Irrigation and stressing trials have helped develop the best strategy to synchronise flowering. This strategy requires a reliable dry period of two months between September and December and an efficient irrigation system (preferably drip) over which you have precise control. It is an advantage if the irrigation system can be controlled separately in different parts of the plantation to allow for differences in irrigation requirements between cultivars, tree ages and soil types.
Six stages of floral bud development from initiation to cherry set, the cherry in stage six is at the pinhead stage
Procedure:
1. Irrigate trees regularly during floral initiation and development from March to September i.e. while floral buds are in stages one to three (see above image of stage 1 to 3)
2. As flower buds start reaching maturity from August to October, make sure there are no large fluctuations in soil or plant moisture (see above image of bud stage 4) It is at this stage that any large changes in soil or plant moisture will trigger flowering.
3. Begin water-stressing by stopping irrigation around mid-October. Most floral buds should be in stage four at this time (see bud stage image above). Continue until you can see the first signs of wilting. If you have access to a pressure bomb, stress the trees until the leaf water potential reaches -2.5 MPa. This may take three to five weeks, depending on temperatures and soil type. Tensiometers cannot be used to determine this point because the soil is too dry for them to operate properly.
A well managed plantation (Catuai Rojo) two years after planting: seedling misses have been replanted, plants are single stemmed and uniform in size, ground cover and windbreaks are established and there is a 1m weed free strip around trees that have been purposely water stressed and rewatered to synchronise flowering.
4. Maintain trees at this level of stress for three to four weeks by applying one to two litres of water per tree every day with very short irrigations. Irrigate in the morning rather than at night so that the trees do not recover from wilting during the night. This small amount of water daily keeps the plants water-stressed but prevents them from becoming too dry. Water-stressing in this way is a compromise between stressing trees for a long time (to maximise flowering) and minimising tree damage by keeping the severity of stress low Minimising tree damage, especially leaf fall, is critical to maintain tree health. Maintaining tree health prevents dieback from over-bearing (see Coffee Tree Pests & Diseases article) and prolongs cherry life (see also Synchronising Coffee Cherry Ripening article), increasing the proportion of cherry that can be harvested at one time.
5. After eight to ten weeks of water-stressing, bring the soil water back to field capacity as quickly as possible by irrigating heavily for five to six days. Irrigating in the late afternoon will further improve the flowering response. Water-stressing for longer than ten weeks does not improve flowering and causes unnecessary tree damage.
This system of water-stressing and heavy irrigation has been used on the Atherton Tablelands and works very well when the stressing period is not interrupted by rain (see image above). When rain does interrupt stressing the course of action depends on several factors: how long the trees have been stressed, the degree of stress, the amount of rain, and the level of flowering likely to be triggered. The guidelines below outline action needed when rain falls at three different times during the stressing cycle.
Rain early in the cycle:
Drip irrigation system
If rain falls in the first three weeks of stressing, then it is likely to trigger some flowering. At this stage the trees and soil will still have enough moisture to allow these flowers to open and develop fully, so do not rewater the trees. Continue stressing as described above.
Rain in mid-cycle:
If rain falls four to six weeks after stressing begins, it may or may not trigger flowering, depending on the amount of rain and how stressed the trees are. You can determine how much flowering is likely to be triggered by observing how many flower buds start to enlarge two to three days after the rain. If little or no flowering is triggered, do not rewater, and continue stressing the trees as described above. This may occur if there was less than three millimetres of rain and the trees were starting to wilt. If the rain seems to have triggered heavy flowering, you may need to partially rewater the trees for two or three days. If trees are not partially rewatered, the soil and plants will not have enough moisture to allow the flowers to develop properly and little cherry will set. The aim in rewatering at this stage is to provide enough water for the partial flowering but little enough so that the trees can be stressed again quickly to trigger flowering of the remaining flower buds. To achieve this, partially rewater the trees by wetting the top 10 to 15 cm of soil. After rewatering, continue stressing until trees reach wilting again, then rewater fully to trigger the remaining flower buds. In this case there will be two flowerings separated by three or four weeks.
Rain late in the cycle:
Flowering can be delayed to move period of cherry expansion
If rain falls seven to ten weeks into water-stressing, stop stressing and fully rewater the trees as described above. Nearly all flower buds should flower after this length of stress. The flowering response is usually very good if the stressing period is brought to an end by heavy rain.
The overall aim of water-stressing to synchronise flowering in North Queensland is to trigger most of the flower buds on the trees to flower over the shortest possible time. This may require a series of stress cycles if rain falls during stressing. By delaying stressing, flowering can be delayed until mid-November to December. This moves the period of rapid cherry expansion (7 to 17 weeks after flowering) to the wettest months of the year, which will help improve bean size. As well, cherry ripening is delayed until the coolest, driest months of the year (June, July and August), which greatly improves cherry life, helping to synchronise cherry maturity. Choose late-maturing cultivars such as Catuai Rojo or Mundo Novo to help delay flowering and maturity.
Source:
James Drinnan
