Karl Foord, UMN Extension Educator
Most apples trees today consist of an upper part, the scion, and a lower part, the rootstock, to which the scion has been grafted (Exhibit # 1). The scion produces the apples with which you are familiar such as Honeycrisp or Haralson. The rootstock confers a number of important traits on this 'compound' plant such as precocious flowering and size reduction. A tree on its own rootstock would produce a full sized 25' high tree and could take 6 to 7 years to flower. A dwarfing rootstock can reduce the size by 50% or more and reduce the time to flowering to 2 or 3 years. Size reduction makes more efficient use of space and facilitates numerous operations such as picking, pruning, and spraying. Rootstocks confer other traits on the plant such as disease resistance, stress tolerance, and ability to tolerate low
Thus apple breeding and improvement efforts must not only create and test scions for fruit quality and disease resistance, but also create rootstocks that have the desired dwarfing qualities, cold tolerance, and favorable reaction with the scion. To this end many rootstocks have been created with different degrees of dwarfing. The standard is a tree on its own rootstock which will be a full sized tree. Dwarfing rootstocks are classed by the percent reduction in size conferred by the rootstock relative to a tree on a seedling rootstock (Exhibit # 2). The size variation can range from a tree you could grow in a large pot on rootstock P22 to a full sized tree on a seedling rootstock. Exhibit #'s 3 and 4 show tree size variation conferred by three commonly used rootstocks.
All well and good but how is such a tree produced? A bud from a cloned version of the original tree is T-bud grafted on a cloned version of the original rootstock. In August a dormant bud in the axil of a leaf from the scion say Zestar!TM is harvested from trees pruned in such a way as to produce a lot of these kinds of buds. A T shaped cut is made on the rootstock tree about six inches above ground level, and the bud is inserted to pair the cambium cells of both materials. The cut is wrapped with a paraffin tape and left to overwinter. In the spring when the bud breaks the rootstock tree is cut above the graft and the scion becomes the upper part of the tree. This process is described in greater detail with very illustrative pictures in the following document: http://www.kuffelcreek.com/GrowingApples/GraftAppleOrchard.pdf
The rootstock is also produced in a fascinating way. A cloned rootstock tree is planted and allowed to grow for a season and then is cut off at ground level. The next season a number of sprouts will arise from the rootstock and have the appearance of a bush. These branches are then mounded with sawdust covering around 10 inches of the new stem. The stem will develop roots between the uncovered part of the stem and the covered part still attached to the stump. At the end of the season the sawdust will be removed and the stems cut from the stump and the new shoots with roots will be planted as a new tree ready for the T-bud graft. These mounded layers are called stool beds.
If you go to buy an apple tree, purchase one where the scion and rootstock are both identified on the plant label. It used to be that the label on apple trees for sale would only indicate the scion and not the rootstock. At a big box store, I recently saw trees with labels that only indicated that it was a fruit tree. The label did not even identify the tree as an apple let alone as a Honeycrisp on M26. There is a big difference between Zestar!TM , Honeycrisp, and Haralson apples and as you now know a big difference between the rootstocks on to which they are grafted. To purchase a tree labeled in this fashion would be like buying a pepper plant and then waiting to see if it was a sweet bell pepper or a jalapeno.
All exhibits created by Karl Foord.