Pruning apple and other fruit trees is especially important in a tree’s first five years, as it allows one to create a structure best suited to the space allocated. Best undertaken just before a tree emerges from hibernation, pruning involves three types of cuts, namely heading, thinning and notching, which can produce highly varied training systems both free standing and high density.
Why Should I Prune?
Pruning is not a particularly time consuming process, as it’s only necessary a few times a year. It allows one to maximise value, promoting plant health and reducing size. It’s essential for many fruiting plants, such as raspberries, gooseberries, and many fruit trees, which often only produce on new wood, with old unproductive wood unable to fruit.
How Does Pruning Affect Plants?
To understand how pruning affects plants, you first have to understand how plants function.
Plants harness the sun’s energy to produce sugars through the process of photosynthesis, which primarily occurs in leaves. They absorb water and nutrients through roots, which flow up by capillary action. As sugars and water move around plants, so can many other substances, such as hormones, which control plant processes.
The most important hormones are auxins and cytokinins, which act to give plants their structure. Auxins are produced in plants’ leaves and flow downwards, while cytokinins are produced in the roots and flow upwards. Auxins act to simulate root and shoot growth and cytokinins the production of lateral buds, which are initiated in a shoot’s tip. Auxins also act to inhibit cytokinin through a process of apical dominance.
When you prune a plant, the auxin-cytokinin pathway is interrupted. Without auxin flowing downwards, apical dominance is broken and dormant buds emerge at the shoot tip. Hence, pruning encourages branching. It also temporarily upsets a tree’s balance between top and bottom, causing cytokinin to become dominant.
Types Of Cuts
The most common cuts practiced are known as heading and thinning. Heading removes part of a shoot or limb, while thinning removes an entire shoot or limb. As pruning causes branching, heading cuts alters a tree’s natural habit, causing unnatural branching. As thinning cuts completely remove a limb, a tree’s habit is unchanged, albeit with missing branches. Heading isn’t necessarily bad, and is commonly used with fruiting plants, but it’s important to note by heading trees you are altering its natural habit.
Training vs Pruning
It’s important to practice pruning and never let a tree get out of hand, as remedial pruning is more challenging and detrimental to the tree’s health than bi-annual maintenance. In the early years of your tree’s life, it’s best to practice formative pruning or training to establish an ideal form. For example, with hedging plants you may cut down to 15-30cm to encourage branching at ground level, giving you a thick screen from bottom to top. Training can take a long time, up to 4-5 years, but gives you the best value for the rest of a tree’s life.
Pruning is more about function and size. It can be used to encourage fruiting, by removing old unproductive wood, or simply maintaining an ideal size. In early years, you may reduce vigour, removing weak or crowded growth. In later years, you may try to promote vigour, removing old unproductive wood, and remove shading branches, maximising sunlight penetration.
Establishing a Framework
Note: most trees will arrive 2 years old and have already been pruned at the nursery. Some steps below may be unnecessary.
The point in the trunk from which the first branch emerges is known as the head. Trees can be low or high-headed depending on what height the first branch is maintained. Low headed trees are structurally stable, easy to harvest and necessarily produce earlier (because less branches are removed). While high headed trees are disadvantaged, they are grown for their ornamental value.
The major branches that emerge close to the head are known as the scaffold branches (and branches growing off the scaffold branches secondary scaffolds). Selecting the best primary scaffold branches is key to establishing productive fruit trees, as because any tree has limited resources it’s best to concentrate on a few scaffolds. Any young tree will have dozens of shoots to choose from but only select the strongest, removing weak growth and those too close together. Optimal scaffolds have wide crotches, characterised by gaps between the trunk and scaffold, which ensures they are mechanistically strong, grow horizontally and have a reduced risk of disease. Wide crotches can be established by weighing down branches.
Left unpruned, trees grow into structurally stable forms able to withstand strong winds and bear fruit load. However, they grow too tall, with shading of the interiors. What is required is known as codominance, whereby the main stem is relegated in importance, and more energy is transferred into the scaffolds on which the bulk of the fruit is produced. Multiple, training systems were developed to establish codominance.
