Plant walnuts closer. Unthinkable? Think again!

Nuts Sept. 26, 2021

*Growers of walnuts may not realise that the methods of managing a high-density planting of fruit trees can also be applied to walnut trees.

Like walnut trees, fruit trees used to be tall and were slow to come into production. Tall fruit trees were also costly to maintain, and yields were low.

The problem with large walnut trees is that once carbohydrates are produced by photosynthesis, they are used to form the structure of the tree. While shoots are the most visible form of vegetative growth, and they are extremely important in managing sunlight use in the tree, they actually have relatively little effect on the overall use of carbohydrates and nutrients by the tree.

The major above-and-below ground competitor with nuts for carbohydrates and nutrients within the tree is the structural wood mass. Therefore, the amount of structural wood of the top and roots in a walnut tree create an internal limiting factor on the potential productivity of that tree. By limiting and regulating the woody structure of the walnut tree, you can significantly influence its productivity.

Walnut young tree

'Chandler' and a few other walnut varieties can have up to 90 per cent fruitful lateral buds, but young trees often do not push these lateral buds (see image above). This situation can be further aggravated when trees are grown under vigorous conditions to maximise early tree growth.

The failure of lateral buds to grow can lead to blind (bare) wood, which may cause reduced fruitfulness of young bearing trees. To overcome this problem, trees are sprayed in late winter with a restbreaker, such as Dormex, trees are then painted with Cytolin in spring, scored and buds are notched.

To keep trees in their spaces, it is also important to control apical dormancy, delay-head branches and apply regulated deficit irrigation to control vigour and increase fruitfulness. These are all methods successfully used on high density fruit trees.

Like fruit trees, walnut trees can be planted closely, and become productive early, provided modern orchard management practices are carried out, such as tree training, summer pruning, deficit irrigation and applying a plant growth regulator if necessary.

High denisty walnuts

Tree size can have an interesting effect on the productivity of a walnut tree. A small closely planted walnut tree has the same capacity to produce carbohydrates by photosynthesis as a large walnut tree when their performance is compared by the canopy of each tree.

However, because a small tree has a much smaller structural wood mass, it can convert a higher percentage of its photosynthetic production into nuts rather than wood. This difference in tree structure helps to account for the improved yield efficiency and productivity usually observed in smaller closely planted fruit and nut trees.

Size-controlling rootstocks for apple trees revolutionised the way apple orchards were managed. Productivity of apple orchards also dramatically increased.

While breeding and evaluating size-controlling rootstocks for pome and stone fruits are continuing all over the world, methods to manage tree vigour and increase fruitfulness were developed, which enabled orchardists to plant fruit trees on vigorous rootstocks at high density.

Some of these methods can give walnut growers the incentive and the confidence to plant their walnut trees closer than they did in the past.

Growing walnuts has certain advantages which fruit growers do not have, making walnut growing even more attractive. These advantages are:

1. Present projections of costs and returns indicate that growing closely planted walnuts can have a positive cash flow after year five

2. Walnut trees are free-standing – no trellis is required. (Staking young trees may be required in areas that are not protected from strong winds)

3. Excellent varieties on precocity-inducing rootstocks are available

4. Densities of between 500 (5m x 4m) and 660 (5m x 3m) trees per hectare are possible to induce early fruitfulness and obtain high yields

5. The central leader system of growing closely planted walnut trees permits a uniform tree complexity, high light interception per hectare, and good light distribution throughout the canopy for sustained high production

6. Medium to heavy-textured soils, such as the duplex soils in the Goulburn Valley of Victoria, are suitable and provide some vigour control. Hilling-up of topsoil is required for surface drainage and increased depth of topsoil

7. Methods to induce branching, tree vigour control, and maintaining fruitfulness similar to fruit trees, have already been researched, published and can be implemented

8. There is minimal risk of frost damage to flowers and nuts, because walnut trees bloom late (November)

9. No honeybees are required for cross-pollination because walnuts are wind pollinated by polliniser trees

10. Walnuts are mechanically harvested and processed

11. Pest and disease pressures of walnut trees are much less than for fruit trees

12. Closely planted mature walnut trees under micro-jet irrigation need about 6ML of water per hectare

13. Walnuts are not affected by fruit fly quarantine regulations

14. Walnuts in shell have a long storage life. They do not need to be cool stored

15. Walnuts have health and cosmetic properties

16. Walnuts have good export potential

17. The wood of walnut trees has commercial value.

The same principles and practices of managing close-planted walnut trees could apply to pecan, pistachio and macadamia trees.

*Written by Bas van den Ende a Tatura-based author, consultant and former researcher who was involved in pioneering research into managing fruit tree growth and productivity at the Tatura Research Institute, Department of Agriculture Victoria.

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