The potential of late budding walnut varieties

In recent years, we have seen a strikingly increasing interest in growing walnuts among Flemish and Dutch growers and agroforestry pioneers. The timing of budding and flowering in walnuts is variety- and season-dependent, with for some varieties, buds break as early as the end of March and others not until the second half of May or later. In the latter case, we speak of late budding walnuts or LUW trees (laat-uitlopende walnoten, in Dutch).

Nut experts and growers pointed out the untapped potential of these LUW trees in agroforestry and in more intensive nut orchards. Later budding can further reduce shade pressure on intermediate crops in agroforestry, as well as reduce the risk of damage and production loss from late spring frosts, which we assume to occur no later than May 15 (Ice Saints). Varieties with a late bud break are also believed to be less susceptible to some common diseases and pests in nut trees.

LUW varieties are not yet commercially available, so in collaboration with Flemish and Dutch nut connoisseurs, scions were collected from mother trees determined to be late budding. These trees were propagated, raised and finally planted scattered across seven trial sites during the winter of 2020 - 2021. The long-term objective of this trial is to compare phenology, growth, flowering, health, and finally nut production and quality between these different varieties. This both between the LUW varieties themselves as well as relative to the commercial control varieties.

During the first three trial years, five trial sites were eventually retained for more detailed monitoring of the trees. Parameters that were monitored were leaf budding, flowering, leaf deterioration and tree growth. Several protocols were developed for this purpose.

The first three trial years showed that the selected LUW trees effectively did not start sprouting until after May 15, although there was some variation in this even within the LUW varieties with the latest varieties not budding until early June. The common control varieties invariably sprouted before May 15. Beginning monitoring of flowering also indicated flowering after May 15 in the LUW trees, again crucial in avoiding production losses due to frost damage. Due to the limited number of trial years, we still need to be cautious about these conclusions. Further monitoring in future years should confirm them. This is even more the case for observations around leaf infestation, where the LUW trees were found to be slightly less susceptible to leaf infestation by diseases and pests.

The various trial plantings have already been successful, so there is confidence that continued monitoring in the coming years will provide many interesting insights. This includes yield, taste and other relevant nut characteristics of the LUW varieties and their susceptibility to diseases and pests.