Evaluation of flowering and fruit set on almond allows for within-season assessment of orchard productivity; however, understanding the vegetative growth dynamics of almond allows growers to consider parameters affecting productivity years into the future.  Vegetative growth of almond has two main components: vegetative shoot growth and spur production. Vegetative shoot growth provides the overall architecture of the canopy, and spur production generates the tissues that give rise to the majority of fruit in subsequent seasons.  Both vegetative shoot growth and spur production are key components to the development of an economically sustainable and productive orchard.

Timing of vegetative growth.  All buds (vegetative and flower) are formed during the prior season. Because almond has one of the lowest chill requirements of permanent crops grown in California, the chill requirement is generally fulfilled by January 1. As temperatures increase, growth initiation is induced, and bud break ensues, with flower buds breaking in advance of vegetative buds. Vegetative shoot growth proceeds at a somewhat uniform rate throughout the season on young trees, but the durationof spur elongation is short and generally complete by April or early May.

Vegetative buds. On almond, vegetative buds can be distinguished from flower buds by shape. Flower buds are thick and oval; vegetative buds are pointy and triangular. On shoots, flower buds are generally formed on either side of a vegetative bud. On spurs, the apical bud is always vegetative (Figure 1A), and this bud can give rise to either further spur growth (Figure 2) or a vegetative shoot (Figure 1B). Spurs in positions with high light interception are more likely to give rise to vegetative shoots than new spur growth. 

Vegetative shoot growth. Vegetative buds may give rise to long vegetative shoots that support future spur production. During the early years of orchard establishment, long shoot growth is the main component of vegetative development on almond (Figure 3A, B, C). On mature trees, vegetative shoot growth occurs under conditions of low crop, high vigor, and in regions of the canopy where there is excessive light interception. Canopy regions with excess light include external/exposed areas and empty spaces resulting from broken limbs.

Spurs. Spurs are short, compact vegetative shoots, approximately 0.5-2 inches long (Figure 2). Spurs arise on vegetative shoots or on spurs produced in the prior season (Figure 2). Within a season, the duration of spur growth is generally short, with spur extension completed by April or early May. Spurs are always formed on the prior year's wood, and remain vegetative for 1-2 years prior to flowering. As a consequence, the process from vegetative shoot growth to spur production and flowering may take 4 seasons.

Spurs support approximately 80% of the total almond yield in a given year, yet only about 20% of the total spur population on a tree supports nut production each year.  The fact that only 1 in 5 spurs bear fruit in a season is explained by the dynamic status of spurs between years. A portion of spurs remain only vegetative in a given year (Figure 1C), whereas others may support 1-5 flowers that may develop into single fruit-bearing spurs or multiple fruit-bearing spurs (Figure 1D). Due to the reliance on a localized carbon economy, individual spurs tend to alternate bear, meaning that spurs that bear fruit one year tend not to flower or bear fruit the following year.

Comprehensive view on vegetative growth. In new almond plantings (Figure 3A), growers should expect the mainstay of vegetative growth to be production of long vegetative shoots (Figure 3B and C). Although the majority of the future crop is produced on spurs, it will take time for bearing spurs to be represented in the canopy.  Consider that spurs are produced on the prior year's wood and will remain vegetative for 1-2 years before entering productivity. Patience is needed as these vegetative spurs store carbohydrates to support future nut development.

Select References

Tombesi, S., Lampinen, B.D., Metcalf, S., DeJong, T.M. 2016. Yield in almond related more to the abundance of flowers than the relative number of flowers that set fruit. California Agriculture 71: 68-74. Online: https://doi.org/10.3733/ca.2016a0024

Lampinen, B.D., Tombesi, S., Metcalf, S.G., DeJong, T.M. 2011. Spur behavior in almond trees:  relationships between previous year spur leaf area, fruit bearing and mortality. Tree Physiology 31:  700-706. Online:  https://doi.org/10.1093/treephys/tpr069

Kester, D.E., Martin, G.C., and Labavitch, J.M. 1996. Growth and Development. In: W.C. Micke, Editor, Almond Production Manual. Oakland, California, University of California, Division of Agriculture and Natural Resources (pp 90-97).

