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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11/18/2010

FLOWER BUD INDUCTION ADVISORY #1 for 2010-2011-11/18/10

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 

This is a fortnightly service to our citrus growers posted on the CREC website.  The indicated Expert System on intensity and time of bloom can be accessed anytime at the designated Web Site.

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).  A period of high temperatures in winter can then initiate bud differentiation which after sufficient days of warm springtime temperatures leads to bloom.   The meteorologists predict that this winter in Florida will be an ENSO-La Niña year, above average temperatures and lower than average rainfall.  Under these conditions, enough hours of low temperatures below 68 degrees F. usually accumulate to induce an economic level of flower buds.  Conditions that can interfere with good flower bud induction include: 1) several warm periods interrupting the induction process or 2) the previous crop was exceptionally high or 3) leaf loss from hurricanes, freezes or other causes (canker) were excessive and tree recovery was not complete.  Excessive leaf loss leads to low carbohydrate levels in developing buds which reduces their ability to become flower buds.  Except for a few trees with freeze damage, none of these adverse conditions appear to be in play for the coming season’s flower bud induction.

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 > 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 high, 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 8 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.shtml.  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 hrs < 68 degrees F.  Warm events just 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 watered.  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 mid-December.  Then after more cool temperatures additional flower buds are induced and a later warm period starts their growth and repeats of this process result in multiple blooms.  Presently, the only management tool available to eliminate or reduce the chance of multiple blooms is sufficient drought stress to stop growth.   This 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 quality.  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.  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 the greener shaded bands should be broader (require more hours for the same level of flowering).  If the current crop is lighter or tree condition better, then the colored bands should be narrower as the level of flowering will be as large with fewer total cool temperature hours. Although this representation does not appear on the working screen, recommendations (bottom text box) 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 system is available on-line: http://orb.at.ufl.edu/DISC/bloom

1999-2000_bloom 
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.

Previous responses – In the winter of 2001-2002 following a good crop, cool temperature accumulation was very slow and few hours accumulated (640 hours), warm temperatures persisted and most buds started to grow by 20 December, particularly in well irrigated blocks, leading to excessive vegetative buds.  This resulted in few buds remaining for a second flowering wave and a relatively small crop occurred in the 2002-03 harvest season.  By late December in the winter of 2002-2003, 850 hours of uninterrupted cool inductive temperatures had accumulated with a low current crop on the trees. The subsequent warm period initiated growth of almost all the buds on all of the spring and summer flush with bloom in early March.  We had a fairly leafy bloom of very short duration (slightly more than 2 weeks).  In spite of the high temperatures during and following bloom, an excellent fruit set occurred in all round oranges resulting in the highest Florida citrus crop forecast by the Florida Agric. Statistical Service (2003-04 crop).  In the winter of 2003-2004, there was good flower bud induction and reasonably good fruit setting conditions, although the heavy previous crop probably reduced flowering levels and set.  Even though fruit size was small, it looked like we were headed for a 220 million box orange yield before the 2004 hurricanes significantly reduced the 2004-05 crop.  Since then, we have had Hurricane Wilma in 2005 and a long period of tree recovery from the 2004 and 2005 hurricanes.  Since the hurricanes, flowering levels have been lower and appeared to require more hours to get adequate bloom.  This has usually resulted in the main bloom occurring later (late March).  There is some indication that tree recovery after the multiple hurricanes took several years.  For the 2008-2009 crop season, accumulated hours below 68 degrees F were more than acceptable by the second warm period (over 1000 hours) but flowering and crop per tree was still low resulting in an estimate of only 134 million boxes of oranges.  This low yield probably indicates that the trees still were not fully recovered from hurricane effects.  Some details of the hurricane effects can be reviewed in the 11/01/2006 summary-introduction for the previous year’s flower induction cycle.

