Balance is Key to Managing Alternate Bearing Cycles

Unlike deciduous fruit trees that have defined seasons and go dormant annually — avocado trees are subtropical/tropical fruit trees that never go dormant. They are always growing and developing — even in winter. Consider a deciduous apple tree. It buds in spring, the fruit matures in summer and is harvested in fall and the trees go dormant for the winter season.

In contrast, it takes two years to develop an avocado fruit from bud break of a vegetative shoot to harvest. The amount of fruit harvested in the current year is a result of cumulative weather and cultural management influences in the preceding two years.

Because an avocado tree never goes dormant, there are three cycles occurring simultaneously on the tree. The tree is simultaneously:

• Developing the current year’s fruit — providing nutrients and resources that mature the fruit for harvest.
• Supporting the needs of the year two fruit set that will be harvested the following year.
• Producing new shoots and buds that will produce either vegetative or floral buds for the year three harvest.

That means branches in one part of the growth cycle are competing for resources (nutrients, water, sun) with branches in other parts of the cycle. Fruit and flowers require a lot of resources (sugars, starch, water, minerals and chemicals) from the tree. If there is an abundance of fruit or flowers (on-crop cycle), the tree may shed some fruit and may conserve resources by inhibiting the growth of new shoots, flowers, fruit set or roots. Fewer new shoots on the fruit-heavy tree can lead to an off-crop the following year because avocado trees only flower, set fruit and bear fruit on peripheral new shoots.

Consider this example:

Two farmers each have a tree that will produce 400 fruit this year. Grower A’s crop will be followed by an off year. Grower B’s crop will be followed by another crop of 400 fruit. Why? Grower B’s tree has 400 shoots without fruit on them. These will produce summer and fall vegetative shoots and a strong bloom with determinate floral shoots. In contrast, Grower A’s tree has very few shoots without fruit. That means his tree doesn’t have enough fruit-free shoots to produced vegetative shoots in the fall and spring.*

Achieve balance with cultural management         

The following cultural management techniques can help balance shoot growth, fruiting and root growth.

• Monitor your trees and take notes — keep records of what you see and what you do. Determine when different growth cycles are primary — when root, shoot or fruit growth is active — and base your cultural management decisions on those cycles and their needs.
• Provide the right nutrients at the right times. Avocado trees have a variable demand for nutrients depending on where the tree is in its cycle. To determine the exact amounts and times of applying fertilizers monitor your grove, take notes and use a specialist who will perform soil, leaf and water tests. The specialist will help determine nutrient amounts based on fruit load, cropping history, growth cycles, soil/leaf/water tests, and existing regulations. Remember — irrigation water contains nutrients as well and this must be factored into irrigation strategy.
• Provide the right amount of high quality water. Avocado trees don’t search for water and their roots are only 6” under the soil, so it’s important to provide the right amount of water at the right times.

Establish a pruning system that balances fruit set with new shoots. Pruning removes wood with flower buds, and thus potential fruit, so yield from pruned trees is nearly always less than yield from non-pruned trees — but fruit quality is improved by pruning. Pruning improves fruit size by increasing the amount of leaf area per fruit. Pruning also  improves light distribution throughout the tree, which is important for the development of fruit. Prune to increase shoot numbers and increase the complexity of branching. This will increase floral shoot numbers and yield.

* Lovatt, Carol J. Alternate Bearing of 'Hass' Avocado. California Avocado Society 2010 Yearbook, 93:125-140.