Fall and Winter Irrigation Management for Nut Crops

Mae Culumber-Farm Advisor UCCE Fresno

After all the nut harvests are through for the year, some managers may ask “how much water do trees need when leaves are still on the tree? How much do they need through the winter?”  Central Valley’s Mediterranean climate allows deciduous tree crops to hold on to their leaves until months after harvest, usually until the first hard freeze. With optimal conditions, transpiring trees will continue to photosynthesize, the process by which plants capture energy from the sun and carbon dioxide (CO2) from the air to produce sugars and carbohydrates that supply the building blocks for tree growth and next year’s crop. Maintaining a healthy canopy post-harvest is critical to building carbohydrate reserves and fruitful bud development, and many orchard managers have reported rapid defoliation following preharvest regulated deficit irrigation (RDI) and mechanical shaking. Regulated deficit irrigation (RDI) during hull-split in almonds and before harvest in all nut crops, depletes available moisture reserves and can cause stress to develop more rapidly during the post-harvest period. A post-harvest irrigation is typically applied immediately after harvest to relieve trees of stress. However, soon after, irrigation is often greatly reduced or ended for the season to reduce vigor and provide frost damage protection for the winter. As the weather cools tree water requirements will decrease, but fall is an important time to continue site specific, calculated irrigation decisions that factor in tree water stress (see UCANR pub 8503, Fulton and Buchner, 2014), the moisture in the soil profile, and either historical or real time evapotranspiration (ETc) rates (http://cekern.ucanr.edu/files/98834.pdf).

Almond, pistachio, and walnut may have different water needs after-harvest due to phenological differences in fruitful bud formation, the process that determines if a bud will flower or produce vegetative growth in the following season. It is well established that resuming irrigation in the post-harvest is critical for almond, as severe stress during this developmental stage has been associated with yield reduction in the following season (Esparza et a. 2001). In almond, bud differentiation occurs in the time before and after

Table 1. Monthly reference evapotranspiration (ETo) for five points based on a 30-year average from 1997 to 2007, with corresponding zone 15 crop coefficients and ETC rates for almond, pistachio, and walnut.

Month	30-year avg ETo	almonds Kc	almond ETc	pistachio Kc	Pistachio ETc	walnut Kc	walnut
	(5-Points)	(Zone 15)		(Zone 15)		(SJV)	ETc
January	1.34	0.4	0.54	-	-	-	-
February	2.17	0.45	0.98	-	-	-	-
March	4.18	0.69	2.88	0.11	0.46	0.12	0.5
April	6.12	0.81	4.99	0.45	2.76	0.61	3.71
May	8.07	0.93	7.46	0.93	7.46	0.84	6.78
June	8.79	1.03	9.05	1.17	10.28	0.97	8.48
July	8.9	1.08	9.57	1.19	10.59	1.14	10.14
August	8.21	1.06	8.72	1.17	9.63	1.14	9.36
September	6.4	0.99	6.33	1.01	6.46	1.03	6.56
October	4.41	0.83	3.64	0.82	3.63	0.7	3.07
November	2.22	0.62	1.39	0.56	1.25	0.28	0.62
December	1.28	0.4	0.51	-	-	-	-

harvest (depending on variety) and can extend several weeks afterward (Doll and Shackel, 2015). Looking at the 30-year average for ETc rates (Table 1), an almond block in the Five Points region would require 11.9” of water from September to December after a harvest in late August. Over half of the water loss occurs in September, the critical period for harvest stress recovery and fruitful bud differentiation.

Less is known about the late season irrigation needs for pistachio and walnut, as the main period of next seasons flower development is completed by June and July in mature trees. Pistachio briefly resumes fruitful bud formation in the fall, but little research has investigated the effect of water deficit on this development (Catlin, 1998; Ferguson et al., 2015). Research by Spann et al. 2008 found pistachios acquire the necessary reserves for kernel fill from carbohydrates accumulated in the current season. It is not clear what previous season carbohydrate storage has on next year’s crop, and what affect a late season water deficit has on these processes. In pistachio and walnut, Goldhamer and Beede (2004) found a 70 to 75% reduction in irrigation (Oct. 1 - Nov. 15 in the San Joaquin Valley) did not adversely affect Kerman pistachio yields in the following season. For many growers who adhere to an October 1st cutoff for irrigation, this would amount to a full deficit of 4.88” of water from October through November. Kerman, is usually harvested in the second or third week of September, close to the October 1st cutoff” and the impacts of withholding water at this stage may be minimal. Other earlier varieties like Golden Hills and Lost Hills are sometimes harvested as early as a month before Kerman and may be very susceptible to water stress if irrigation is either cutoff or applied at a severe deficit. The average ETc needs for Five Points pistachios is over 16” in August through September (Table 1), indicating early varieties may be particularly vulnerable to early irrigation deficit after harvest.

