Happy Easter! This week’s topic was prompted by a reader after a couple of her fruit trees were girdled by voles this past winter. If you have topics that you would like me to write about, send me an email email@example.com. I do enjoy the correspondence. I cannot promise I will write about the topic right away, but I will try to do so in a timely manner.
Intentional grafting is one of the oldest horticultural practices known to humans. It dates back thousands of years and was widely practiced during the Roman Empire and very likely predates that era as well. Generally speaking, it is a technique to join two different plants together in a manner that allows them to grow together as a single plant. Grafting is also used sometimes to join the same plant back together when the plant has been damaged. For example, an apple tree that has been girdled by rodents can sometimes be saved by creating “bridge grafts” that connect the healthy tissue below the damaged area to the healthy tissue above the damaged, by bypassing the damaged tissue. Small diameter branches from the upper part of the tree are grafted onto the trunk, both below and above the damaged tissue, creating a “bridge” to bypass the damage.
Most of us see examples of grafting every day, if we live in a community that has street trees. Street trees of all species are usually grafted in the nursery where they are grown. We also eat tree fruit on a daily basis that are the result of grafting. Almost all of our apples and most other tree fruit are grafted varieties.
The process is really quite simple. It is a manner of connecting the plumbing from one plant to the plumbing of another plant by aligning the pipes properly. Plant plumbing consists of tubes or vessels that are like vertically connected straws. Xylem vessels are the water connecting pipes and phloem vessels are the food connecting straws. In between the pipes (vessels) are cells and tissues that can regenerate new pipes and allow fusion to occur. In order for a graft to be successful, the two different plants generally need to be pretty closely related. Apples graft easily onto other apples and pears graft easily to other pears, but apples and pears don’t graft well to each other. Sometimes the grafted partners really don’t look all that much alike, nor are they always very closely related. Pears are often grafted onto quince roots and lilacs are often grafted onto privet (a common hedge plant) roots. I am not sure how these “odd couples” were discovered, but I imagine there has been a lot of trial and error.
There are many reasons for grafting ranging from economic to purely esthetic. Street trees are grafted to insure a nice straight trunk that you can walk under without side branches hitting you in the face. Grafting a bud onto an existing trunk above a well developed root system insures that the shoot growing from that bud will grow fast and long. Typically a bud from the desired cultivar is grafted to the trunk just above the root system during the dormant season. When growth commences and the bud begins to grow a new shoot, the remaining top portion of the tree is removed so that all the energy, water and nutrients the root system is capable of producing is channeled into that single stem arising from the bud. Other shoots that may arise from the roots or trunk are removed so as not to compete with the desired one.
In nature it may take many years for an apple tree to bear its fruit while growing on its own roots, but you can graft a mature branch of a different variety onto that tree when that tree is young and get it to produce fruit in a single year. There are well over 2,000 different named cultivars of apple trees and hundreds of different rootstocks they may be grafted onto. Grafting does not just represent the characteristics of the scion, i.e. the part that is attached, but also is influenced by the characteristics of the plant that it is attached to (the stock). Some apple rootstocks limit the size of the tree to 6 feet or less, regardless of what variety is grafted onto it. Other rootstocks may tolerate adverse conditions such as excess moisture or drought. Others may confer disease resistance, anchorage ability in wind and other traits.
Next week I will go into the process of grafting.
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