The weekend started at Lincoln University. Clive Kaiser, who has recently been appointed Associate Professor of Horticulture, talked us through plant responses to pruning and the influence of growth hormones and environmental factors.
Pruning and vegetative growth
There are many different pruning methods and there is seldom a single option that is unarguably best. Even the experts will have more than one opinion on what might work, depending on various other factors. “Window pruning” is common in many mature orchards, where a large cut is made to open up a gap for light. Healthy trees will respond to pruning by filling in the gap quickly, so it’s not a long-term answer.
Fruiting growth requires light
Clive gave us a crash course in plant physiology, explaining the different stages of bud development before fruiting can occur. By default, growing buds will develop as vegetative growth. Flower bud induction is the point in time when direct sunlight on the bud causes the bud to switch on certain genes and change from a vegetative bud to a floral bud. This is followed by flower initiation, changes within the bud on a microscopic level (internal changes that cause flowering buds to be rounder, vegetative buds to be pointy), and then differentiation, which is the characteristic visible change in bud morphology where different organs (e.g. stamens, ovaries, styles, etc.) of the flower develop.
Induction requires direct sunlight on the bud for a minimum length of time at a critical point in time – but we don’t know for sure when the window for exposure is for hazels, or how much sunlight is required. Clive’s best guess is that the time will be late summer, after the flush of extension growth has finished. It probably requires a full day’s worth of direct sunlight within a 7-10 day exposure window (but the exposure might not have to be continuous, it could be accumulated within the window – this isn’t certain). Exposure is reduced if there is too much foliage covering the buds (shading), or even by dreary weather at the wrong time, and indirect light isn’t sufficient. Therefore, up to a certain date it’s advantageous to prune because this improves light penetration to induce other buds. After that date, pruning removes buds that have already been induced. Further research is still needed to work out the exact timing points, and even then it will probably vary according to local conditions, individual cultivars and from season to season.
We’re starting a mini-trial from summer 2022 to gather data on some of these factors. Click here for more information about the trial and how you can participate.
Hormones compete to control plant growth
The two main plant hormones, auxin and cytokinin, work together but also in competition to control plant growth. Understanding their roles and how they interact helps us predict how pruning will affect their balance.
- Cytokinin is produced in the root tips and encourages cell division and growth of the plant. This drives shoot growth above the ground surface.
- Auxin is produced at the apical meristems – the growing tips of shoots – and suppresses lower buds from growing vigorously. The effect of auxin is easiest to see when a growing tip dies back or is pruned off. When auxin from the terminal bud is no longer moving by gravity back down the plant, another bud will become dominant and a replacement leader will emerge.
Interfering with the production or flow of auxin (by pruning) increases the influence of cytokinin until balance is restored. We see this as the growth response to pruning.
Advantages of different tree formations
There are advantages and disadvantages to different tree shapes, and no single pruning strategy is best for every situation.
Clive favours keeping the working area at a height that can be reached easily from the ground. This removes the need for airborne delivery systems and increases coverage by fan-assisted (airblast) delivery mechanisms for spraying. The ability to pick the crop from the tree is important for fruiting crops but doesn’t apply to tree nuts.
We looked at other mature trees in nearby blocks and Clive pointed out that the heaviest crops were on the trees that had all round exposure to sunlight. In effect they have 5 growing faces: north, south, east, west and top surface. Compared with a vase shape where lower growth is constantly shaded (and thus lacks flower buds), a more domed-shaped tree has more surface area and therefore more potential fruiting space.
One of Clive’s guiding principles is complexity in the plant form. Unchecked vegetative growth results in height at the expense of fruiting, whereas regular pruning gives more potential fruiting stems. This suggests a multiple leader system rather than a central leader, but this could still be grown from a single stem up to knee height (for ease of orchard floor management and to avoid nuts being caught between multi-stems). Above that, Clive favours pruning the top growth off at “pocket height” – a convenient metric that inherently adjusts to individual operators.
The trial block at Lincoln: the canopy far above head height makes maintenance difficult, and low light penetration prevents fruiting growth lower down
A possible strategy to maximise fruiting growth
Clive suggested that as an orchard approaches canopy cover, it might be advantageous to top every 2nd row at pocket height. This would restore the light to the alternate row, and subsequent regrowth could be more actively managed to encourage branching complexity.
As the new structure becomes established, removing the most dominant branches each winter would continually renew the growth to maximise fruiting potential. This requires consistent annual management, rather than a three- or four-year cycle.
Points to bear in mind when pruning
- Snipping off a small portion at the end of a growing stem gives a small regrowth response. To create a more vigorous response with several replacement stems, prune back more vigorously (remove several buds – ½ to ¾ of the length of the shoot)
- Topping upsets the balance of growth. Root pruning (with a wing ripper or mole plough) should be considered to compensate.
- If you prune heavily, make sure you retain active growing points (a nurse branch) on the main stem to keep the sap flowing upwards. Without active growth, the tree may die.
- Bark is susceptible to sunburn if it is suddenly exposed to a much higher level of sunlight. Consider painting with a preparation of copper mixed into acrylic paint (eg undercoat).
- Rubbing out buds is a fast alternative to pruning.
- Removing all low growth early on can result in spindly trunks. To encourage a strong lower trunk, leave lower branches on until the growth has stabilised and prune up progressively later.