Uitlopende knoppen walnoot op agroforestry perceel

Digging Deeper: growth and flowering of walnuts

Through the 'Digging Deeper' series, you can follow the developments and results of Inagro's own agroforestry plot from the front row. Field work on cultivating leeks between the rows of trees is still a long way off, but our trees have already made a start. In this edition, you can read more about the growth and flowering of the walnuts, tree row management and other work on the plot. We also take a little side trip to our greenhouse, where we are currently conducting a trial on shade tolerance in summer cereals.

Buds are sprouting

As from mid-April, we score the bud sprouting and leaf development of the 47 nut trees on our plot every week. The earlier the buds sprout and flowering starts, the greater the risk of frost damage. In our regions, it’s best to choose varieties that sprout earliest in late April - early May. These are the 'mid-term', and 'late' varieties. Broadview, the variety we planted, belongs to the first category. Coming into leaf later is also a plus in agroforestry because the struggle for light between tree and crop is more limited that way. However, the exact timing of sprouting and flowering is variable, even within varieties. By monitoring sprouting and growth in detail every year, we also learn about the impact of location, soil and climate on budburst.

Scoren uitlopen walnoot agroforestry perceel

This is how we score the leaf development of nut trees

As from mid-April, we look at all trees every week and each tree is given a score of 0 - 4. We saw the very first bud sprouting on 25th of April. By early May, half of the trees had sprouted, with the highest score being '2'. By mid-May, over 80% of the trees had budded and 15 trees were already at the furthest stage, i.e. score '4'. By the end of May, however, 7 trees had not yet budded and a number of trees remained at stage 1, 2 or 3. The consequences of an extremely wet spring after planting? Or a difference in the 'budding' of the trees after planting? We will follow it up further!

Verschillende stadia bij uitloop van knoppen walnoot

And what about flowering?

It often takes 4 to 5 years for a grafted walnut tree to bear its first fruit. So this year, we do not expect much flowering on our trees. Nevertheless, it's best to think about flowering before planting. This is because the timing of flowering is an important variety-dependent characteristic. It's best if walnuts do not flower too early to avoid frost damage. Walnuts are wind pollinators. Both male catkins, pictured left, and female flowers, pictured right, occur on the same tree, but not necessarily simultaneously. In the Broadview variety, there is usually enough overlap between male and female flowering to allow good self-pollination. If you choose a variety with no or minimal overlap, it's best to also mix with another pollinating variety when planting so that cross-pollination is possible.

Walnoot bloei

First mowing tree strips

Last October, we sowed the 3-metre-wide strips under the trees with a perennial mixture of grasses and herbs. This to attract beneficial insects to the plot. The tree strips here thus serve both tree and crop. In the first year after sowing, a first early mowing in April/May and a second mowing after the summer, in October, are necessary to suppress the unwanted annual weeds and allow the mixture to establish itself properly. In the coming years too, we plan two mowings per year in May/June and October. Last month, the strips received their first mowing. We used a ride-on mower for this. Taking a closer look at the mowed strips, we can already see many herbs ready to grow and flower!

Recent gemaaide boomstrook met gras en kruiden

Shadow test with cereals

We take a little side trip to our greenhouse. There, we are currently running a pot trial looking at the shade tolerance of spring cereals. For cultivation in agroforestry systems, this is an important varietal characteristic. In total, we sowed 11 varieties of spring barley and wheat in pots. Half of the pots we placed in optimal light conditions, the other half in shade conditions. To create shade, we use a net that approximates foliage as closely as possible.

We are monitoring the growth, alloy, disease susceptibility, yield and quality of the grains. The results will help us make smarter variety choices in the field in the future and improve our agroforestry advice to farmers. We are conducting this shadow trial as part of the ongoing VLAIO project 'Agroforestry 2025'.

Schaduwproef granen in serre

What has happened on the plot since the last edition of 'Digging Deeper'?

We monitored the sprouting of the walnuts and the tree strip was mown. But additionally, we also paid attention to extra protection of the trees against the wind. The persistent strong wind causes our young trees to bend too much, which even caused one of them to crack. Where necessary, we lined the trees with a bamboo pole against the trunk.

Insect monitoring also continued last month. We placed LED buckets for the first time to sample moths. In the meantime, the plot also received a lot of international interest. Farmers, researchers and consultants from Poland, England and the Philippines visited the plot.
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