Heading for trouble
How to keep your trees calm.
Tree pruning is one of the most important horticultural practices in an orchard. When you prune in winter, you are tested on how you can balance vigor and fruitfulness. The key to this balance is sunlight, and how it is distributed through the tree canopy. Making the wrong pruning cuts can increase shading and decrease fruitfulness in the next growing season. This can cost you dearly.
Shoots and roots grow in balance. When you prune fruit trees in winter, you interfere with this balance. The shoots start to regrow in spring, but to do this, they need energy and carbohydrates. These come from carbohydrates stored in the roots produced in the previous growing season. In this way, the tree reestablishes the balance between shoots and roots.
When you make a heading cut in winter, cytokinins (hormones) move from the roots in spring, and along with carbohydrates and nutrients, accumulate in the buds immediately below the cut. These buds develop into strong shoots, which then establish apical dominance, that is, the strong shoots control the growth of lower shoots. Regrowth from heading cuts is very strong after you have headed into one-year-old wood in winter. The new shoots grow straight up, create shade, have no fruiting potential, and are costly to remove. You must avoid this.
New shoots grow strongly when you make heading cuts in winter. Heading cuts remove the tips that used to dominate growth or inhibit lower buds. Heading cuts also create forks and branches-on-branches in young trees. Forks create crowding, and crowding creates shade. Forks belong on your dining table, not in your orchard. Remove forks with thinning cuts, and singulate your leaders.
Use heading cuts with care, especially on young, vigorous trees, because heading cuts in winter disrupt the natural growth and shape of the trees. Postpone topping the heads for as long as possible. Keep the heads upright and soft. The very tip of the leader is the "rudder of the ship." It communicates with the rest of the tree, including the roots. The less you interfere with the head, the calmer the tree will be. A calm apple or pear tree has a soft head with many small fruiting branches and spurs.
The head or highest point in an apple or pear tree must not end in pruning cuts. The head must end in weak, slightly upright branchesnot horizontal or pendant branches. Avoid pruning the head to a one-year-old vegetative shoot, and thereby avoid shade in the canopy.
Avoid making many small heading cuts in winter, because they irritate the tree and it will be harder for you to keep the tree calm. If you have to make a heading cut in an apple or pear tree, only cut into wood that is more than one year old. If the branch is strong, cut to a spur or weak lateral that faces downwards. If the branch is weak, cut back to a spur or lateral that faces upwards.
When you look down on your trees that you pruned in winter, you should see as few heading cuts as possible. To do this, remove whole branches or limbs with thinning cuts. Do not remove part of these branches or limbs with heading cuts. Thinning cuts do not interfere with apical dominance, but heading cuts do. Pruning cuts that you can see when you look down onto a tree generate strong, upright shoots, also called risers. These are vegetative shoots (growth shoots) that cause shading and do not become fruiting wood.
If you have to make heading cuts, to limit tree height for example, make these heading cuts in spring or early summer. The later you make these heading cuts (but before the longest day), the weaker the regrowth will be.
Van den Ende is a tree fruit consultant in Australia's Goulburn Valley.