The dynamic state of spurs in almond
Elizabeth J. Fichtner, UCCE Farm Advisor, Tulare and Kings Counties, and Bruce Lampinen, CE Specialist, UC Davis
Over 80% of the almond crop is borne on short, compact vegetative shoots called spurs. Each season, however, only a portion of the spur population on a given tree supports fruit production. Because of their role in supporting productivity and yield, maintenance of a healthy spur population contributes to the economic sustainability of an orchard. Understanding the dynamic states of spurs between seasons and the conditions promoting spur productivity and survival may enhance orchard management practices to maintain or increase yields in future years.
What are spurs? Spurs are short, compact vegetative shoots (approximately 0.5-2 inches long) that are borne on the prior season’s wood. Spurs are either formed from lateral buds on vegetative shoots (Figure 1A) or from vegetative buds on spurs (Figure 1B). When spurs give rise to further spur growth over sequential years, it may be difficult to visually evaluate the age of a spur due to the compact nature of growth (Figure 2). The apical bud on a spur is always vegetative (Figure 1B); however, spurs can also support up to 6 flower buds in a season (Figure 3B). The duration of spur growth on almond is short and generally complete in April or early May.
Spurs exhibit a localized carbon economy. Spurs are considered semi-autonomous with respect to carbon supply, meaning that spurs serve as both the main source and sink of carbohydrates utilized in vegetative and reproductive growth. As a result, spurs remain vegetative (Figure 3A) for 1-2 years prior to flowering. Although not immediately productive, vegetative spurs with adequate leaf area produce and store carbohydrates for support of future flowering and nut development. In fact, the leaf area of spurs is a better predictor of potential for flower bud development than the number of leaves per spur. Spurs with less than 10 cm2 leaf area are unlikely to support viable buds (floral or vegetative); spurs with 10-12.3 cm2 leaf area are likely to support only vegetative buds; and spurs with >12.3 cm2 have a higher probability of supporting flower buds. Due to the carbohydrate demand of setting fruit, few spurs flower the year after bearing.
Spur leaf area influences flower bud development. Flower buds can be differentiated from vegetative buds by both shape and position. Flower buds are generally positioned on either side of a vegetative bud on shoots (Figure 1A), or in lateral positions on spurs (Figure 1B). Vegetative buds are triangular and pointy, whereas flower buds are thicker and more oval than vegetative counterparts. In early summer, buds manifest in leaf axils, but it is impossible to differentiate between floral and vegetative buds until late August or early September. Even in late summer, identification of flower versus vegetative buds may require bud dissection and microscopy.
Flower bud development does not proceed at a uniform rate in a given block or tree, but varies dramatically between spurs. The rate of floral bud development is positively related to leaf area. Consequently, spurs supporting high leaf areas exhibit more rapid flower bud development than spurs with lower leaf areas.
Prior year spur leaf area affects flowering and nut set. Spurs supporting high leaf area in a given year have enhanced potential to support flowering and nut set in the subsequent year. In fact, non-bearing spurs with >50 cm2 leaf area have over an 80% probability of flowering the following year. Non-bearing spurs in lower light positions in the interior of the canopy may require more years in a vegetative state prior to supporting flower and fruit production.
Spur survival is influenced by prior year leaf area and exposure to light. The literature suggests that spurs remain viable for 3-5 years; however, the survival potential of individual spurs is related to light exposure, bearing status, and prior season leaf area. We have found that spurs in well managed orchards in outer canopy positions can remain productive for more than 10 years. Regardless of bearing status, spurs with higher leaf areas are more likely to survive into the following season. Bearing spurs are more likely to survive into the following season when occupying light-exposed positions in the canopy. Conversely, the mortality of non-bearing spurs is generally not influenced by light interception in the canopy. These relationships are all explained by the reliance of spurs on a localized carbon economy.
Orchard management for enhanced spur survival and productivity. Following best orchard practices, particularly in irrigation scheduling and nutrient management, will allow for canopy development and maintenance of tree health. However, consider that practices supporting excessive growth may cause shading, which may be limiting to spur survival. Promotion of modest annual growth will allow for production of new spurs, but be patient because new spurs may take 2 years to support flowers. Last, when managing the tree canopy, overlapping branches and dead wood should be removed to prevent shading and promote spur survival and productivity.
Select References:
Lamp, B.M., Connell, J.H., Duncan, R., Viveros, A.M., Polito, V.S. 2001. Almond flower development: Floral initiation and organogenesis. Journal of the American Society for Horticultural Science. 126: 689-696.
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
Polito, V.S., Pinney, K., Heerema, R., Weinbaum, S.A. 2002. Flower differentiation and spur leaf area in almond. Journal of Horticultural Science and Biotechnology. 77: 474-478.
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