Thousands of acres of California walnut orchards located primarily in the Sacramento Valley were affected by flood waters last winter and spring, leaving considerable uncertainty about the trees’ long-term viability.

University of California Cooperative Extension farm advisors Janine Hasey and Luke Milliron have been scouting orchards waterlogged in Sutter, Yuba, Colusa, and Butte counties over the last seven months to assess tree health and assist growers with recovery efforts.

Waterlogging or anoxia occurs when tree roots lack oxygen for a significant period of time.

Hasey and Milliron have examined tree root status, soil conditions, shoot growth, and overall tree vigor of the surviving trees to make decisions about either tree removal or strategies to help the trees recover.

“There are varying degrees of recovery,” Hasey said. “Some trees are trying to recover and many aren’t going to make it.”

Not knowing the long-term outlook for an orchard has been an issue as growers cannot receive federal assistance in a tree replacement program until 18 percent mortality per block is reached.

The California Walnut Commission estimates bearing California walnut acreage at about 320,000 acres.

According to Hasey, walnut orchards in San Joaquin County were also affected by floodwaters yet later in the spring as snowmelt caused rivers to rise. Some orchards near the Sacramento and Feather rivers were flooded January through May while orchards in San Joaquin County were underwater from April to June.

Some orchards were inundated by floodwaters from overflowing rivers and some were affected by under-levee seepage.  

Some growers with significant tree mortality or who know the long-term health of the trees was compromised have pulled trees or orchards to prepare to replant. Others are waiting until next spring to decide which trees to pull and replant.

Hasey says there are physical signs on whether a tree can recover. She has assessed affected orchards by looking at roots through backhoe pits and evaluated rootstock and tree vigor to determine which trees might survive or which may eventually die or become unproductive.

“If the trees are pushing enough adventitious buds on the lower limbs with vigorous shoot growth there is a good chance they can come back,” she said.

These are the trees growers should consider topping in November. They should know by next spring if these trees will recover or collapse. From experience, many of these trees can become productive again if upper non-vigorous growth is removed.

Hasey is encouraged by the number of trees which survived flooded orchards, even producing a crop this year and showing good signs of recovery. Through the summer, walnut trees in several orchards under her watchful eye flooded with seepage still produced shoots from adventitious buds. These latent buds can sprout when needed from many parts of the tree.

She says many trees also produced suckers from the rootstock where the English scion showed symptoms of waterlogging to total collapse. Under normal conditions, sucker removal is recommended.

Yet this last summer Hasey thought that leaving these there might produce carbohydrates for root regeneration. Depending on the rootstock and the severity of damage by this summer’s end this approach gave mixed results.

Whether walnut trees are in standing or flowing water can make a difference. Hasey says cold, flowing water has oxygen and doesn’t have the same effect on trees as standing water. When water remains in swales or trees sit in seepage water for a long period anoxia can occur.

However, the ‘water mold’ Phytophthora is found in surface water so there are disease concerns with flood-affected trees in river bottoms or where rivers overflowed into adjacent orchards.

Hasey said several Phytophthora species were identified from bleeding root and aerial trunk cankers in river bottom areas where each tree appeared to have some infection in some walnut orchards. She said the long-term effect on infected trees remains unknown.

Hasey will continue examining tree roots and crowns to look for patterns of root regeneration, but she believes the impact of prolonged waterlogging may be seen for several years.