How many trees did you plant last year? I will bet more than you think; probably almost twice as many trees as you think. Where were all these additional trees? You only need look at the bottom of every tree you planted: It is the rootstock.
You might never have even considered this as a separate tree, but it is. You know a lot about the cultivar, in fact you selected it specifically for its autumn foliage color, flowers, form or another attribute, but what about the rootstock?
The number of landscapers who have never really given this question much thought surprises me. If they were an apple grower, they would not have a dismissive attitude about the bottom of their trees. They would know as much about the belowground tree as they do about the aboveground.
Apple cultivars vary considerably in their fruit color, flavor, texture and ripening times, but an orchardist also knows the characteristics of the rootstocks. An apple cultivar might be placed on an M.7a rootstock if a mature height of about 60 to 70 percent standard is desired or a P.16 if a much smaller tree, about 20 to 30 percent standard size, is needed. If the soils are droughty, perhaps a MM.111, or if it’s in a cold climate the hardy rootstock M.26 might be used. The choice of rootstock is as important a decision to an orchardist as the cultivar placed upon it. Unfortunately, we rarely give this a thought with landscape trees, ignoring the influences and potential of the rootstock.
Taking care with budding
We have a small number of tree cultivars propagated from cuttings, root sprouts and even tissue culture, but the vast majority are budded, an operation where the cultivar is reduced to a single bud. Budding involves inserting a bud from the desired cultivar into a seedling that will function as the roots. The bud is slipped into the bark of the seedling a few inches above the seedling roots. The seedling stem is pruned off before the bud opens and in the spring, a new stem is produced from this bud. Because this new shoot originates from the side of the seedling stem, it results in the slight crook so commonly seen on the lower trunks of young ornamental trees.
Different cultivars of the same species can generally be budded into a seedling of that species. There are a few exceptions, such as red maple (Acer rubrum), where some cultivars are incompatible with rootstock from even other red maples. Many cultivars of red maple and Freeman maple (A. × freemanii) are propagated by cuttings to avoid issues with delayed incompatibility between the scion and rootstock. However, for many cultivars budding between species in a genus is usually possible and is frequently done.
Back in the day when ash (Fraxinus) was a popular landscaping choice, it did not matter if you purchased a black ash (F. nigra), blue ash (F. quadrangulata), green ash (F. pennsylvanica) or white ash (F. americana) cultivar, you received the same rootstock – green ash. I used to refer to green ash as the universal donor, as it seemed to be used on every ash cultivar.
If the rootstock sent out sprouts from the trunk of the rootstock, the leaves and bark would appear different. There was an interesting “clump” ash tree growing in the Rapid City, South Dakota, arboretum labeled as Autumn Applause white ash (F. americana ‘Autumn Applause’), but only one of the four trunks was a white ash. The rest were green ash. This made for an interesting autumn foliage color!
I am now seeing Siberian elm (Ulmus pumila) being used as the rootstock for some of the hybrid elms. Many of these hybrids are outstanding ornamental trees (if properly trained), but I am occasionally called upon to look at a “clump” elm consisting of one hybrid elm stem crowded in among spindly Siberian elms. This makes for a very ugly tree.
The true definition of ugly I reserve for trees in which the scion is budded onto the stock at about four to five feet above the ground. There are a number of lilac “trees” where a dwarf Korean lilac (Syringa meyeri) or other shrub lilac is budded onto a Japanese tree lilac (S. reticulata) stem at about four feet. I have never been fond of these trees, but beauty is in the eye of the beholder so I am sure my opinion is not shared by all. However, I am certain that no one finds attractive a Japanese tree lilac pushing up through the scion’s lollipop crown. This frequently occurs if the plant owner is not vigilant about pruning off any shoots that arise along the trunk.
Consider the environment
Another challenge with this mixing and matching scions and rootstocks is: On what do you base the tree’s environmental requirements, the top or the bottom? I remember one instance where maples planted in Minnesota were budded on stock that originated from the South. While the cultivar was adapted to northern climates, the rootstock was not. When the planting experienced an open winter, one in which the soil and lower trunk were not insulated by snow, the trees died. Interestingly, the top started to leaf out in that spring and then quickly wilted. All the tissue above the bud survived the winter; all the trunk below the bud did not.
This same problem can occur when a less hardy species is used as the rootstock. I also have seen State Street® maple (A. miyabeli ‘Morton’) planted on hedge maple (A. campestre) rootstock. State Street® maple is hardy in zone 4; hedge maple is not. When we have an open winter, we can lose a tree because of this rootstock choice.
Which growth phase?
There is another issue with budded trees, but it involves the bud, not the rootstock. There are three growth phases in trees: juvenile, transition and adult. The lower trunk of a tree is juvenile. It does not produce flowers or fruit, a signature of an adult. Instead its energy is devoted to protection. In species that produce thorns, the thorns generally occur along the lower trunk. Unless a tree is browsed by giant sloths or modern day giraffes, there is no need to have thorns 30 or more feet above the ground. The lower trunk often has thicker and corkier bark to protect from environmental extremes. Juvenile tissue will often retain foliage during the winter.
The vegetative propagation of a meristem reproduces the phase of the source. This means that if you selected a bud from adult wood, the tree arising from that bud will grow differently than one from the juvenile wood. As an example, a tree originating from an adult bud will flower and fruit sooner than a tree selected from juvenile wood since it skipped a phase.
Another characteristic of juvenile tissue is a thicker bark. An interesting study done several decades ago illustrates the problem of starting trees as adults rather than as juveniles. Two rows of European beech (Fagus sylvestris) were lined out in the field, one started from buds from juvenile wood and the other from buds from adult wood. The only trees that suffered sunscald bark were the trees that originated from adult buds. I wonder if we could reduce the incidence of sunscald on some tree species prone to this disorder if we made sure that the buds came from juvenile wood.
There are many questions about the influence of the rootstock on the tree. Clearly this is an area that deserves more investigation, as knowing more about how the rootstock influences a tree’s performance might improve the quality of our ornamental trees. Perhaps one day we will be as familiar with roots as we are with cultivars, and just as orchardists order by cultivar and rootstock, we will ask for a specific rootstock for a cultivar. The characteristics of the bottom of the tree may someday be as important as the top.