Current status for 2010-11 Fall-Winter - The light to medium crops and general tree recovery without a hurricane have finally led to more typical flowering responses in Florida.  This is supposed to be an ENSO-La Niña winter with below average cool temperature accumulation and less rainfall.  Warm periods can interrupt the accumulation process but lower than average rainfall could make it easier to impose drought stress to prevent an early flowering wave.  Currently, citrus locations have accumulated low temperatures < 68 degrees F of 350 to 570 hours from southern to northern areas, respectively.  The next 7 days will be moderate for cool temperature accumulation with another 70 to 90 hours.  Continued accumulation of cool temperatures and prevention of growth during a winter warm spell are important for good 2010-11 citrus production.  Therefore, start to monitor irrigation amounts so drought stress can occur if a warm period occurs between November 20 and Christmas or occurs before reaching an acceptable level of over 750-800 hours of cool temperatures.  Prepare to make groves relatively dry by withholding irrigation if a warm period is predicted.  Keep track of induction hours in your area and watch for the next advisory after December 3rd.

See a previous background introduction for previous important yield responses to cool temperatures: FLOWER BUD INDUCTION ADVISORY #1 for 2012-2013. 
If you have any questions, please contact me (albrigo@ufl.edu


12/07/2010

FLOWER BUD INDUCTION ADVISORY #2 for 2010-2011-12/07/10

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

This is a fortnightly 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.

Current Status: In spite of projections that this is a La Nina year, the weather has been very cool with projections for another week with highs below the threshold for induction, <68 o F. This means that another 168 hours will be added to the current 540 to 830 accumulated cool hours from southern to northern citrus areas.  The minimum hours in southern areas will therefore be about 700 at the end of another week (December 13th), which is near the minimum required for an economic bloom if the current crop is low to moderate..

In order to improve the induction level beyond a minimum, trees should remain at rest for at least another two weeks after that.  With any help from the weather, even trees in the south should have been exposed to at least 900 hours of inductive temperatures.  Remember, watch the weather reports and if daytime high temperatures are projected to go back up into the 80 degree range after next week and before January 1st be sure that soil moisture is low to avoid initiation of bud growth.

If cool temperatures continue past this week, flower enhancing sprays may not be needed in most cases.  The exceptions could be trees with a heavy crop and/or weak root systems due to high water levels this past summer and fall.  More about this next report, but if you anticipate this need be sure you have the urea or phosphorous acid product on hand or readily available from your supplier.

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


12/21/2010

FLOWER BUD INDUCTION ADVISORY #3 for 2010-2011-12/21/10

This is a fortnightly 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

Current Status: The weather has continued to be cool with projections for another week to 10 days with highs near the threshold for induction, <68 o F. This means that another 130 to 150 hours will be added to the current 835 to 1150 accumulated cool hours from southern to northern citrus areas.  The minimum hours in southern areas will therefore be above 975 hours of cool temperature induction at the end of another week (December 28th ), which will be 100 hours  above the minimum required for a good economic bloom if the current crop is low to moderate.  Therefore, it appears that bloom should be more than adequate for next year’s crop on healthy trees.  One negative aspect of the recent weather is that at least 50 hours were less than 41 degrees F.  These hours were probably not effective because the induction process apparently doesn’t proceed at temperatures colder than 41o F.

Trees in low areas may have sustained some damage from the recent freezing temperatures.  Remember, if damaged leaves persist after a freeze, it indicates that the green tissue of the shoot was also damaged.  The associated buds will be lost.  If damaged leaves quickly abscised, the buds should be ok and will grow normally and most of these buds will be flower buds this year.

Even though we should have good flowering with the cool temperature induction that will have occurred by New Years, it is still a good idea to delay the beginning of bud growth until after 10 January. This will delay flowering to a more normal bloom date and delay loss of cold hardiness.  This can be accomplished by keeping soil moisture low and having the trees in a slightly droughty condition so that the buds will not grow if a warm period occurs.  With the high induction levels that are now present, the buds will begin to grow if daytime high temperatures reach the high 70s to 80 o F for just a few days. Remember, watch the weather reports and if daytime high temperatures are projected to go back up into the 80 degree range after next week keep soil moisture low to avoid initiation of bud growth.

