 days of a warm spell. Sprays should be delayed until this cold spell is nearly over. In spite of the early December warm spell, we now have enough additional induction conditions to expect to get a fairly good second bloom even if some buds were starting to differentiate from the first warm.

We have now reached an induction level where gibberellin sprays may help reduce flower intensity and improve set of excessive flowering seedless cultivars like navel and Ambersweet. Expected on-year mandarin blocks may also benefit from reduced flower levels by producing fewer flowers with larger ovaries. Look at the gibberellin spray advisory posted on this same page.

In summary, induction by cool temperatures is now in the upper-moderate range and enhancement sprays may be beneficial for flower bud induction, particularly in Valencias, Hamlins and grapefruit, if applied before the next warm spell in spite of one warm spell having already occurred. Daytime temperatures should be near 70 degrees F before sprays are applied to assure good uptake. We should expect a fairly concentrated bloom this year in blocks where sufficient water stress was maintained in early December's warm spell. For other blocks, two blooms or a protracted bloom should be expected. Foliar nutrition or PFD control sprays during the bloom period may be more difficult if a protracted bloom period occurs in blocks that had good soil moisture in early December.

Watch out for Sunday mornings predicted freeze temperatures.

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


1/3/2001

FLOWER BUD INDUCTION ADVISORY #6 for 2000-2001-1/3/01

Welcome to the new Millennium. It is about time to apply Winter sprays for enhancement of flower bud induction. The progress of Florida citrus trees toward induction of citrus flower buds from cool temperatures continued to be favorable this week and will continue most of next week. The cool temperature accumulation below 70 degrees F at Lake Alfred through yesterday is now 1450 hours through Tuesday 2 January, while Fort Pierce and Immokalee have 1162 and 1281 cool inductive hours, respectively, and Tavares had 1496 inductive hours. This is a (lower) upper level of induction temperature accumulation for Florida.

Therefore, it is a good flower induction year, especially considering the predicted cool weather for next week.. The forecast of temperatures for the next 8 days indicates that inductive temperatures will continue with some daytime high temperatures being slightly higher than ideal. The WebSite for these temperature predictions (www.crec.ifas.ufl.edu , then connect to Weather and 8 day forecast for Florida locations near you) should be examined to determine if the cool temperature period will continue into mid-January. Mid-January is a traditional freeze period in years such as this (El Nino-La Nina neutral). Enhancement sprays should be effective by next week since daytime temperatures are predicted to be above 70 degrees F. Lower temperatures will reduce uptake of the applied chemical. Sprays can be delayed or eliminated this year if cool temperatures continue after next weeks slight warming spell. A urea spray next week may have the benefit of stimulating the initiation process, providing a more uniform bloom that can be 5 to 7 days ahead of non-sprayed blocks.

Urea sprays should be at the rate of 25 to 28 lbs N as urea per acre (53 to 60 lbs of urea). If phosphorous acid formulas are used, they should be applied at 2.6 quarts of 26-28 % P product or equivalents per acre.

We have now reached an induction level where gibberellin sprays may help reduce flower intensity and improve set of excessive flowering seedless cultivars like navel and Ambersweet. Expected on-year mandarin blocks may also benefit from reduced flower levels by producing fewer flowers with larger ovaries. Look at the gibberellin spray advisory posted on this same page. Application rates of 20 to 30 grams per acre are recommended.

In summary, induction by cool temperatures is now in the upper range. Enhancement sprays may still be beneficial for flower bud induction, particularly in Valencias, Hamlins and grapefruit. Next weeks temperatures appear favorable for application of sprays. Daytime temperatures should be near 70 degrees F or higher before sprays are applied to insure good uptake. We should expect a fairly concentrated bloom this year in blocks where sufficient water stress was maintained in early December's warm spell. In blocks with moderate water stress in early December and a moderate crop this year, bloom may be adequate without an enhancement spray. For other blocks, two blooms or a protracted bloom should be expected. Boosting the flower induction of the second bloom and advancing its date of bloom may be beneficial.

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


1/10/2001

FLOWER BUD INDUCTION ADVISORY #7 for 2000-2001-1/10/01

Currently, we are at the highest level of winter cool temperature accumulation for flower bud induction compared to 20 years of data that has been analyzed. The warming spell expected this weekend and early next week will likely trigger the initiation of differentiation of flower buds. As discussed earlier, this will be the second set of initiated flower buds for many trees. The first set of flower buds started initiation in early December with the warm spell of December 7 to 9. One of the unknowns in this year's flower bud induction temperature accumulation is ‘How much does that first warm spell diminish the total effect on flowering?'. We have not experienced a comparable winter since we started this work on flower bud induction. We expect that most blocks of citrus will not need help from a winter urea spray in order to have good flowering. However, successful use of winter urea has been reported from California even though they accumulate many more hours of cool temperatures than occurs in Florida.

Another unknown this year is the possible effect of slight freeze injury in many blocks that has led to leaf but not wood injury with subsequent losses of 50 to 80 % of the leaves. There may be a slight stress effect associated with this level of injury which could enhance flowering, make it more synchronized and possibly advance the date of bloom.

Of the years that we have studied, it may be least likely this year that urea sprays will enhance flowering or that enhanced flowering is needed. The most likely trees to benefit from a winter urea spray applied now are Valencia trees with a good current crop that had good soil moisture in early December when the warm spell could cause induced flower buds to initiate growth. There have been only about 850 hours of cool temperatures accumulated since that warm spell. This is less than the 900 which normally relates to a good urea spray response. However, we do not know how much flowering response we will get in the second set of flower buds from this intense level of induction.

Trees that are more likely to benefit this year from adjustment of flowering levels are seedless cultivars that flower excessively almost every year (see the last advisory). These cultivars will likely benefit from a gibberellin spray (see gibberellin advisory on this page). The timing of these sprays should be later next week or early the following week. If a urea spray is to be applied it should be put on from tomorrow (Thursday) through the first day or two of next week.

This will be the last advisory for this year. If you have any questions, please contact me (albrigo@ufl.edu)