Most fruit and nut trees available today are actually two different plants that have been grafted together. Where those two plants come together is called the graft union, or graft collar.
Graft unions are usually easy to spot. They tend to be lumpy, raised areas. The shape of the graft union depends on the type of graft used to merge the two plants.
People have been cloning plants with grafting for thousands of years, and this method of plant propagation is not limited to trees. Your store-bought tomatoes and other vegetables may also have been grafted.
Grafting is done by inserting a shoot or twig, called the scion, into a slit cut into the trunk or stem of another plant, called the rootstock. This allows the vascular bundles of the two plants to merge, allowing water and nutrients to move between the two.
Grafting allows us to take advantage of one plant variety’s strong root system and another plant’s heavy fruit, nut, or flower production. Grafting allows us to select plants for their combined characteristics of size, hardiness, growth habit, growth rate, disease and pest resistance, and flavor.
The name of the plant you buy usually refers to the aboveground portion of the graft, though you may also see mention of the rootstock. Grafting is what allows you to have a tree that produces multiple varieties of fruit. These graft unions are found higher in the tree canopy.
Bare root trees
Bare root trees commonly start appearing in garden centers in January in the Bay Area. If you shop from knowledgable, reputable growers, the plants they have available will be suited to the local climate. If you are shopping bulk discount stores, well, you may be getting something else entirely. Wherever you get your fruit and nut trees, be sure to inspect the graft union for signs of damage, disease, vine mealybug nymphs, and other insects. Graft unions are delicate, vulnerable areas. Until relatively recently, it was suggested that the graft union be positioned below soil level. We now know that this is a really bad idea, leading to several problems.
Graft union problems
Improper planting depth is currently the primary reason why trees fail. Part of this is due to bacterial and fungal diseases entering the tree through the graft union, causing root rot, crown gall, and phytophthora root and crown rot, among others. These problems can be avoided by ensuring that your plants are installed at the proper depth. This generally means that the graft union will be 2 to 4 inches above the soil level, or more. [The higher the graft union, the smaller the mature tree will become.] If stem growth starts occurring below the graft union, it is called graft union suckering.
Graft union suckering
Graft union suckering occurs when the graft is less than successful. These suckers start growing out of the root stock portion of the stem, which may sound fine for the root system, but it is bad for the overall plant. Very often, rootstock plants are highly susceptible to aboveground pests and diseases, and they rarely produce desirable crops. If graft union suckers appear, remove them as close to the branch collar as possible, without damaging the collar.
Take a closer look at the trees, shrubs, and other plants in your garden. Do you see any graft unions? Are they above the soil line? I hope so!
You can grow a surprising amount of food in your own yard. Ask me how!