Drought

Severe drought management recommendations for almond

by David Doll, farm advisor, UC Cooperative Extension, Merced County

UC researchers have urged growers not to take aggressive actions in reducing tree size or crop load in response to the west side water shortages this year. Severe pruning will increase new growth which would increase the leaf surface and evapo-transpiration rates (ET) of the tree. Crop thinning has a similar effect and is also not recommended. By reducing crop load, the source/sink ratio of the tree is disturbed, causing the tree to put nutrients into vegetative growth instead of the nuts. Furthermore, data suggests that less than 10 percent of ET may be attributed to crop load. The tree should thin naturally when it undergoes a late-spring water stress period. Furthermore, in season nitrogen applications should also be reduced in order to reduce vigorous shoot growth.

In scheduling irrigation, the pressure chamber should be used to determine the stem water potential of the trees. Orchard irrigations should not be initiated until the trees reach -15 bars. Irrigations should be at the percentage of ET that can be afforded - for example: if 15 percent of water available for the season, water at 15 percent ET at each irrigation. Research by David Goldhamer suggests that almond trees can survive through the year on as little as 6 to 8 inches of water (5- 10 percent ET). This includes the 2 to 4 inches of water available within the soil profile.

Further reduction of inputs this coming year is advised for the West side growers facing water restrictions. Reductions of in-season fertilizers and foliar nutrients will help decrease the vigor of the tree. Use judgment in making these cutbacks as the goal is to reduce tree vigor, not to make the trees deficient. Post harvest fertilizer applications are still recommended. Furthermore, it is not advised to cut back on miticides. With severely stressed trees, mites can flare up easily, causing defoliation and adding to tree stress. If the orchard has a history of pyrethroid use, miticides will most likely be necessary for the coming growing season. If softer chemistries have been used, sprays may be limited or unnecessary. In these cases, monitoring the population of mites and beneficials will be needed throughout the season to see if they approach the treatment threshold.

A light pruning or topping, may be a feasible practice to stop new shoot growth in the spring. Once the trees push new growth, heading cuts would terminate shoot growth, thus reducing leaf surface area. This may work if nitrogen rates were reduced - otherwise the tree would push again causing more tree stress. There is no direct data that supports this practice, and it may not be worth the effort or expense.

In general, if the tree is able to maintain some of its leaves until the fall, the tree probably will survive. Yields will be affected severely for the next 2 to 3 years. This year would be a good year to remove older blocks with declining production and divert the water to younger blocks, if possible.