FLOWER BUD INDUCTION ADVISORY #1 for 2004-2005-11/1/04
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
Overview of flower bud induction in Florida – It is time to start following citrus flower bud induction conditions for the coming year's bloom. Low temperatures first stop growth and then promote induction of flower buds as more hours of low temperatures accumulate. A period of 5-12 days 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 a weak to moderate El Niño year, cooler and wetter than normal. Even if this winter is only slightly cooler than normal, enough hours of low temperatures below 68 degrees F. should accumulate to induce a moderate to good level of flower buds.
Under normal Florida conditions, sufficient flower bud induction should be achieved when total uninterrupted, accumulated hours of low temperatures exceed 850 hours below 68 degrees F. if the current crop is heavy. If the crop load is light, sufficient flower bud induction can occur after 750 hours of accumulated low temperatures. A warm period of 7 to 12 days, with maximum temperatures > 75 to 80 degrees F., after some low temperatures have accumulated can trigger growth (bud swelling). Fewer days of higher temperatures and lower daytime highs are required to stimulate growth if the accumulated cool temperature hours are high later in the winter. Current and previous seasons weather information is available on the Florida Automated Weather System (fawn.ifas.ufl.edu) for locations near you. The 8 day forecast from the National Weather Service predicts Florida weather for several sites around the citrus belt and is linked to http://www.crec.ifas.ufl.edu/crechome/crecweather.shtml
Some flower buds will be induced in the range of 300 to 600 accumulated hrs < 68 degrees F. Warm events after these levels of induction result in weak flowering intensity, and therefore many buds remain that can be induced by later cool periods. This situation results in multiple blooms. During the years from 1963 to 2002, multiple blooms occurred in over half of the years. Historically, the time period in which a warm 7-12 day period can lead to some bud growth and then result in multiple blooms is roughly Thanksgiving to Christmas. Presently, the only management tool available to eliminate or reduce the chance of multiple blooms is to promote water stress by stopping irrigation before these predicted warm periods occur. If the warm periods(s) are of the typical 7 to 10 day duration, mild water stress will have little impact on current crop development or quality. Mild water 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 mild stress condition of citrus trees, buds will not grow in response to high temperatures. If an extended warm period has passed, trees again can be watered to minimize current crop water stress. Although no weather prediction is guaranteed, rains in the winter usually come on the fronts of cool periods. Therefore, the chances of being able to use water stress to prevent an early flower bud differentiation event is reasonably good for most warm periods. A difficulty that occurred 3 years ago, which resulted in a very small crop, was that daytime high temperatures were continuous through the fall until December 18 th . If trees were allowed to be water stressed for this extended a period, this could lead to low photosynthesis, little fruit growth or sugar accumulation and probably excessive fruit drop.
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 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. This may be risky for ‘Hamlin' or other early maturing cultivars not yet harvested 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'.
(Figure 1, with added information in color or bold lettering, is from the ‘Flowering Expert System for Florida Citrus' and represents the different aspects of apparent flower induction.)
The level of potential flowering would be greater (orange line) with a light crop or less (green line) with a heavy crop for the same amount of hours of induction. 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. This system will be tested again this winter with several growers that helped us refine the system the past two years. Future advisories (usually weekly) will update accumulating hours of related temperatures and other weather effects on flower bud induction plus methods for enhancing or reducing flowering intensity as conditions and cultivars dictate. Read the archived advisories from previous years (link at top of page) for more background.
Previous 3-year's results – In the winter of 2001-2002 cool temperature accumulation was very slow, warm temperatures persisted and many buds started to grow by 20 December, heavily to vegetative buds. This resulted in few buds remaining for a second flowering wave and a small crop occurred. In the winter of 2003-2003 by late December we had 850 hours of uninterrupted cool inductive temperatures with a low current crop on the trees. The following warm period initiated almost all the buds on all of the spring and summer flush to differentiate and 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, a good fruit set occurred in other round oranges resulting in the highest October crop forecast for Florida that FASS has ever predicted. Last year (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 somewhat. Even though fruit size was small, it looked like we were headed for a mid-200 million box orange yield before the hurricanes.
The new season's situation – If you have citrus that was not in the path of the hurricane, two years of heavy to moderately heavy crops means that a high level of induction is desired to produce adequate flower buds for next year's crop. If the block was in the path of one to three hurricanes, then many scenarios exist and most of my conclusions are guess work, but here they are:
If you lost fruit but not many leaves, the trees may need less inductive temperatures, but fruit losses were after most of the summer drain on carbohydrates had occurred. If fall temperatures stay in the mid-70s to low 80s, trees may build up carbohydrate reserves under a low crop situation providing good bud condition for flower induction.
If many leaves were lost along with the crop, then trees may have fairly low carbohydrate levels and need high levels of inductive temperatures to produce a good bloom.
If heavy fruit and leaf loss was followed but many buds stimulated to flush this fall, then fewer buds are available on last year's spring and summer flush for flowering. However, if these fall flushes matures sufficiently before bud growth stimulation occurs (usually in early to late December), then their buds can become flower buds. I think that 2 to 2 ½ months for flush development may be necessary for this to happen.
Therefore, best results this fall-winter may come from reasonable development temperatures until mid-November followed by above average cool temperature accumulation (> 850 hrs).
So far this Fall, little cool weather has occurred to slow down or stop vegetative growth on mature trees, only about 110 to 160 hr < 68 degrees F., from southern to northern districts. Also, the National Weather Service (NOAA) predicts that there will be about 80 more hours below < 68 degrees F. during the next 8 days. To view specific FAWN data for a location near you in the citrus growing areas, use (www.crec.ifas.ufl.edu) and click on FAWN or for NOAA's 8 day forecast go to Weather Links on our CREC homepage and then 8 day forecast.
The major concerns for the next 60 days are the possibility of 1) continuous warm weather that will push vegetative buds to grow as occurred 3 winters ago or 2) an extended warm period, 10 to 12 days with max. temperatures > 75 to 80 degrees F, following an inductive period of 300 to 500 hrs < 68 degrees F that will initiate differentiation of easily induced flower buds. The first condition will lead to low flowering and the second to multiple blooms. By next week the 200 to 250 total hours will be ½ way to a weak bloom potential. Continued accumulation of cool temperatures or prevention of growth during a winter warm spell are important to a good start to the 2005-06 harvest season. Therefore, keep irrigation amounts low to moderate (if fruit are still present) to minimize growth possibilities. Prepare to keep groves relatively dry, keep track of induction hours in your area and watch for the next advisory.
( Request for potential cooperators) – Although rains this past winter negated attempts to use water stress to delay bloom in many areas and collection of meaningful yields was severely disrupted by the hurricanes, we are still interested in tests to delay bloom by managing irrigation to delay initiation of flower bud growth. This might be accomplished by using water stress to prevent growth during warm winter periods until mid-January has passed. If you are interested in putting a block or a few rows of grapefruit, ‘Hamlin' or ‘Valencia' trees under this protocol.