Many different types of propagation are used in the walnut nursery industry to create finished propagated trees for orchard planting including sexual propagation (rootstocks only), micro-propagation, cuttings, budding, and grafting.
Sexual propagation, i.e. growing saplings from seed, is only used in the production of rootstocks. Two common types of seedling rootstock are Northern California black walnut (Juglans hindsii) and Paradox hybrid (usually a hybrid cross of J. hindsii x J. regia). Seeds are collected in September through early November by mechanically shaking mature walnuts off the tree onto tarps, or picking them off the ground after they fall naturally. However, collecting nuts from the ground to be used in sexual propagation has been shown to be linked to increased incidences of infection by Agrobacterium and crown gall in seedlings (Yakabe et al. 2010).
The nuts are commonly allowed to air dry in the hull, however hulls can be removed prior to drying. Planting seeds in nurseries in the fall allows for the necessary stratification period to occur over winter in the field. If nuts are planted in the spring they needed to be cold stratified for at least two months to ensure germination.
Vegetative propagation can be used for production of clonal Paradox rootstocks and for developing a finished tree with an English walnut cultivar (scion) grafted or budded onto the rootstock. Clonal Paradox rootstock can be propagated by hardwood cutting or micro-propagation techniques although micro-propagation is the preferred technique.
Micropropagation has increased in popularity over the past ten years because walnut is difficult to propagate clonally from cuttings. To micropropagate walnut, surface-sterilize individual stem segments containing one bud (no leaves) to remove surface bacteria and fungus. Next, place the basal portion of the stem segments into an agar medium containing a mixture of plant hormones, nutrients, and sugars to promote bud growth. Many clonal microshoots can be produced from a single bud. These microshoots can be used for rooting (clonal rootstock or own-root trees). For rooting, the bases of microshoots are treated with potassium indolebutyric acid (KIBA) in vitro for 5 to 7 days and then stuck in a peat:perlite medium on a fog bench in a greenhouse (Vahdati et al. 2004).
Micropropagated plants need to be hardened off after planting in liner sized containers by gradually reducing humidity (from 100%) over a period of weeks in a greenhouse. In the spring or fall, hardened plantlets can be transferred to a nursery field and then budded or grafted when they reach an appropriate size.
Although hardwood and semi-hardwood cuttings can also be used for rootstock propagation, they are not commonly used because success rates are often low. Such cuttings have rooting percentages from 30 to 80% and often have poor initial survival. Use cuttings from only vigorously growing shoots for propagation and root them in individual liners to promote uniform and deeper root branching. The bases of cuttings are treated with KIBA at 8-12,000 PPM before sticking. Semi-hardwood cuttings are rooted in greenhouses on bottom heated mist benches in mid to late summer and hardwood cuttings on bottom heated mist benches in late fall and winter outdoors. The use of broadcast flats yields lower rooting percentages, shallower roots, and poor root development. Plant liner-sized rooted cuttings in the field in late February or early March. Direct field rooting is not commercially successful.
Once the rootstock is established, growing well in the nursery, and has a diameter matching scion wood, the English scion variety can be grafted or budded on the rootstock. Trees are typically propagated in the nursery by fall-budding the rootstock between late August and mid-September (patch or T-bud, see below). A finished tree will be grown by the next fall if the buds heal over, remain dormant until the spring, and grow out in the following summer. The rootstocks that were initially too small, or had failed fall buds, are whip and tongue grafted (see below) in the spring, and the scion is encouraged to grow immediately following grafting. Both techniques create a finished tree in two years.
A more recent technique, June budding, creates a tree in only one year. Rootstocks to be used in June budding are grown in very fertile conditions and typically reach budding size by June, coinciding with the time current season scion buds develop to a condition suitable for use as budwood. The budded trees are then managed intensively for the rest of the summer to create a tree large enough to sell.
Some growers choose to do much of what is done at the nursery in their own field. In-field budding or grafting has the advantage of establishing an orchard with less up-front cost if a grower has the necessary propagation skills.
Patch & T-budding
Patch budding is the most common budding method used for walnuts. However T-budding can also be employed successfully. Collect budsticks when bark is slipping on both the rootstock and scion budstick. To ensure the best take, the budwood can be prepared by removing the leaves while still on the tree a few weeks before use.
Patch budding is done using a double bladed knife to cut a square piece of bark from the rootstock which is replaced with the same size patch from the budstick containing a well-developed bud. T-budding uses a single bladed knife to slit the rootstock bark in the shape of a T and a shield shaped piece of stem including a bud from the budstick is cut and placed into the opening. After placement, the bud should be covered with budding tape to prevent desiccation. The tape is removed when the bud has healed, usually after a few weeks.
In California grafting is done in the late spring after the rootstock has produced leaves and the risk of graft failure due to bleeding has somewhat diminished. Bleeding (sap flowing from the rootstock) is the most difficult step to manage when grafting walnuts. To reduce the likelihood of bleeding, avoid grafting during times of the year with strong temperature fluctuations or two weeks before or after heavy rains and irrigation. Prior to grafting, the tree should be “bled”. To bleed a tree precut the rootstock two to three inches above the grafting site, and make several cuts below the graft site (without girdling the tree), 7-10 days before grafting. If bleeding occurs after grafting, restrict watering or make cuts on the stock below the graft.
Whip and tongue grafting is the most common graft used to propagate walnuts. Collect scion wood from dormant trees between December and February from the basal portions of the previous season’s growth with well-developed buds. Place collected scion wood in moist wood shavings or a plastic bag and refrigerate. Choose a piece of scion wood that closely matches the diameter of the rootstock. The scion piece should only be long enough to contain two buds. Make a diagonal cut on both the scion and the rootstock and cut a small slit or tongue directly in the center. Place the two matching cuts together with the tongues locking them in place. Then, wrap the graft with grafting tape and seal the top of the scion with grafting sealer. Paint the whole graft and rootstock with white paint to prevent sunburn.
Because grafting tends to produce multiple leaders, select the strongest shoot and stake it while keeping the other shoots trimmed low so that they do not compete with the chosen leader. Grafting can also be used to top-work a mature walnut tree, though this is an unusual practice and only used when there is a need to change the cultivar or add pollinizer cultivars in an orchard.