Walnut

Planted-out walnuts overwinter in the Central European lowlands without protection; older trees tolerate well below -20 °C. Only young trees benefit in the first few years from leaf mulch over the root area and a hessian band on the trunk against frost cracks in the thin bark; after that you no longer need winter protection. Pay more attention to late frost in April/May: the early shoots can freeze, which costs you the harvest but does not kill the tree. A wind-sheltered position without a frost pocket helps. Young plants grown in containers should be placed in a bright, frost-free spot, because the roots in pots are less protected than in the ground.

The walnut can be propagated in three ways, which serve different aims.

Sowing (cold stratification): Harvest ripe nuts in autumn and remove the green outer husk. Stratify the kernels for three to four months at 2 to 5 °C, either in moist sand in the fridge or by sowing directly outdoors in autumn (5 to 8 cm deep), where the winter provides the cold phase naturally. The first seedlings appear in early spring (March/April). Important: trees grown from seed do not come true to the variety; they later bear nuts that differ from the mother tree, and often need many years before the first crop. This method works well for raising rootstocks for later grafting, or when a vigorous seedling tree with no variety tie is the aim.

Grafting (true-to-variety cultivars): If you want to carry on a particular variety, such as ‘Franquette’, ‘Lara’ or a regional farm variety, true to type, grafting is the most reliable way. Two- to three-year-old walnut seedlings serve as rootstock. For indoor grafting (whip-and-tongue grafting), lift the rootstocks in mid-December, pot them up and let them start into growth for a few weeks in the warm; December to January is the ideal time. Alternatively, you can work in summer with chip budding (bud grafting), ideally in July, when a well-developed bud from this year's shoot is inserted into the bark of the rootstock. The scion, that is, one-year-old, ripened shoots of the variety you want, is cut in winter and stored cool until grafting time. Walnut contains a lot of tannic acid, so the cut surfaces oxidise quickly: work quickly and cover the union immediately with grafting tape. Walnut grafting is considered more demanding than with apple or cherry and works best with a little practice.

Layering: You can also get true-to-variety plants through layering, provided the tree has flexible shoots close to the ground. In autumn or spring, bend a young, low-lying shoot in an arc to the ground, fix the part touching the soil with a clip or a stone and mound it with loose soil. Roots form at the covered spot over the course of the season; once the layer is well rooted, separate it from the mother plant and transplant it. For young trees with low branches this is an inexpensive and manageable method; on a full-grown standard tree without shoots near the ground it is hardly feasible.

Note: Cuttings are not practical for walnut; the shoots' ability to root is severely limited and is regarded as the main bottleneck in walnut propagation.