A remedy for blind wood
Delayed heading can stimulate bud break.
The branches of this young Fuji tree suffer from blind wood.
Blind wood is a growth characteristic of certain fruit varieties, where buds at the basal ends of shoots remain dormant or blind. Apple varieties such as Granny Smith and Fuji and the pear variety Beurre Bosc suffer badly from blind wood.
Blind wood hinders the development of fruiting units. Fruiting units are pieces of two-year-old and older wood that are attached to scaffold limbs and branches, and form the basis of the bearing capacity of apple and pear trees.
The development of fruiting units starts when vegetative buds break and form laterals. Laterals spur up in the second and third year and bear fruit. These spurred-up fruiting units are then cut back in the third year to produce laterals again (the 1, 2, 3 rule). This cycle ensures that the bearing wood is always young and productive.
The word “shoot” or “new shoot” is used when the shoot is long and vigorous, and the tip bud remains vegetative. When it is short and the tip bud is a fruit bud, it is referred to as a lateral or new lateral.
In a high-density orchard, every part of the structure of the trees must be dressed with fruiting units as quickly as possible to maximize early production. Blind wood is unproductive wood.
Many orchardists believe that dormant heading cuts will remedy blind wood. This is often not the case, and if you look closely at dormant heading cuts, you will see that only two or three buds immediately below these cuts break and form strong shoots with narrow angles. But when the heading cut is delayed until spring, many more blind buds break and form laterals with wide angles. This method is called delay-heading.
What is the reason for the difference in tree response between dormant heading cuts and delay-heading cuts?
In spring, hormones from the roots move to the tip buds of the shoots. This is a sign that dormancy has broken, and trees start to leaf-up. The young leaves in the tips of the shoots produce hormones called auxins, which move down the phloem to the roots. As they do so, they stop buds lower down the shoots from developing into new laterals. This phenomenon is called apical dominance and is strong in many types of fruit crops, especially in pear, cherry, and some varieties of apple.
The trick is not to cut the shoots in winter, but to let the hormones from the roots get to the tips of the shoots in spring. Then, cut off part of the shoot when the young leaves at the tip start to produce auxins. The shoot must be at least pencil thick where the cut is made. Hormones now move from the roots to the blind buds, which are no longer under the influence of auxins from the tips. The blind buds break and form laterals.
Since delay-heading is still classed as a heading cut, there is a tendency for the three buds immediately under the cut to form new shoots that are stronger than the rest lower down. Pinch the second and third new shoots when they are about 70 mm (3 inches) long, to allow more new laterals of equal size to develop lower down. The response, however, is restricted to no more than 400 mm (16 inches) below the cut.
Delay-heading, done two weeks after full bloom, devitalizes trees. Early delay-heading, done at green tip, devitalizes trees less than does late delay-heading. The later the date of delay-heading, the more buds will break, but the resulting shoots will be short. The time to delay-head depends, therefore, on the vigor of the trees. Delay-head vigorous trees late. Delay-head early when shoot growth is moderate. Do not delay-head weak trees.
In extremely vigorous trees with lots of blind wood, you could force breaks by making two delay-heading cuts (double delay-heading). After the first delay-heading cut, make a second delay-heading cut when the three buds under the cut have developed new shoots that are about 70 mm (3 inches) long. Cut the piece off with these three new shoots. Double delay-heading requires a bit of work, but will devitalize your trees and calm them down.
This explains simply how you can manage certain natural hormones in young trees to promote fruiting units on branches, where normally buds remain dormant. Trees with plenty of fruiting units become productive early and remain calm.
What Delay-heading can do:
- Force breaks on shoots
- Overcome the blind wood that many apple and pear varieties suffer from
- Convert strong shoots into fruiting units
- Stiffen the wood early to protect the fruit from getting sunburnt
- Cause the fruiting zone to develop progressively from inside to the outer regions of the canopy
Much of the delay-heading technique was developed in South Africa by Professor D.K. Strydom of Stellenbosch University.