Managing Trees as a Collection of Spurs in a Drought Year

Understanding the biological processes by which almond trees make fruiting and vegetative selections is critical to managing water in a drought year. Stress at the wrong time can have an impact on future years’ yields, UC Davis plant sciences Professor Ted DeJong told almond growers at the Nickels Field Day in May. The strategic deployment of water is key to ensuring enough photosynthetic energy is available at the right times of year to maintain shoot and fruit growth.

Late-season irrigation water is especially vital to next year’s shoot and fruit growth as individual spurs differentiate between vegetative and fruiting buds during late summer and early fall. Trees can tolerate moderate stress during this period with little effect on future yields, but severe stress dramatically reduces development of fruiting buds, which reduces bloom, fruit set and yield the subsequent season.

Dr. DeJong also advised growers to prevent trees from completely defoliating during the summer. The tree needs to store some carbohydrate energy for growth in the following spring. Stored carbohydrates fuel growth during the 30–45 days beyond bloom. Without leaves on the tree and the energy (or carbohydrates) provided by photosynthesis, the tree’s ability to grow in the current year and store carbohydrates for future use will be diminished.

The other consequence of severe stress is the death or lack of growth of almond spurs. Almond Board–funded research on spur dynamics led by UC Davis almond specialist Bruce Lampinen found that on average between 10 and 15% of the total number of spurs in a tree die annually, largely due to shading in the lower canopy. Higher-than-average spur loss — given the drought stress that many orchards are under this year — will have negative consequences on future yields if new growth to replace dying spurs has not been maintained in the orchard.

Also at the field day, pomology consultant Wes Asai explained that typical almond spur renewal is a three-year process from new vegetative growth to blossoming, with blooms primarily occurring on third-year wood. Anything that imposes a major stress on the tree will negatively impact the renewal cycle. Whether the stressor is severe drought, nutrient deficiency, disease outbreak or chemical injury, two to three years are required for the regrowth of significant replacement spurs that can bear nuts.

However, Dr. DeJong cautioned growers not to take the idea of promoting spur replacement after a high-stress, high-spur loss year too far. He explained that, in general “strict topping, if the tree is healthy with adequate nitrogen, will cause a witch’s broom at the top of the canopy and accelerate spur death in the lower canopy.”

Additionally too much nitrogen will cause a similar explosion of canopy growth, shading out the interior and expediting lower-canopy decline. Careful management and balance of spur growth will be required to maintain yields following the drought stress that many orchards are experiencing this year.