Earlier this year, I wrote about
top-working a young tree by grafting both the central leader and three side limbs. Just to remind you of my grafting efforts, the photo at right shows the tree right after I completed the grafting process. By mid-summer all 4 grafts have grown new shoots but at very different rates. Watching this tree grow has taught me a valuable lesson about the nature of apical dominance in pecan trees.
By mid July, I had made several trips to this tree to trim off trunk sprouts and to prune each graft to a single growing shoot (photo at left). Each new shoot has been tied to its associated bamboo stake to prevent wind damage. What impressed me most about this tree is the fact that the scion placed on the central leader has far out grown the scions place on side limbs. The new shoot growing from the central leader is near 3 feet in length and still growing while side-limb scions had shoots less than half that length.
The difference in growth rate can also be seen by comparing the diameters of the scions. The photo above shows the growth of the scion on the central leader as compared to one of the side shoots. I placed a quarter in front of each scion to give you a sense of scale (a US 25 cent coin is roughly 1 inch or 25mm in diameter). Remember, when I placed the grafts on the tree the scions were roughly 7/16 inch or 11mm in diameter. As the scion grows in length, it also grows in diameter. What is especially impressive about the growth of the scion on the central leader is how quickly the scion will cover over the wound created by the grafting process.
Obviously, a pecan tree will expend most of its energy to quickly re-establish the central leader. This is why, when top-working, a bark graft is traditionally placed it on the central leader. Although the grafts placed on side limbs were successful, these limbs will continue to grow at a slower pace and will eventually become completely dominated by the scion growing on the central leader.
Many novice growers dream of growing several pecan cultivars by limb grafting several types of scions on the same tree. Yes, multiple grafts can be made but a graft placed on the central leader will always dominate the rest of the tree. This is why it is always best to graft a single cultivar on a tree and make that graft on the tree's central leader.
