Late Budding Walnut varieties - dataset of a long term monitoring program in Flanders

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 four trial years (2021-2024), five trial sites were eventually retained for more detailed monitoring of the trees. Parameters that were monitored were leaf budding, flowering, leaf infestation by diseases or pests, tree growth and health. Several protocols were developed for this purpose.

Per site at least 30 LUW trees were planted (10 varieties and at least three individuals per variety) and some control varieties (commonly available, non-LUW trees). Each LUW variety was planted in at least two sites.

Control (commercial) varieties used, are: nr 16, Buccaneer, Broadview, Germisara, Bella Maria, Coenen, Lange van Lod, Plovdivski, Proslavski, Rita and Wunder von Monrepos.

The five long term monitoring sites are located in Merelbeke, Rumbeke, Hulste, Herent and Pajottegem. They are located on sandy, loamy sand / sandy loam or loamy soils.

We maintained a minimum planting distance of five meters in the row, the trees were protected from chewing with a net, the treebed was weeded and provided with extra nutrition (compost, champost or farmyard manure) and the trees were originally tethered to two support poles (bamboo poles). In some locations, these were later also tethered to sturdier chestnut stakes.

In this record, you can find documents with the monitoring protocols, the site information and the (raw) data.

The results of the first three years have been published in an initial report, which can be found here: https://zenodo.org/records/14197756 (in English and in Dutch).

More info via https://www.agroforestryvlaanderen.be/nl/nieuws/het-potentieel-van-laat-uitlopende-walnootrassen