Since cool temperatures will continue past next week, flower enhancing sprays are probably not needed.  The exceptions could be trees with a heavy crop and/or weak root systems due to high water levels this past summer and fall.  If a warm period that is forecast to last 5 or more days with maximum temperatures above 75-80 degrees F does occur before January 10th then with adequate soil moisture for growth, 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 in the first 3 to 4 days at the beginning of the warm period. These products apparently increase the stress level and enhance the amount of flowering induced by the cool temperatures.

Have a Merry Christmas and New Year.  The next advisory will be the first week of January.

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


1/05/2011

FLOWER BUD INDUCTION ADVISORY #4 for 2010-2011-1/5/11

This is a fortnightly 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

Current Status: Happy New Year and Surprise-Surprise. While we celebrated the holidays, apparently Mother Nature was busy.  The Flowering Monitor Model indicates that a first wave of flower buds was initiated to grow between December 17th and 28th depending on the location.  The inductive cool hours had reached 800 to 940 in southern areas and 1000 to 1250 in central and northern areas of Florida’s citrus industry.  The full bloom dates for this first wave of flower buds is about February 24 to 25 in southern, flatwoods areas and March 2 to 7 for central and northern areas.  Since that time another 300 hours have accumulated so that hours of inductive temperatures have now reached 1100 to 1500 hours from South to North in the citrus belt.  Induction levels are now good to excellent for all citrus areas 

The initiation of bud growth has occurred with temperatures in the 70 to 80 o F range lasting 6 to 7 days, often with a couple of days in the 60s. This doesn’t seem to be a strong initiating warm period for bud growth, and therefore I expect another wave of flower buds to initiate growth when the next warm period occurs.  The one exception may be the south western citrus area around Immokalee.  This area had 10 days from 68 to 79 o F and will have warmer weather this week.  All other citrus areas will probably respond to the next warm period, which isn’t predicted for the next 8 to 10 days.

Some growers still want to apply urea or phosphorous acid to boost flowering.  Trees most likely to respond are on shallow soils where root systems are usually compromised.  To obtain enhanced flowering from one of these sprays in the south western areas requires immediate application.  All other areas should wait until several days in the 70 o F range are predicted by the weather service and the spray should be applied at the beginning of the warmer period. With the high levels of cool temperature induction, I question whether any benefit will occur. For more details regarding these sprays, see the last advisory (#3).

As a reminder about damage from the recent freezing temperatures, remember if damaged leaves persist after a freeze, it indicates that the green tissue of the shoot was also damaged.  The associated buds will likely be lost.  If damaged leaves quickly abscised, the buds should be ok and will grow normally and most of these buds will be flower buds this year.  An assessment of how much reduction in crop may occur next year may be gained by evaluating how much of last year’s spring and summer shoots appear to be damaged based on non-abscising dead leaves.  Actual damage is usually not clear until the spring growth starts and is or is not sustained by the supporting wood below the buds.

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


1/18/2011

FLOWER BUD INDUCTION ADVISORY #5 for 2010-2011-1/18/11

This is a fortnightly 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

Current Status:  The Flowering Monitor Model indicated that a first wave of flower buds was initiated to grow between December 17th and 28th depending on the location.  The inductive cool hours had reached 800 to 940 in southern areas and 1000 to 1250 in central and northern areas of Florida’s citrus industry.  The model now indicates that a second wave of flower buds has been initiated to grow in all areas from Sebring and to the south with 1030 to 1220 hours below 68 o F. The full bloom dates for the first wave of flower buds are about February 22 to 25 in southern, flatwoods areas and March 1 to 8 for central and northern areas.  The second wave will bloom about March 4th to 11th. 

Induction levels are now excellent for all citrus areas and raise the possibility of too many flowers on mandarin and seedless cultivars such as navel and Ambersweet, particularly if the current crop was low.  For areas south of Sebring it may be too late to reduce flowering levels with a GA3 (gibberellin) spray, but for areas north of Sebring this week may be an excellent time to prevent more flowering.  Temperatures are projected to be in the mid-70s for 3 to 4 days.  This will likely cause initiation of more flower bud growth where only one wave has started so far.  A single spray of 20 ppm in 80 to 125 gal of water per acre is the recommended rate.  Pro-Gibb will have an active ingredient (ai) rate/acre on the label.

