CULTURAL TIP: READING YOUR TREES DURING WINTER
Now that Halloween has passed, the weather has taken a distinct turn to the cool side and there has even been some rain. At a time like this, the avocado grower should be looking at their trees in order to evaluate what is likely to happen in spring. It used to be thought, that for avocado trees, winter was a period of not quite dormancy, the trees are evergreen subtropicals after all, but quiescence as the trees waited out the cooler weather until spring where they could start to grow again. There is more and more evidence that for avocado trees winter is not a quiet time at all. A great deal happens within the tree over the next few months, which will become apparent in how well the trees flower and set fruit in spring. Over winter the trees can develop flower buds, continue to size fruit, the fruit accumulate dry matter, roots can grow and some types of sugars can accumulate. The result is that the weather at this time of year and over the next few months can strongly influence the way the trees develop flowers and are set up for fruit set in spring.
Quite a bit of the development of flowers within the flower buds occurs over winter. If the winter has warm spells some flowering and fruit set can even occur. This says that despite the cooler weather, the trees are primed to be ready for flowering and fruit set should conditions be right. There is also a time course of development for the flower buds on shoots that is influenced by when the shoots grew in spring, summer or fall and internal tree factors. The latest research from California and grower experience tells that where there is a heavy crop on the trees, the flowering in the spring is often less than what is needed for a good fruit set. It is not so much that the tree does not have the flowers, but rather the research suggests that while the flowers are formed and start developing over winter, the flower buds never break in spring.
Once the trees are into winter there is not much that the grower can do to influence how many flowers there will be in spring. The hard work needs to have gone into the trees before winter sets in. The most easily recognizable issue for the grower is to ensure that the tree grew enough of a new flush in summer so that there is the potential for good flower numbers in spring. A very useful skill for a grower is being able to look at a tree and have a good feel for how much new growth there is that will be the flowering wood in spring. Cultural management practices a grower can undertake to help with the number of flowers depends on the root health of the tree, the type of shoot growth, crop load on the tree and the tree nutrient status. Trees with healthy root systems and strong shoot growth are candidates for girdling in fall to force some flowering on branches that may not flower at all. Trees with very heavy crop loads should be targeted for early harvest once fruit maturity is reached. Above all the tree must grow some new shoots in the summer if it is to have a decent chance of flowering with a good chance to set fruit.
By the end of November it should be possible to see flower buds on the avocado trees and to follow the change in shape and size of the buds through to flower bud break. The buds to look for can be found where the leaf stalk is attached to the shoot and start at the tip of the shoot and go back down towards the main branches. Vegetative buds are small and triangular in shape while flower buds are round and plump. Most of the flower buds are clustered at the tips of shoots and in the joints of the first five to six leaves on a shoot. Very often the most flower buds are found on the warmest and sunniest parts of the tree. If the trees are tall it will not be easy to see where most of the flower buds are, as flower development is likely to be concentrated at the tops of the trees. Small trees, well exposed to light all around, are more likely to develop flower buds all over the tree and it is easier to see how many flower buds there are.
Having a good idea how many flower buds are developing and the progress of flower bud growth are useful pieces of information for growers in their decision making. By determining if flowering will be stronger than expected a grower can look to alter harvesting decisions of when to remove fruit from the trees. If the crop is heavy the fruit will compete with the flowering and new fruit set. There may be a need to introduce bees earlier and in greater numbers than first anticipated. When flowering and fruit set is excessive the trees can fail to grow enough new fruiting wood resulting in a poor return crop. It is possible there could even be a need, should the flowering look to be excessive, to prune trees to better balance flowering to shoot growth for the following year.
For a grower, following flower bud development through winter can be part of a proactive cultural management process. It allows the grower to anticipate problems and opportunities that can be corrected. Careful observation of the growth and development occurring on avocado trees, even during times of the year when the trees appear to be doing little, can be used to indicate what cultural management activities are needed for the best yields.
