Same seed source--big differences in rootstock growth

   When we were harvesting a cultivar trial, I came across a plot of four Major trees that were fairly uniform in size and all bearing a good crops (photo at right). These trees were field grafted back in 1985 to Giles seedling rootstock trees. The grafts were placed at 18-24 inches above the soil surface and after 30+ years you can still see the graft union on each tree (note the abrupt change in bark texture).

  
    As I walked down the tree row, I noticed significant differences in the diameter of the rootstock as compared to the trunk diameter of the scion (photos above). For tree "A", the Giles rootstock has over-grown the Major scion. Tree "B" has a smooth transition between rootstock and scion. Trees "C" and "D" are more typical of trees grafted with a Major top--the scion overgrows the rootstock.
    Major is a vigorous growing tree, often producing the largest tree in a planting of several cultivars. That's why it so common to find Major scions overgrowing their rootstock. However, I wanted to show you these photos to make two points. First, no matter the seed source for the rootstock, there will always be variation in growth among rootstock trees. Each pecan rootstock tree has a unique genetic composition created by a known mother (in this case Giles) and a unknown father (pollen blown to the stigma on a puff of wind). This variation may cause differences in the appearance of a graft union but it appears to have little impact on the scion's performance and yield.
    The second point I wanted to make is that is not that critical to plant a particular seed source to grow rootstock trees. In northern pecan areas, you should use seed from either local native trees or nuts produced by a northern cultivar. The resulting trees will have the best cold hardiness for your location.  Many years ago, we had some Giles trees growing at the research station that had been propagated by a southern nursery that used a southern pecan cultivar for growing their rootstock trees. In 1989, temperatures dropped to -26 F (-32 C) in mid-December.  The Giles tops survived the cold but the rootstock portion of the tree was killed by the cold. With a dead root system, these tree had to be removed.

Importance of a vigorous rootstock

    We were harvesting pecans today and couldn't help but notice the graft unions of two of our older trees. As I clamped the shaker onto the trunks,  I asked myself, "What happens when the scion cultivar out grows its rootstock?"
    In the photo at right, a Major scion was grafted about 4 feet up into a native pecan tree. Note the slight bulge at the graft union and the fact that the  Major top has a larger diameter than the native rootstock. This tree was originally grafted way back in the late 1950's and is currently about 2 feet in diameter. This is also the only Major tree on the research station that has out grown its rootstock.
    Major trees tend to grow more vigorously that other northern pecan cultivars. Major trees that we grafted on to Giles seedling rootstock back in 1983 are nearly the same size as the Major tree pictured here. It became obvious to me, that the Major tree in the photo, was being dwarfed by a inferior rootstock.
    I shook the tree and found its nut production to be similar to other Major trees of equal trunk size. The problem is, at 55 years old, this Major should be closer to 30 inches in diameter.  As far as I can tell, the inferior rootstock has had only one effect on the scion cultivar--it has slowed tree growth. However, slower tree growth means the tree is slower to come into bearing age and has less cumulative nut production over time.

     The Major tree is not the only tree that has shown scion over growth. In the photo at left, a Peruque scion was grafted into a native tree also sometime in the late 1950's. This tree is even smaller in diameter than the Major tree pictured above. If you take a close look at the photo of the Major tree, you can see the Peruque tree just to the left of the Major trunk in the background. Since these two trees are only 100 yards apart, it is likely that the seed parent for both inferior native rootstock trees could be the same.
     On my farm, I have hundreds of volunteer pecan seedlings that could be grafted. But which ones should I use? Will I be condemning certain scions to a life of perpetual slow growth if I choose the wrong seedling? How can I tell?
     I only graft seedlings that have demonstrated a strong growth rate--2 feet of new growth in a single year. If a tree seems to be struggling to grow as a sapling, it is a pretty sure sign that tree will not provide a vigorous enough rootstock for any scion cultivar.