In walnut, controlled deficits are thought to help harden-off new shoots and decrease the susceptibility of these young shoots to damage by early frost. Conversely, extreme deficit may cause winter die-back on new growth as well as older wood in walnut trees. With a seemingly narrow margin between the risk of frost damage and winter kill, it seems fall is a critical time for attention to tree water stress as an indicator of the appropriate application of water. Tree water status measurements can be combined with soil moisture and ETc monitoring to make calculated decisions about when and how much water to apply. Dendrometers continuously monitor daily cycles of shrink and swell of the tree trunk in response to dehydration and hydration. Less water in soil and or increased ETc rates will cause the trunk to shrink more each day. The sensors also provide a trend of trunk growth through the season. Dendrometer tree water stress and tree growth monitoring in a UC Davis trial (brown trend line, Figure 1) resulted in an overall leveling-off of seasonal tree growth in mid-October. This may demonstrate a physiological shift from growth to storage of carbohydrates for the following year and may also indicate when it may be appropriate to pull back on irrigation for the season.

Preharvest RDI irrigation practices may not be the only cause of yellowing leaves and defoliation late in the season. Irrigation trials in walnuts (Lampinen, personal communication), found trees that received an early season irrigation with higher application rates throughout the season began to defoliate up to two full weeks prior to normally irrigated trees (Figure 2).

Figure 1. Dendrometer mean daily shrinkage and overall growth (brown line) from the end of September to end of October 2014 for Howard Walnut trees in a trial at UCDavis. The blue arrow indicates the point where tree growth levels off for the season.

Figure 2. Picture of late season (November, 2016) health and vigor of young walnut trees in a variable irrigation trial. Trees on the left received normal irrigation to match ETc throughout the season, and trees on the right received excess water. Trees on the right defoloiated a full two weeks before those on the left. Photo credit: Bruce Lampinen

Over and under irrigation can both be catalysts for stress in tree crops. With an increased tendency for stress this time of year, it is important to monitor tree water stress and soil moisture content to ensure that trees maintain a healthy green canopy after harvest through the end of the season.

After leaf drop, ETc rates for nut crops are very low. Since tree water use is minimal during the winter months, water use needs may be met by the last irrigation around the end of October or beginning of November, combined with rainfall. In almond, the 30-year average (Five-points) for ETc in almond (Table 1) is estimated to 1.4” in November and 0.5” December. Without rain, irrigation may be recommended in some regions to ensure adequate moisture will be available at bloom. The effects of winter drought on almond bloom development in the following year is currently being researched and preliminary data suggests bloom timing may be delayed in drought winters. Most of the precipitation in the Central Valley comes in January and February and in many years may provide enough water without supplemental irrigation. For those who choose to irrigate in winter, the amount of water needed to fill the profile depends on the soil type, usually 3 to 6 inches of water should be adequate. Irrigators should make sure to keep irrigation sets to less than 24 hours to avoid the risk of soil saturation and Phytopthora. A good strategy is to begin applying irrigations in January (one inch per event) leaving some ability of the soil to absorb any rainfall that occurs before flowering and leafing. If no additional rainfall occurs, continue applying one inch every few weeks with the goal of reaching a full profile by the time of flowering.  

References:

Doll, D., and K. Shackel. (2015). Drought management for California almonds. ANR Publication 8515 10

Esparza, G., Dejong T., Weinbaum S., and I. Klein. (2001). Effects of irrigation deprivation during the harvest period on yield determinants in mature almond trees. Tree physiology. 21. 1073-9.

Ferguson, L., Polito, V., and C. Kallsen. (2015). The Pistachio Tree; Botany and Physiology.” Fruits and Nuts. UC Davis. 31-39.

Fulton, A., Grant, J., and R. Buchner (2014). Using the Pressure Chamber for Irrigation Management in Walnut, Almond and Prune. UC ANR 8000 Series Publications 27

Goldhamer, D.A and B. H. Beede. (2004). Regulated deficit irrigation effects on yield, nut quality and water-use efficiency of mature pistachio trees. Journal of Horticultural Science & Biotechnology. 79 (4) 538-545.

Polito, V., (1998). Floral Biology: Flower Structure, Development, and Pollination In: Ramos, D.E. (Ed.) Walnut Production Manual. University of California Division of Agriculture and Natural Resources. Publication 3373. pp. 127-132.

Spann, T., Beede, R., and T. DeJong. (2008). Seasonal carbohydrate storage and mobilization in bearing and non-bearing pistachio (Pistacia vera) trees, Tree Physiology, 28, (2), 207–213, https://doi.org/10.1093/treephys/28.2.207