The open centre system involves severing the main trunk up to a 1m of newly planted trees. Early severation reduces the dominance of the main stem and leads to branching. From these branches 3-5 are selected, growing in different directions. The system is ideal for growing and harvesting fruit, but isn’t mechanically strong as the heavy scaffolds are so close together.
The delayed open system involves first selecting a few scaffolds and then severing the main trunk, higher up. A problem with this method is that adequate branching (to be selected as scaffolds) is not guaranteed.
The modified leader system is an intermediary between the two extremes. Here, the main trunk is left to develop until it reaches 2-3m and growth is restricted. Scaffolds are selected as normal, however laterals higher up the trunk are selected to compete with the central leader, reducing apical dominance. The central leader and the laterals are periodically pruned, maintaining the tree at one size.
High Density Training Systems
Better suited to the average garden are high density training systems such as fan, cordon, espalier and stepover, although the latter two are only suitable for apples. Most high density systems require rigid horizontal wires run at intervals, bamboo canes, weights and twine or ties to keep branches in place.
Fan and espalier require more vigorous rootstocks such as the M26/MM106. Stepovers require very dwarfing rootstocks, specifically the M27. With cordons the M27 is again recommended, but the M26 will suffice. It’s important you choose spur fruiting trees as shortening any tip bearers cause a drastic loss in fruit output. Where possible, choose maiden trees; they are not only easier to shape, but also cheaper.
Espalier training involves repeatedly severing a young tree’s main trunk to establish tiers of branching on one horizontal axis. A first cut is made about 40cm above the ground, above a few strong buds. Once the buds have grown, two opposing branches are then tied to galvanised wire, and weighed down, ensuring they grow horizontally. Another branch is then selected to be the new central leader. This one is then headed around 30cm from the first tier, and the process repeated. Up to 4 tiers can be established this way.
With fan training, the first cut is again made about 40cm from the ground, but this time the resultant branches are trained at 45 degree angles, attached to bamboo canes. From each of these two branches, four shoots are chosen, two growing upwards, one on the lower side and another to further extend the arm, and these are again attached to bamboo canes in turn. No central leader is tied in.
Cordons are very simple to set up and are often grown at 45 degree angles. Every year, the leader needs to be shortened by about a third, and branches shortened to three or four buds.
With stepovers, you are carefully bending the central leader until it grows horizontally, tying it to a low-lying wire. The key is to tie at multiple points as to distribute pressure.
When Should I Prune?
Pruning during the growing season allows quick recovery. Just before the end of dormancy is ideal, although pruning in this period may remove flower buds. It’s common to wait till after flowering, but you can continue tidying to midsummer. Avoid pruning late summer and early autumn as it will make plants vulnerable to frost injury.
Avoid pruning during times of drought. Leaves produce a plant’s food, and severe damage to a plant’s production capability can result in death.
Remove the 3 D’s: dead, dying and diseased wood. These will only worsen your plant’s health.
Remove water shoots and suckers. Water shoots are vigorous shoots that grow from the top of a branch upwards. They will not fruit, but cause crowding and waste energy. Suckers emerge from a tree’s roots, and will compete with the central stem for resources. With grafted trees, suckers will be a completely different variety entirely.
Notching is another cut, similar to heading, but more precise. It involves making a notch into the bark, above a bud, interrupting the flow of auxins, causing the bud to grow into branch. It can be used to force secondary and tertiary scaffolds in the optimal locations.
Consider how large the secondary scaffolds will end up. If two secondary scaffolds are close when young, they’ll definitely cross when old. Remove one.
Consider biomechanics. A tree adapts to its exposure. Leaving it lopsided, or without too many branches, can cause a plant to sway in the wind, and potentially topple.
Consider shading. Often branches are left on the south side to shade the trunk during peak heat.
Consider rootstock. If you want to experiment choose a vigorous rootstock. Extreme dwarfing rootstocks do not grow fast enough to establish an elaborate structure and are best left as a bush.
Skillscult is an invaluable resource for pruning, and expands upon all the concepts in this post.