by Bas van den Ende - Consultant in fruit production (ret.)
To obtain early and high yields from a modern high-density orchard, you must start with planting good quality nursery trees. Tell your nurseryman what type of tree you need (i.e. unheaded, feathered, whip), and what orchard system you are going to use. You have to have a plan how you are going to establish and train the trees. Do your homework first before you order your trees.
Once planted in the orchard, the trees must fill their allotted spaces as quickly as possible. It is the way you manage your young trees that determines how well the trees grow and with what type of wood they fill their spaces. Good soil conditions, micro-irrigation, fertilisers and a long warm growing season promote strong growth. It is therefore not difficult to get fruit trees to grow, unless weeds and/or too much or too little water impede growth of the trees. In many instances, strong growth creates strong wood, which is often exacerbated by a vigorous rootstock. So, how would you deal with vigour?
Take advantage of the strong growth by ‘steering’ this growth in the direction of a) building a simple permanent tree framework, and b) dress this framework with fruiting wood. To lay this foundation, train and prune your trees in summer, not in winter.
When you train and prune your trees in summer you direct growth and do away with potential big, unwanted wood. In summer you have control over your trees, in winter you have not. Why is this so?
In summer, the top grows in harmony with the roots. With the top I refer to the part that grows above the ground. The roots take up water and nutrients from the soil and produce certain plant hormones that are necessary for growth. In return the top grows leaves to produce carbohydrates and plant hormones that are necessary for continued growth of the top and roots. Ultimately, the roots control the top. This harmonious relationship continues until growth stops in autumn. After one year in the orchard, the balance between roots and top has been established and the tree will maintain this balance for the rest of its life.
If you prune young trees in winter, instead of summer, you correct growth. In doing so, you unconsciously upset the balance between roots and top. The tree does not like this and will first restore this balance with new vigorous shoot growth around and below the cuts that you have made, before the tree will produce additional growth. Most of this compensatory shoot growth creates forks. Forks cause crowding and shading unless you eliminate these forks and other wasteful growth. This battle will go on for the rest of the life of the trees and can be very costly for you.
Much money, and time can be saved and headaches prevented, if you understand how trees grow and how you should manage the trees accordingly.
The key is to manage young fruit trees in such a way, that you need to do as little or no correctional work in winter. You should aim for ‘finished’ trees before they go dormant. ‘Finished’ trees need very little or no training and pruning in winter. Your trees remain calm and will respond with an abundance of flower buds. If you set yourself this goal, you are well on the way of setting the orchard up for a long and productive life.
Pruning young trees in winter such as shortening laterals of peaches or removing superfluous spurs in apple, pear or plum trees, does not upset the balance between roots and top.
How can you direct growth in summer to make young tree productive?
To answer this question, you must first understand something about the role of hormones in a fruit tree. One hormone is called auxin. It is produced in developing buds and young leaves. Auxin promotes growth of cells in the tips of young shoots. Auxin moves down by gravity through the phloem. As it does so, it stops buds below the tip from developing into shoots. This phenomenon is called apical dominance. Phloem is a thin layer of cells between the bark and the cambium layer and transports photosynthates and auxin produced in leaves down to the roots.
Another hormone is cytokinin. It is produced in root tips and seed of the fruit. Cytokinin is responsible for bud break and cell division. It moves up to the top through the xylem with water and nutrients. Xylem is a thin woody layer immediately under the cambium layer.
A group of hormones are called gibberellins. These are produced in new leaves and young seed. Gibberellins are responsible for cell division and expansion, and can repress the development of flower buds.
When you cut a new shoot in half in summer, you temporarily remove the growing tip and the source of auxin and gibberellins. Now cytokinin moves from the roots to the highest buds on the shoot under the cut. These buds form new shoots and the new shoots produce auxin and gibberellins again. Your pruning cut has created more shoots and more of these hormones, which only promotes more growth. Cutting new shoots in half stimulates a lot of regrowth. This regrowth produces a lot of auxin and gibberellins, which retard development of flower buds. With other words, if you indiscriminately cut shoots in summer, all you do is promote more growth which delays cropping.
If you remove whole shoots in summer, called shoot rubbing, you do not create more growth. The shoots that you want to keep are trained in the direction that you want them to go. Shoot rubbing does not increase the levels of auxin and gibberellins in the tree. The remaining shoots can form flower buds as soon as the shoots stop growing.
It is often necessary to manage some strong watershoots in young trees. You can convert strong watershoots to fruiting wood if you use the four-finger cut. This cut leaves one or two weak buds on a short stub. The bud under the cut, which faces downward, develops into a new shoot with the help of cytokinin, auxin and gibberellins. You have created more fruiting wood, without a whole lot of unwanted regrowth.
Think about these three hormones in your trees when you train and prune. Use them to your advantage in summer by removing unwanted growth (shoot rubbing), directing growth (training), converting strong growth (four-finger cut), which will stimulate growth in the tree parts that you want to keep, and is necessary to fill the trees’ spaces. In doing so, your young trees will grow fast, remain calm and become productive at an early age.