Apple Top-Working: Managing the New Grafts

Top-working is the name for grafting established trees to another variety. The existing tree, the “stock”, is cut back, leaving just one or two limbs (nurse limbs) to supply the tree with energy.
Apple Top-Working: Managing the New Grafts - Articles

Updated: May 25, 2018

Apple Top-Working: Managing the New Grafts

Short sticks of 1-year-old wood of the new variety (the scions) are inserted (grafted) into the cut surface. Once the tissues of stock and scion make new connections, the scions begin to grow into a new tree. One of the first questions to address is how many scions to leave? The typical case is that two scions were grafted, and often both of these grow. If only one scion takes, then train it as a tall spindle/vertical axe/ central leader tree. If both scions grew, then there are some options:

  1. Select the biggest and trim down to one. However, this isn’t necessary, and it slows healing of the grafting wound. The wound where the stock tree was cut off will cover over from the base of the scion. If there are two, the wound heals from both sides and covers over twice as fast.
  2. You can leave both scions and bend the smaller of the two down across the center of the trunk to a horizontal angle. This essentially makes that scion into a scaffold limb. This works okay, but you should wait until the end of the first season to do this, as the strain of bending can cause the graft to break. This is a good way to manage more vigorous trees at moderate spacing. Always bend over the center of the trunk. Don’t pull the scion away from the trunk as it is easily broken when pulled away.
  3. The best way to train top-worked scions for high-density plantings with trellis is to leave both scions to grow as a bi-axis (two leader) tree. After the first growing season, crisscross them by bending them over the middle of the trunk, and allow the two scions to form an in-line V-shaped canopy (Figure 1). Two leaders will help to make a calm tree with a smaller, narrower canopy. Minimal pruning promotes earlier production. Top-worked trees re-establish very quickly, and it is often possible to get a partial crop in the second or third year.

The graft union is very brittle when the trees first start to grow. Provide support when the trees are still very small. Attach bamboo stakes/ vertical wire and tie the biggest shoot of each scion to this support structure. Initially, this will help prevent birds from landing on the tops and breaking them off. As the scions grow very rapidly, they will establish a large leaf area while the graft union is still very brittle. Without support, these can break in the high wind. After the biggest shoot is safely secured, rub off competing buds on each scion. Only one shoot per scion is needed to create the two leaders.

Figure 1. The Bi-Axis, or two leader tree is an ideal way to train top-worked trees in trellised plantings. Photo: Jim Schupp, Penn State

What if the buds were floral (Figure 2)? There will still be a vegetative shoot that will come out beneath the flowers. These buds will lag behind at first, but they will grow to make a perfectly acceptable leader, and you probably won’t be able to tell them apart in a couple years. Use caution if you feel the need to pick off the flowers, as the graft is brittle and subject to breakage if you yank on them too much. Protect any open blossoms against fire blight.

Figure 2. Sometimes the buds on the scion are floral. Although somewhat slower to grow, these will still produce a satisfactory shoot. Photo: Jim Schupp, Penn State

Prune out/ rub off any suckers that sprout from the trunk or the base of the nurse limb. Keep the scions open and exposed to full sunlight and spray coverage. If the scion(s) grow successfully, the nurse limbs should be removed after the first growing season.

First season pest management is similar to other vigorous non-bearing trees. Careful monitoring for fire blight is warranted, with the possibility of late opening bloom from the scion buds, and the overall vigorous growth. Monitor for, and control potato leafhopper (PLH). PLH seems to seek the tender foliage from top-worked trees, and too many of them causes chlorotic/ necrotic leaf margins (hopper burn) and stunted growth.

Top-worked trees grow rapidly, and there normally is no need for supplemental fertilizer. Provide supplemental irrigation to prevent water stress. This will keep the tree growing, so that the trees will fill their space by the 3rd or 4th leaf (Figure 3). Tree training is minimal. Top-worked scions grow extremely fast, and usually the lateral buds will break and form new side branches on the current season’s growth. Side branches formed this way have naturally wide branch angles, so limb bending isn’t needed. Keep the terminal growth attached to your vertical support structure, and re-tie these every 2-3 weeks, as needed.

Figure 3. These top-worked trees have filled their allotted space at the beginning of the 4th leaf. Photo: Jim Schupp, Penn State

Starting in the second season, the vigor and dominance of the terminal growth can be managed by adjusting its angle. More vertical terminals will grow taller and have smaller side branches. Terminals that are bent to 30-35 degrees from vertical will be somewhat shorter and will have some stronger side branches. An angle of about 25 degrees from vertical often provides a good balance of vigor, and is wide enough to allow adequate space for both leaders. After the second year, the leaders are permitted to grow vertically.

More information on grafting

Fruit Tree Propagation - Bark Grafting 

Also available in Spanish - Injerto de Corona o Corteza  

Authors

Tree fruit production Orchard management systems Crop load management of tree fruit Fruit tree pruning and training

More by James Schupp, Ph.D.