Becky Wheeler-Dykes, UCCE Farm Advisor Glenn, Tehama and Colusa Counties
Luke Milliron, UCCE Farm Advisor Butte, Glenn, and Tehama Counties 

With the recent good prices, many growers with prune orchards at the end of their productive lifespan are opting to replant back into prunes. With ever increasing input costs, yield maximization is one of the key objectives you will likely have. Here are some things to keep in mind while designing and preparing for a new orchard. 

Nematode testing: Nematodes can not only damage trees and reduce vigor, but they can also vector or increase susceptibility to diseases like bacterial canker and prune brownline. Knowing the nematode population levels in your soil will inform whether pre-plant fumigation, a fallow year, or an alternative rootstock may be necessary. For more information on nematode sampling, listen to this podcast from UCANR’s Growing the Valley. 

Soil preparation: Thorough root removal by chiseling and deep ripping of any underlying compaction will be necessary between plantings. Even with very deep soils, years of production without soil disturbance can lead to compaction that may limit root growth for young trees. Roots can carry pathogens and pests over to the next generation of trees. After chiseling and ripping, discing, and if necessary, fumigating, the field should finally be leveled before berms are made.  

Pre-emergent herbicides: Pre-emergent herbicides allow young trees to get a head start on growth without competition from weeds. However, pre-emergent herbicides used in orchards can have plant-back restriction periods of up to 20 months. It’s important to check the dates and products used and plan ahead to ensure no damage to young trees. 

Variety choice: The prune breeding program at UC Davis has developed several promising new varieties with the potential to improve grower profitability with early bearing and reduced drying costs. If you would like to help with the field testing of these varieties by planting part of your orchard with one of these new selections, please contact Sarah Castro at scastro@ucdavis.edu. 

Rootstock choice: Perhaps the most important decision when planting a new orchard is which rootstock to use. Factors to consider when tailoring a rootstock for your site include vigor, anchorage, reduced suckering, bacterial canker history and risk of waterlogging and Phytophthora. The Brownline Alert article in this newsletter also details very recent findings of brownline in prunes on the popular K86 rootstock. Viking™ rootstock was second to only K86 in UC testing of anchorage, although its brownline susceptibility remains unknown.  

Tree spacing: Total yield and the bottom line of the orchard can be greatly affected by tree spacing. Maximizing the trees per acre and light interception can be a delicate balancing act. Equipment, soil quality, rootstock vigor, and worker safety at harvest, all play into the decision of tree spacing. For more information, check out this article. 

Early pruning choices: Long pruning has been gaining in popularity for management of young prune trees. Without a follow-up plan, leaving scaffolds un-headed and only making thinning cuts during training can risk limb breakage. Roping, tipping, and selective fruit removal are all ways growers avoid limb breakage while also maximizing yield. Whichever pruning strategy is used, it’s important to protect pruning wounds to avoid fungal canker infections that can cut an orchard’s lifespan and profitability short.