Some growers may still want to apply urea or phosphorous acid to boost flowering.  Only trees north of Sebring may show enhanced flowering from one of these sprays but application would be required this week while temperatures are in the mid-70s, however a response is less likely since induction levels are fairly high. For more details regarding these sprays, see advisory #3.

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


2/3/2011

FLOWER BUD INDUCTION ADVISORY #6 for 2010-2011-2/03/11

This is a fortnightly 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

Current Status:  The Flowering Monitor Model indicates that in addition to a first wave of flower buds with growth initiated between December 17th and 28th a second wave of flower bud initiation occurred for locations south of Lake Alfred, both Ridge and Flatwood Districts, but no second wave of flower bud development is indicated by the model for Lake Alfred or northern locations. The inductive cool hours were 800 to 940 in southern areas and 1000 to 1250 in central and northern areas of Florida’s citrus industry for the first wave of growth.  The second wave of flower buds had growth initiated after 1030 to 1220 hours below 68 o F. The full bloom dates are about February 19 to 27 in locations from Sebring south including the flatwoods. Central and northern areas should have full bloom March 3 to 7.  The second wave will bloom about March 1st to 9th, 2 or 3 days earlier than the model projected in January. 

Induction levels are now excellent to excessive for all citrus areas.  The buds are breaking at Lake Alfred.  Pinhead flowers could be seen this week on navel and Hamlin trees.  Buds are pushing on some shoots in Valencia and grapefruit.  No bud push was observed yet on mandarin trees, which usually bloom 10 to 14 days after oranges. 

The projected bloom dates would be delayed if cooler than normal weather occurs the rest of February.  In another two weeks we should be able to tell if a second wave of flower buds was induced, but the model has been fairly accurate in projecting number of flower bud waves that will grow in a spring season 

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


2/15/2011

FLOWER BUD INDUCTION ADVISORY #6 for 2010-2011-2/15/11

This is a fortnightly service to our citrus growers posted on the CREC website.  The internet Expert System on intensity and time of bloom is not functioning due to loss of the server on which it was housed. If it becomes accessible it should be available at: http://orb.at.ufl.edu/DISC/bloom

Current Status: According to the Flowering Monitor Model there has been no change in the predicted bloom dates.  An earlier warm period was off-set by a cooler than normal period according to the model. The inductive cool hours were 800 to 940 in southern areas and 1000 to 1250 in central and northern areas of Florida’s citrus industry for the first wave of growth.  The second wave of flower buds had growth initiated after 1030 to 1220 hours below 68 o F. The full bloom dates projected were about February 18 to 27 in locations from Sebring south including the flatwoods. Central and northern areas had full bloom projected for March 3 to 7.  The second wave was projected to bloom about March 1st to 9th, 2 or 3 days earlier than the model projected in January.

Observations in the field indicate that the projected bloom dates are too early as the most advanced buds still have a few tight white flowers, but most are still pin-head or smaller.  The Ft. Pierce and Immokalee projections were for full bloom in 2 to 3 days, which can’t happen at the present stage of bloom.  The weather service is projecting a warmer 7 days, which should accelerate flower development.  Full bloom in some areas could be late February or early March, but not this week.

Flower buds are now inflorescences with many new stems of 2 to 3 cm length. In trees observed locally, pin-head flowers, usually with leaves, were observed at 4 to 6 nodes.  Some of the inflorescences had as many as 5 flowers and leaves.  Developing leaves associated with the flowers indicates a better chance for set.  In judging how good a bloom will occur on your trees, remember that in most years at least 80 % of the flowers occur in the first 4 buds of Hamlin or Valencia orange trees.  If you find on average that 6 or more leaf axial buds are flowering, then you can expect a better than average flowering intensity.

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