Farm Progress

Floods in northern California left peach, walnut orchards with disease which could lead to tree death

tfitchette, Associate Editor

June 18, 2017

7 Min Read
Standing water was still present in Brad Foster’s orchards in mid-May. The low-spot behind Foster has several rows of pecans, planted there by the previous owner because pecans are thought to weather sustained wet conditions better than walnuts.Todd Fitchette

Janine Hasey has lived through several northern California floods. The county director for the University of California Cooperative Extension offices in Sutter and Yuba counties witnessed several major flood events in the region including in 1997 but none rival last winter.

Most notably is the evidence of orchard collapse, or the widespread death of orchards because of long-term flooding.

“I’ve never seen this kind of collapse in 35 years,” Hasey said.

Orchard collapse is in peaches and walnuts as the death of entire orchards begin to unfold. The cause of death can be traced back to long-standing water in the orchards, which remained as trees came out of dormancy and began to bloom, producing leaves and expanding root systems.

Record rain

Yuba and Sutter counties are located near the confluence of four major river systems, including the Feather, Sacramento, Yuba, and Bear rivers. The Sutter Bypass, part of a natural overflow area called the Sutter Basin, also plays a major role in the region. The basin is incorporated into a flood control project that takes pressure off the Sacramento River during high flow events. Yet by doing so it spreads water across thousands of acres, including farm land.

By June, the city of Sacramento boasted its third-highest rain season in recorded history at nearly 34 inches, not far behind 1982/1983 and 1889/1890, at first and second, respectively.

Exacerbating the season for growers in the south Sacramento Valley, particularly those along the Feather River, was the situation at Lake Oroville and the spillway at Oroville Dam in February.

In early February, the concrete spillway that channels water from Oroville Dam to the Feather River collapsed when the hillside that supports the spillway eroded from weeks of heavy rain. Soon after, the initial failure of the main spillway, inflows into Lake Oroville from the Feather River watershed exceeded the volume in controlled flows. This caused the lake to rapidly fill and spill in uncontrolled releases over the emergency spillway for the first time since the earthen dam was built in the late 1960's.

In the weeks following, the California Department of Water Resources attempted to control lake levels. The Feather River rose and fell rapidly, and caused river banks in the Marysville area to sluff off into the river channel.

With each rise above flood stage a new pulse of water flowed across farmland in the region. As river releases were abruptly lowered, millions of cubic yards of earthen river banks were released into the river channel.

River Oak Ranch

Brad Foster leases River Oak Ranch north of the town of Marysville. The 400-acre orchard of mostly walnuts with some pecans were affected by three separate flood events - water at times greater than 10 feet deep flowing rapidly through the trees. The high flows left a field of debris from upstream farms that included trees, soil, irrigation pipe and other materials.

Though the impacts were significant, Foster said his damage was mitigated because he allows weeds and grasses to grow as cover crops during the winter months. This helped preserve some of the soil in his orchards which otherwise would have flowed downstream with other debris.

That his orchards flooded this winter isn’t a point of contention with Foster; he understands and accepts the risk of farming in the river bottom along the Feather River.

His point is the management decisions made by state officials that rapidly raised and lowered the Feather River caused significant river bank erosion.

By late spring, Foster lost over two acres of buffer zone that protected the river from his farming operations. The zone was once a 200-foot riparian strip with native oaks and grasses that helped keep spray drift from his 400-acre walnut block from getting to the river.

“This just caused the river to get wider and shallower,” he said.

In spite of the river-bank losses, Foster says he lost no trees directly from the floods, though there was significant erosion around the base of some trees in his orchard due to eddies created by fast-moving water. He intends to fill the holes with soil deposited on his ranch from upstream sources.

He will spend the summer moving soil to rehabilitate his orchards which he still irrigates with sprinklers.

Foster’s fungicide costs were high this year due to the wet season.

“We put on more copper this year than we’ve ever put on,” he said.

Foster pointed to an eight-acre orchard section converted by the previous owner to pecans more than 10 years ago because pecans are said to weather “wet foot” issues better than walnuts. The pecans were still under water in late May, but appeared to be doing well.

Foster says the walnuts are just coming back to good health. The trees last year produced yields of three tons per acre.

A similar flood inundated the same trees in 1997.

“This was a young orchard then,” Foster said. “They weathered the flood fairly well.”

Yet another impact of the flooding was bleeding cankers associated with Phytophthora, a water mold fungus in rivers, according to Hasey. Hasey and Greg Browne, USDA plant pathologist at UC Davis, sampled standing water in Marysville-area orchards, and took tree tissue samples to confirm the presence of Phytophthora in the orchards.

Hasey says the extent of the damage won’t be known until at least late summer and perhaps not fully realized for up to two years.

In one sampled walnut orchard, one-third of the Chandler variety on RX1 rootstock experienced seepage from January to early May. In this case, Hasey says the flooded trees leafed out about a month later than non-flooded trees.

Management considerations

The following information is from a University of California publication. This section was written by Astrid Volder, Plant Sciences, UC Davis; and, Bruce Lampinen, UCCE walnut specialist, UC Davis, to help growers affected by the winter floods with management decisions they will need to make related to the floods. It is reprinted with permission.

"Flooded orchards will likely respond to irrigation differently than normal since root systems are compromised.

In saturated soils, fine roots die, and depending on the extent of flooding, larger roots can die as well. It takes time for the root system to regain functionality and re-start new fine root production after flooding.

If the water table level below the soil surface drops gradually, trees may be able to continue to produce functional roots at increasingly deeper levels over time. Initial irrigation could be delayed for some time, as deeper roots may maintain water uptake.

However, if the soil remains saturated at some level below the surface, the only functional roots may be at a very shallow depth and irrigation may have to be initiated earlier than normal. In this case, shorter but more frequent irrigations could help avoid further damage to roots in the upper zone.

Carefully monitor both soil water levels and tree water status so that the trees can be gradually brought back to health. Soil based monitoring, using Watermark soil moisture sensors installed at different depths, will let you know the level where the soil is saturated as the water level drops, as well as the amount of water that is being moved up above this level of saturation by capillary rise.

Apply enough water to bring the soil to field capacity to enable plant water uptake from the upper soil, but avoid prolonged periods of saturation that may damage roots. It is easy to saturate the soil, particularly when the water table is close, so irrigate judiciously, ideally using both soil moisture monitoring and plant pressure chamber data to aid in determining irrigation duration and frequency.

Do not initiate irrigation until pressure chamber measurements show trees are 2-3 bars below the fully watered baseline (more dry) and only irrigate enough to bring the trees back up to about 0.5 to 1 bar below the baseline by applying short sets of irrigation.

It is not a good idea to prune or fertilize trees after flooding. Pruning will reduce both carbohydrate reserves and leaf area, while fertilizer applications may mostly go to waste and delay or damage root production. The best approach would be to wait, and let the tree adjust to the altered environmental conditions while managing irrigation as described above to avoid further damage to the root system."

About the Author(s)


Associate Editor, Western Farm Press

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