Black Walnut in Oregon - Agroforestry


A tall black walnut tree

Agroforestry could make the economics of black walnut culture more attractive in the Northwest as it has done in the Midwest. Many possibilities are being demonstrated locally through the efforts of Peter Kenagy, Joe Lowe and others in the Walnut Council. In addition to annual crops (e.g. corn) and short-rotation trees (e.g. poplar) already under trial, shade-tolerant, bareroot nursery stock or flower bulbs could be produced between the walnut trees.

While cattle are likely to cause damage by rubbing the trees, it may be possible to graze sheep on grass-clover pasture sown in strips between the tree rows. Nut production is an important source of early returns in many black walnut plantings. However, most of the hybrid walnuts now planted in Oregon are not good edible nut producers because of difficulties with meat extraction, according to Gary Goby. Some thin-shelled varieties, e.g. ‘Cooksie,’ are grown commercially in the state. Goby believes that the threat of blackline disease, which ruins timber quality, precludes the possibility of high-grafting English walnut varieties to hybrid black walnut rootstock for a combination of nuts and timber.

Seed Source Important:

As with any tree crop, careful selection of seed source is a prerequisite to success. A diverse genetic base exists in Oregon, including hybrid crosses and the pure parent species, J. nigra and hindsii. A high degree of variability in both growth and form has been observed among individual trees growing in the Willamette Valley. Goby stresses the need to maintain careful records on the characteristics of parent trees, the soil type and management regime of the plantation, the growth of progeny, and ultimately the quality of the wood they yield.

Black Walnut - Jagans hindsii

In a joint effort with Oregon State University cooperative extension, the Oregon Chapter has identified promising individual trees as sources of seed for new plantations. For example, the ‘BV’ source originates from a 100 ft. tall, approximately 120 year old hybrid tree that is currently growing at a rate of about three-fourths inch in diameter per year and that yields large, heart-shaped nuts. Several test plots have been established since the local chapter was founded three years ago. In order to select the best seed sources for new plantations, there is an on-going need for more nut collections from good local trees, establishment of seedling seed orchards, and progeny trials.

Management Strategy:

The strategy which Gary Goby recommends to landowners is to plant their walnuts at higher initial stocking and then remove the poorer trees at age five, leaving the best trees to grow at a final thinned spacing. The overall objective is to develop a straight, branchless trunk (bole) 16-20 ft. long with a small “defect core” through timely training and pruning. once a satisfactory trunk is developed, then the only effort required is to maintain diameter growth so the tree will add high value, knot-free “clearwood” until it reaches optimal harvest size.

According to Gary Goby, hybrid black walnut in western Oregon grows best in deep, well drained soils. Its soils requirements are similar to those of commercial fruit trees; the OSU extension service has prepared maps of suitable valley soils as a guide to landowners. Black walnut prefers moister north or east facing slopes, and creek banks. Shallow, dry or poorly drained soils should be avoided for new plantations. Landowners don’t need to plant large acreages of black walnut to make a profitable investment, according to Goby.

Farm woodlots as small as 1-2 acres can be sited in odd-shaped areas which are difficult to cultivate for annual crops. Clusters of black walnut trees can also be incorporated into riparian buffer strips.

Planting Black Walnut:

Heart shape walnut dried nuts

Starting with a good seed source, the nuts are first stratified (moist chilling treatment) to increase the rate of germination. Following ground preparation, growers can plant either bareroot seedlings, or one or two pregerminated (or ungerminated) nuts per planting spot. The initial spacing is usually 10-14 ft. between rows, depending on the size of the implements used for cultivation, and 10 ft. within the row. Although individual tree shelters areoptional in the mild Oregon climate, some improvement in growth and protection from deer browse has been observed. Most of the work required to grow black walnut between planting and harvest occurs during the first 10-12 years. Weed control is perhaps the most important task during this establishment phase, particularly to reduce grass competition prior to canopy closure. Where moisture is not limiting, cover crops could be planted and mown between the rows while maintaining an herbicide cleared strip beneath the trees. Fertilization and irrigation during dry periods will promote faster growth. In Oregon, black walnut encounters no significant disease, insect or deer browse problems.

Early training and pruning are necessary for the production of high-value veneer and sawlogs. Training starts in the second year to eliminate branch crotches and other form defects. The aim of pruning is to remove shoots along the trunk in stages (“lifts”) to 16-20 ft. high, yielding two branchless 8-10 ft. logs at harvest. Modified farm equipment such as a tractor-mounted cherry picker can be used for pruning. To minimize knot size, limbs should be removed before they reach an inch in diameter.

Thinning at about age five is also recommended for black walnuts. The best formed, apically dominant trees are selected to grow on after removal of the poorer trees. The Oregon Chapter of the Walnut Council presents special workshops on pruning black walnut. Taxation and harvest regulation are important considerations for farm woodlots. Black walnut is considered an exotic species in Oregon, and is not recognized as a forestry species in all counties where it is grown. Under state forestry regulations, certain species qualify for special tax consideration, property tax deferral, and severance tax, payable at harvest. Gary Goby suggests there is a need for interested landowners to lobby their local county governments to recognize black walnut as a forest crop.