Things to Consider When Establishing a New Walnut Orchard

Kari Arnold, PhD, UCCE Area Orchard and Vineyard Systems Advisor, Stanislaus County

Planting a walnut orchard can be daunting. The decisions involved impact a long-term investment and remediation can be difficult. If you are plan- ning to plant this year, hopefully you have already discussed soil/site preparation and plant selection with your local University of California Coopera- tive Extension (UCCE) Farm Advisor. Below, is a summary of a few critical points when planting bareroot or potted plants, irrigation, fertilizer, and weeds in new orchards.

Planting Bareroot Trees

Although bareroot trees look tough, they are quite vulnerable to the environment and must be treated with care. After pick-up or delivery, plant trees immediately and only pick up as many trees as you can plant that day. While planting, keep trees moist, cool, and protected from sunlight by placing in a warehouse or under a tarp.

When planting, remember that proper soil depth can be tricky with bareroot trees. Preparing the soil during the summer or fall allows time during the winter for settling. Planting into moist soil is best. Avoid planting into overly wet or overly dry soil. Dig holes only as deep as the roots and wide enough to accommodate, roughing up the sides of auger-dug holes with a shovel to avoid glazing.

Spread roots away from the trunk in all directions in the hole, do not twist or maneuver to fit the root ball into the hole, this will lead to girdling later in the tree’s life. Dig a wider hole if possible. Use Gallex or strain K-84 of Agrobacterium tumefaciens as a dip on the root system to reduce the potential for crown gall. Do not wound the roots or trunk, this will provide opportunity for crown gall infection. Plant into previously formed berms or small mounds where drainage is an issue. Fill soil around roots with some light packing (tamping) making sure not to leave air pockets. Mound soil slightly to allow for settling and use a minimum of one-to-two gallons of water, up to five in sandy soils, to tank in trees.

Planting depth is not the only thing to consider when planting bareroot trees. Paint entire trees with whitewash (interior latex paint diluted with 50% water) to prevent sunburn. Wait until the soil begins to dry within the rootzone before applying an irrigation. Do not apply fertilizer inside the hole and avoid planting on hot days. Place stakes at planting 12-inches away from the tree and to the side, which is perpendicular to prevailing winds. Ties should be loose to allow tree movement.

For further information please visit http:// www.sacvalleyorchards.com/walnuts/horticulture- walnuts/planting-bareroot-trees/ for a current adaptation of “Guidelines for handling and planting bareroot walnut nursery trees” by UCCE Farm Advisors Carolyn DeBuse and Bill Krueger.

Planting Potted Clonal Plants

Clonal rootstocks are propagated on a large scale by dividing small plants into more small plants in a laboratory. As plants mature, they are moved to soilless potting medium and cared-for in pots.

Clonal potted plants can be planted almost any- time during the growing season, depending on their stage of growth, but will need an additional year to become established. Green, actively grow- ing plants can be planted May 15th to June 1st; not actively growing plants, September to Octo- ber; and dormant plants, November through March 1st. Generally, it is not advised to plant trees in the middle of the summer due to heat and the risk of sunburn.

Potted plants allow for more flexibility in planting time, but there are risks involved. Trees planted during the dormant season are vulnerable to winter freeze, whereas planting in late spring requires attention to irrigation. Proper irrigation is tricky as the potting medium used at the nursery is great for drainage but dries out quickly in the field. It is important to maintain adequate moisture in this medium for at least the first month until roots have grown into the surrounding soil. Careful monitor- ing is required. The small plants are also suscepti- ble to animals and herbicides and should be protected with grow tubes.

When planting both bareroot and clonal plants, remember to cover the roots, but the other “trunk” portion of the rootstock should be above the soil line. For further information on planting clonal rootstocks please visit http://www.sacvalleyorchards.com/walnuts/horticulture- walnuts/planting-potted-clonal-rootstock/ for an adaptation of “Guidelines for Handling Potted Walnut Clonal Rootstock Plants” by UCCE Farm Advisor Janine Hasey.

Irrigation

Properly irrigating a young orchard establishes a healthy root system leading to a vigorous, uni- form, and productive orchard. It is important to note, young trees do not use much water. Check- ing the soil moisture and utilizing a pressure chamber are effective means of monitoring the young trees’ water status. Since irrigation demands are site-specific, please contact your local farm advisor for further information.

Fertilizer

The three numbers on the package represent the amount of nitrogen, potassium, and phosphorus in the fertilizer in that order. Nitrogen is essential for green growth and plant proteins, while phosphorus and potassium aid in woody tissue development. Although phosphorous and potassi- um toxicity is not typically an issue, nitrogen can be easily over-applied. This leads to excessive growth, toxicity, and nitrogen build-up in our water resources. Young orchards require about 25-30 lbs. of nitrogen per acre, which is often sourced from residual soil nitrogen during the first year. Think of this as an easy way to save money on a new orchard.

Weeds

Weeds should be managed within new plantings, as they can reduce tree growth. Weeds will starve young trees of nutrients and moisture, and if grown tall enough, will shade new trees from the sun. Additionally, small animals love hiding in weed cover, feeding on tree roots and crowns, killing them. Although weed management is important, walnuts are not resistant to herbicide. Avoid planting directly into herbicide-treated soil by reducing or eliminating the previous season’s application. Mounding and filling-in holes with previously treated soil can lead to a higher con- centration of herbicide coming in contact with tree roots. Green tissues like early trunk growth, low branches, and leaves on new trees are vulnerable to herbicide. Be mindful of drift by avoiding windy conditions, using proper spray rig height, nozzle angle, and selection. Know your weeds for proper herbicide selection and use different modes of action to avoid resistance. Certain herbicides are registered for use on new trees. Be sure to check current label listings first, such as Agrian (agrian.com/labelcenter/results.cfm) or CDMS (cdms.net/Label-Database) before spraying anything and do not use 2,4-D.

Hopefully, these tips will help you in planting your next orchard. Please contact your local UCCE farm advisor for further information about planting in your region. Other resources include the University of California Agriculture & Natural Resources (UCANR) Sacramento Valley Orchard Source website http://www.sacvalleyorchards.com/walnuts/ as well as the future publication, the Young Orchard Handbook, from which sections covering irrigation, fertilizer and weeds were derived.