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TNFP1219-LH-irrigation_BT_Edits.JPG Logan Hawkes
University of California Cooperative Extension farm advisor Allan Fulton, seen speaking at a springtime field day in a Red Bluff walnut orchard, says fall irrigation is done for the year.

As winter rains arrive, postharvest irrigation ends

Almond growers will need to revisit their water needs by late January and evaluate the benefits or shortages of winter moisture.

Since the beginning of December, heavy storms began to invade California from the northern Pacific bringing high winds, heavy rain and substantial snowstorms that covered the Sierra Nevada range and even powdered large areas of the Upper Valley floor.

It’s a clear signal that postharvest irrigation is now a thing of the past until next fall and growers begin to look to early spring as their next opportunity to supplement soil moisture across their orchard floors.

“The window for fall irrigation is closed,” Allan Fulton, Irrigation and Water Resources Advisor for Tehama, Colusa, Glenn and Shasta Counties said in a recent telephone interview. “It was dry for most of the fall season but the first major storm earlier this month dropped rain across most of the Valley areas which received at least an inch or more of rain.”

Additional storms have arrived since then bringing more rain, but Fulton said in November many orchard operators were busy irrigating in areas that were not as wet.

“Both October and November were dry months and orchard canopies seemed to hang around a little more than normal, so we had a fair amount of irrigation water going into orchards, especially in late November. Growers wanted to build a little bank of moisture to warm up the soil and rehydrate their trees to guard against cold injury,” he added.

Keeping that moisture in the ground is now up to Mother Nature, Fulton says, and almond growers will need to revisit their water needs by late January and evaluate the benefits or shortages of winter moisture. Almonds are the first tree nuts to bloom. He says pistachio and walnut orchard operators can wait until mid-February or so before deciding whether winter rains were sufficient for the spring bloom.

“The next opportunity to irrigate probably won’t happen until late February or March before orchards can receive supplemental water from irrigation to make up for any winter moisture shortages.”

Prune in March

As far as pruning is concerned, Fulton says almond trees can wait until early March and late walnut variety trees even later by 3 to 5 weeks.

“In terms of nutrients, especially nitrogen, no additional applications need to be made until about 30-days after first bloom. Even then we want to be careful to avoid nitrogen applications if rain is expected as that causes it to leach and as a result, will not really help your orchard.”

The possible exception to delaying nutrient applications until trees are no longer dormant would be potassium as it is not as mobile, and it may be justified in an orchard known for potassium shortages. The winter rains can help you get potassium well into the soil.

On a different note, when asked about cover cropping in tree nut orchards, Fulton says the latest research is taking a new look at the practice as a possible way to help control and manage nematodes in soil.

“I think we are seeing some renewed interest in cover crops in the tree nut orchard, and it’s not just to help build organic matter which is important or about storing nitrogen in the soil. The latest research is also looking at things like can they provide benefits in terms of management of nematodes and things like that.”

Cover crops are crops grown between cash crop cycles or incorporated with cash crops to cover the ground in vegetable fields, orchards, and agricultural sites. They are mainly planted to improve soil fertility, soil structure, decrease soil erosion, and suppress weeds, insects, nematodes, and other plant pathogens. They are used to manage nematodes because nematodes can move only very short distances on their own.

Fallow soil also helps to keep nematodes populations at lower levels, but it may lead to soil erosion and other problems that can be largely avoided with the use of a winter cover crop.

“Cover crops are getting a new, strong look and I think I am certainly seeing renewed interest in exploring their use in tree nut orchards. Finding the right cover crop is important as we need to be careful not to increase water demand by much that could challenge moisture levels for the trees,” Fulton said.

Research on the topic continues, he said.

For more news on tree nuts as reported by growers and farm advisors, subscribe to the Tree Nut Farm Press e-newsletter.

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