The California Department of Water Resources and the University of California Cooperative Extension have teamed up to provide Weekly ET Reports to agricultural water users to assist with irrigation scheduling. The reports include water use information for a variety of crops including almonds, pistachios, walnuts, grapevines, citrus, and stone-fruit of mature bearing age. Adjusted on a weekly basis, water use estimates account for the changing growth stage and weather conditions at the Madera, Parlier, Lindcove, Stratford, Panoche, and Five-Points CIMIS weather stations. Each report gives crop-specific evapotranspiration (ETc, total crop water use including soil evaporation) estimates for the previous and coming week.

Beginning the Irrigation Season

One of the objectives of the weekly ET report is to help managers decide when to initiate the first irrigation.Irrigating too much and too early can lead to reduced growth and yield due to loss of fine feeder roots as well as root disease. Stored soil moisture from winter and early spring precipitation will reduce the need to irrigate early in the season. As ET rates increase and the seasonal crop water use exceeds accumulated rainfall, water managers should consider if there is adequate soil moisture to supply the difference or to begin to irrigation. The weekly ET report provides “Accumulated Precipitation” since January 1^st and the “Accumulated Seasonal Water Use” since leaf-out. When water use exceeds precipitation, it may be time to irrigate. Irrigation decisions should be confirmed with field monitoring of soil moisture and plant water status.

Irrigation Frequency and Duration

Managers need to know the application rate of their irrigation system in either inches/hr or gallons/hr and the effective wetted volume (surface wetting and sub-surface lateral subbing) of the root zone as a % of the orchard floor (Figure 1).Different soils will have larger or smaller wetted zones as soils with different texture hold different levels of moisture. This will influence when seasonal irrigation should begin, the appropriate set duration, and frequency of irrigation events throughout the season.

Example: If the effective wetted volume for a sandy loam soil is 25% of the whole orchard floor and crop ET for the week is given as 1.5” (0.21”/day), then the moisture extraction from the wetted area for the week = 1.5”/25% = 6”. A sandy loam soil at field capacity that has approximately 0.8 inches plant available water per foot of soil would have 5” of available moisture in a 5-foot rooting zone. The weekly water use (6” every 7 days) will exceed soil plant available water in about 5 days. In this example, irrigation sets need to be more frequent than once a week, roughly every 3-4 days. The application time for a system with one gallon per hour (gph) emitters that deliver about 1”/day would be calculated as:

Application time = (0.21”/day ET * 3 days)/ 1”/day irrigation * 24 hr/day = 15 hours every 3 days

Irrigation recommendations for common crop spacings are shown in the report in units of inches or gallons of water needed per week. To convert inches per tree to gallons:

Gallons = (Inches * 27,154 gal/acre inch water) / # plantings per acre  

Adjusting for System Efficiency

Different irrigation systems can vary greatly in efficiency. Those with high uniformity such as drip micro-irrigation, are roughly 80-95% efficient and require less output to meet crop needs than other systems like flood-furrow, with efficiencies as low as 50%. The recommended amount of water to apply is corrected for a range of irrigation system efficiencies. If the mature almond orchard in the example was 90% efficient, you'd find the 90% column in the second table of the Weekly ET Report, and put in the almond value at 90% (1.67 inches or 0.24 in/day) into the equation above, instead of 1.5 acre-inches.

If you would like to receive weekly reports, have questions or need more assistance contact:

Mae Culumber UCCE Fresno County Nut Crop Advisor, cmculumber@ucanr.edu, 559-241-7526

Elizabeth Fichtner UCCE Tulare County Orchard Systems Advisor, ejfichtner@ucanr.edu, 559-684-3310

Phoebe Gordon UCCE Madera County Orchard Systems Advisor, pegordon@ucanr.edu, 559-675-7879 George Zhuang UCCE Fresno County Viticulture Advisor, gzhuang@ucanr.edu, 559-241-7506

Steve Ewert, California Department of Water Resources, sewert@water.ca.gov, 559-230-3334