Vigour Control for Fruit Trees and Bushes
Having just been told about putting the vigour back into fruit trees, it may seem a bit strange to be told now how to reduce it. However this is not as contrary as it sounds. One of the causes of unfruitfulness in trees, especially, shall we say, teenage trees, is excessive vigour. If it is reduced, fruiting nearly always follows. There are a number of ways of doing so. Let us start with the easiest and gentlest, then work through to the hardest, most severe and least recommended and used – the horticultural ‘if all else fails’.
The first remedy is to make sure that the union between the trunk and the rootstock is above ground. If it is buried, roots will grow from the trunk and the restricting effect of the rootstock will be reduced, or even completely overpowered. If you find that the union is covered, scrape away the soil and cut off any roots that are growing from the trunk. The soil should not be replaced or exactly the same thing will happen in a year or two.
If the union is not buried, try altering the; assuming, that is, that you feed the tree. A common cause of excessive vigour is too much nitrogen. If, therefore, the nitrogen content of the fertiliser is higher than the potash level, change to one with more potash, such as a tomato or rose food. In cases of extreme vigour, it may pay to give potash alone in the form of sulphate of potash. This method is effective, but it may take a year to show results.
In the section on weeds, mention was made of using grass to control the vigour of trees. Grass seed is sown beneath over-vigorous trees in the spring. As the grass grows, it ‘steals’ some of the fertiliser and moisture from the ground that would otherwise have found its way down to the tree roots. This is a very quick way of altering the vigour of the tree and the opposite (getting rid of grass or weeds) is equally effective at increasing growth. The more ryegrass there is in the seed mixture, the greater the effect. Once the tree starts, or resumes, cropping, the grass can be cut hard or even killed or dug in.
These actions, either singly or in combination, will nearly always induce a tree to crop, but there are times when they will not. In this case, the remedy could be root pruning.
First, dig out a trench around the tree so that the inside is some 3ft (1m) out from the base of the trunk. The trench should be 2-3ft (60cm- 1m) wide. While you are digging out the trench, save the small and fibrous roots and tie them back out of the way. These will be needed later to keep the tree alive; we want to reduce its vigour, not destroy it.
Any roots thicker than about 2in (5cm) across should have a section the width of the trench cut from them. This is needed to stop the two ends joining up again, as has been known to occur where only a single cut was made. It also gets them out of the way and makes any further digging easier. It will normally be sufficient to dig to 2ft (60cm) deep, but undercut any thick roots you find going straight down; these are often the very ones that are causing the excessive vigour.
Once the operation is over, the trench should be refilled and the retained roots released and spread out carefully as the soil level rises. Firm them in well as you go.
The tree will need staking and should be watered during the following summer if there is the slightest indication that it is suffering.
Always root prune in the winter when the tree is dormant.
The final line of attack is ‘bark ringing’. This should only be carried out if all else has failed. It involves removing a complete ring of bark from around the trunk of the tree so that the flow of sap is all but halted (hence its use as a last resort; it can be lethal if carried out carelessly).
The width of the ring (top to bottom) should never exceed 1/2in (1.25cm), even on the largest tree, and the wound must be bound with sticky tape immediately after removing the ring to prevent it drying out In the normal course of events, this has such a dramatic effect on the tree that fruit buds are often formed in profusion in the year after ringing for fruiting a year later. Don’t let this encourage you to bark ring unnecessarily though; it’s still a tricky business.