by Sally Colby
Most orchards are established with carefully chosen, quality stock from reputable nurseries. But what if the trees you thought would appeal to U-pick customers aren’t turning out as you expected, or fresh market customers are asking for different varieties? Perhaps a family member joining the orchard wants to make cider and would like to grow different varieties? Some cider varieties are difficult to obtain as bare root trees, and some even have waiting lists.
“It costs about $30,000 per acre to put in trees, hardware, soil prep and everything else to establish,” said Dr. Daniel Weber, Penn State tree fruit educator. “If everything goes well, you can recoup that investment in about eight years. With topworking, trees are ready to bear in a shorter time.”
Topworking is the process by which a grower can change a tree from one variety to another by grafting. The process usually takes place in spring, prior to bud break, so timing will vary according to location.
The end goal of topworking is simple: healthy trees on good rootstock but with a new variety of scion wood without having to wait several years for trees to bear. Topwork is most often accomplished through cleft grafting, which is typically done prior to bark slippage, or bark grafting, which is done when bark is slipping but before buds start to grow.
In some cases, the best approach is to remove several large, protruding limbs and graft scions onto each of the remaining stumps. After the scions begin to grow, the grower can decide which of the scions will make the most ideal leader. The risk with cutting several limbs is creating multiple entry points for rot.
One-year-old scions for topworking should be obtained from healthy, vigorous dormant wood with plump buds. Select straight, strong scions, and use the central portion for the graft. If scions are obtained ahead of time, store them in a moist medium, wrapped in plastic. Scions can be kept at about 40º until they’re grafted. A dwarf interstem variety left between the base trunk and the new scions can help inhibit the growth of a vigorous variety.
To start the topwork, select a section of the existing orchard. The portion of the tree to be topworked, often the main trunk, should be cut off squarely and cleanly with a sharp pruning saw. Next, split the receiving trunk or limb to prepare it for the graft. If the tree is small enough in diameter, a knife may be sufficient to make the split. Larger trunks should be split with a sharp axe.
Next, prepare the scions with a sharp grafting knife, cutting the lower portion into a wedge about two to four inches deep. The smooth, wide taper goes into the cleft. After securing one or more scions in the cleft, use tape and tightly wind around the entire trunk to ensure the cambium layers have good contact and adequate moisture until the scion begins to grow. Growers have several options for sealing exposed surfaces, including toilet ring material, grafting wax or liquid grafting seal.
“Stick it down into the split so the vascular cambium lines up,” said Weber. “You can also angle cleft grafts to ensure better connection with the vascular cambium. Even if you’re only getting a cross-section of the stem that’s stuck down in there, you’re guaranteed getting actual contact.”
Weber said that when topworking is done in older orchards, pruning and other routine maintenance becomes easier. “Our goal in the industry now is to make super narrow canopies,” he said. “We’re removing almost everything that sticks out (beyond the spindle) to form a very narrow fruiting wall. By doing that, you can move the rows closer together, and mechanization and robotics is a lot easier.”
If two scions were grafted and both are growing equally well later in the season, plan to train them to create a bi-axis tree. After the first growing season, gently bend the two new leaders over the center of the trunk, allowing them to cross, to form a stronger, inline V-shaped canopy. “While the stems are still pliable, if you’re doing biaxial, pull them in so they cross,” said Weber. “Now the strain is all toward the inner part of the tree.”
The graft union is fragile and brittle when the tree begins to regrow, so it’s important to provide support. Bamboo stakes can be tied to the biggest shoot of each of the scions for support, which will also prevent birds from roosting on fragile tops. Support structures also help prevent breakage in high winds. Select and secure the largest shoot on each scion, then remove any other actively growing buds. If just one scion survives the grafting procedure, begin to train it as a central leader as in a tall spindle system.
Weber said topworked trees grow rapidly and don’t require supplemental fertilizer. Watch for signs of fire blight and other diseases and insect pests, and treat appropriately.
In the second season, terminal growth can be managed by adjusting the angle. Terminal growth bent to about 25º from vertical will allow trees to continue growing, with ample space for both leaders. After the second season, supports can be removed and leaders can grow vertically.