Twin leaders fill canopy quickly
The twin-leader system results in a tall, thin, uniform canopy that opens up possibilities for mechanization.
Ray Fuller (right) and Andrew Del Rosario discuss a twin-leader block that Fuller planted in 2008. The trees were whips, which he headed back at planting so he could develop two leaders as the trees grew. The trees are Minneiska (trade name SweeTango) on Geneva 11 rootstocks.
When organic grower Ray Fuller replants his orchard, he has two major economic factors to address. One is how to get the trees to fill their space quickly in order to generate optimum yields, and the other is how to effectively control weeds without herbicides.
In recent plantings at his Chelan, Washington, orchard, he’s planting trees three feet apart and growing the trees with two leaders 18 inches apart on the trellis. Fuller said that’s cheaper than doubling the number of trees planted, and the three-foot spacing in the row allows him to control weeds with an in-row tiller, which would be difficult with closer plantings. A mulch for weed control is not an option because it provides habitat for mice.
There are various ways to develop a twin-leader tree. For a 2008 planting of Minneiska (trade name SweeTango) on Geneva 11 rootstocks, Fuller bought whips, headed them low, and selected the two strongest shoots to grow up as the leaders. For more recent plantings, he used nursery trees specially grown with two leaders.
Dale Goldy, a partner in Gold Crown Nursery in Wenatchee, said that even before he heard about the Bibaum system that is becoming popular in Europe he had been looking for better ways to develop a twin-leader tree—to avoid having to regrow the tree in the orchard and lose time developing the canopy. The Bibaum involves double-budding trees in the nursery so they already have two leaders when delivered to the grower.
However, with double budding, there’s the risk that one of the buds won’t take, Goldy said. Nurseries had poor bud take last year because of winter damage from record low temperatures in November. If he had double-budded trees, it’s likely many would have failed. Some nurseries would have a market for trees where only one graft took, rather than both, but Goldy said his business comes mainly from custom orders.
Another way to grow a two-leader nursery tree is to train two shoots from one bench graft in the nursery, supporting each of the shoots with bamboo sticks. Goldy said that he first tried growing up the top two branches as leaders, but they were usually dissimilar in size. Now, he selects two similar-sized lower sprouts as leaders, although shading of one tree by another can still cause uneven growth.
A twin-leader nursery tree—whether double budded or grown from a bench graft—is more expensive than a standard nursery tree because of the additional labor at the nursery, but it is still more economical than buying twice as many trees, Fuller noted.
“Even if your cost per tree goes up, it’s not double,” he said. “When you plant 2,000 trees per acre and you’re paying full price, it gets to be a big number.”
Tom Auvil, research horticulturist with the Washington Tree Fruit Research Commission, said that with the twin-leader system both leaders tend to be calmer and more productive than a single-leader tree because the vigor is divided between the leaders. The twin-leader system should result in a tall, thin, uniform canopy that opens up possibilities for mechanizing some orchard practices in the future, such as pruning and thinning.
“In this production system, we’re not looking for any branches—it’s all bourse shoots and spurs less than one foot long,” he said.
Auvil recommends supporting the tops of the leaders with kite string (often sold as pea string) or bamboo and removing fruit at the top of the trees to encourage them to grow up to the top trellis wire. “The longer an unsupported leader is flopping in the wind, the more vigor loss will occur,” he said.
In Fuller’s experience, the two leaders rarely grow equally at first in the orchard, but he said he’s not too concerned because the variability lessens as the plantings mature. Auvil said one way to even out the growth is to tilt the most vigorous of the leaders at a slight angle, which will slow it down, while making sure that the less vigorous one is growing vertically.
Fuller favors the Mark rootstock, which he says performs consistently well on his replant sites. He has a block, part on Mark rootstocks and the rest on Malling 9 337, that were planted as Gingergold 20 years ago and grafted over to Ambrosia about 10 years ago, with the two grafts forming a twin-leader system. The trees are 3 feet apart with 12 feet between rows. The trees on Mark filled their space, but those on Malling 9 never have. He used Mark for twin-leader plantings of SweeTango on 3-foot by 10-foot spacings in 2010 and 2011. Fuller is looking forward to using G.41 or G.4214 for the replant tolerance and disease resistance.
On the newer plantings, where the nursery trees came with twin leaders, Fuller expects the trees to reach the top wire by the second leaf. His goal is to harvest 50 bins by the fourth leaf. He estimates it will take a couple of years longer to reach full production on the trees that he had to cut back and regrow in the orchard.
Once the trees reach the top wire of the trellis, Fuller cuts back the tops during the summer to maintain the height at 10.5 feet, just above the wire.
Andrew Del Rosario, horticulturist with Washington Fruit and Produce Company, also experimented with twin leaders in a block of Fuji planted on M.9 rootstocks in 2008. The trees are four feet apart with ten feet between rows, and the two leaders were developed in the orchard from bench-grafted trees.
He’s maintained some strong branches in the lower parts of the tree as “cheaters” to gain early production while the trees are growing, he said. As the trees grow, the larger wood will be shortened or removed to create a two-foot-wide canopy.
In trees where one of the leaders has grown poorly, he’s removed the weakest leader to focus on developing the remaining one and filling the tree space.
Washington Fruit is also training a 2011 planting of Red Delicious on M.106 rootstocks to a twin-leader system with the goal of establishing the orchard more quickly. The trees were planted four feet apart with ten feet between rows. They were headed back at planting, and as the trees began to grow, two similar-sized shoots were selected as the future leaders and supported with string attached to the trellis.
Auvil said the challenge with Red Delicious is to get the trees to grow tall, and any cropping will delay the vertical growth. The planting is on a four-wire trellis, and Auvil indicated the trees should grow five inches per week with a goal of 50-plus inches of vertical growth by the end of the first growing season.