All trees have a shoot system, or top, and a root system. With few exceptions, the top is genetically different from the root system. They are two different plants, genetically distinct, growing separately. The separate plants were united in a way that caused the two to grow together and function as one. This is accomplished through plant propagation.
There are two methods of plant propagation: sexual (seed) and asexual (vegetative). Sexual propagation entails the recombination of genetic material. In nature this results in progeny that differ from each other and from their parents. Vegetative propagation is clonal; progeny are genetic copies of the parent plant.
Budding and grafting are asexual or vegetative techniques used to maintain the cultivated varieties (cultivars) nearly all of which are clones. Due to open pollination, more than 99 percent of all seedlings grown from clones bear fruit that is inferior to that produced by the parent trees. Fruit will be unlike parents in flavor, color, date of ripening, and many other characteristics – typically not “true-to-type” or “true-to-name.” For this reason, it is necessary to graft or bud most kinds of fruit tree seedlings to the desired variety to obtain a “true-to-name” tree of any known variety.
Although all members of the same clone have the same genetic makeup and can be exactly alike, environmental factors can greatly modify the expression of the genetic character so that the appearance and behavior of individual plants can be strikingly different. An orchard of ‘Delicious’ apples that is pruned, irrigated, sprayed and fertilized properly for high quality productivity will appear totally different from an adjacent abandoned orchard of the same cultivar, yet the plants are genetically identical.
Cuttings are used mainly in the clonal propagation of herbaceous and some woody ornamental species. Cuttings are less frequently used for fruit and nut trees. A cutting is a piece of vegetative tissue (stem, root or leaf) that, when placed under suitable environmental conditions, will regenerate the missing parts and produce a self-sustaining plant.
Stem cuttings are of several types. Some species can be readily propagated from hardwood cuttings of stems taken in late winter or early spring. These cutting, which include several nodes (usually 6-12" of stem), are placed in the ground in nursery rows with just the top bud showing. The cuttings will generate roots and can be, after a year’s growth, transplanted as self sustaining plants.
Grapevines can be readily rooted using hardwood cuttings and have long been propagated this way. This is one reason that some of our grape cultivars are so ancient. Some olive cultivars are also readily propagated by stem cuttings. Fig and pomegranate can be propagated this way. Most other fruit and nut tree species will not form roots from hardwood stem cuttings, or will only do so with great difficulty. Some of these can be induced to produce roots by treatment with plant hormones and/or heating the cutting beds; others cannot be induced to form roots under any circumstances.
Layering is another method of rooting stems, in which stems are induced to produce adventitious roots while they remain attached to the parent plant. Some plants (blackberries, raspberries) naturally form layers when the tips of branches touch the ground (tip layering). Others can be manipulated to do so by simple layering or mound layering.
Simple layering is used to propagate filberts (hazelnuts). In early spring a long shoot is bent to the ground, placed in a hole several inches deep, and bent back so that the tip of the shoot is exposed above ground. The hole is then filled with soil. The curved section is usually cut or nicked which promotes rooting. After one season’s growth the rooted layer is cut from the parent plant, dug and transplanted.
Mound or stool layering is widely used to produce clonal rootstocks of apple and plum. Plants are cut back almost to ground level and allowed to sprout new shoots, soil (or a mixture of soil and sawdust) is mounded up around the bases of these shoots and the mound is built up as the shoots grow. Roots develop at the bases of these shoots. The following spring the rooted layers are cut off and transplanted into nursery rows for another season’s growth. The stool bed is handled in the same way the next year for another crop of rooted layers.
Grafting & Budding
Grafting and budding are the most important means of propagating fruit and nut trees for two reasons:
- Species and cultivars that cannot be propagated by cutting or layering can be propagated by budding and grafting.
- Budding and grafting allows the use of rootstocks with desirable characteristics that make them preferable to growing a tree on its own roots.
Grafting and budding involve joining two genetically distinct plants so that they unite to continue growth as a single plant. The two parts of the compound plant are known as the stock (or understock or rootstock) and the scion. The stock refers to the lower part of the grafted plant—the part that produces the root system. The scion is the upper portion that produces the shoot system. In budding, a detached bud of the desired variety is placed under the bark of a seedling tree. In a few weeks, the bud shield and the seedling heal together, then the bud of the desired variety grows to produce the new tree, which is genetically like the parent tree from which the bud was taken and which produces fruit true to the variety.
In grafting, a short section of a shoot taken from a tree of the desired variety is inserted into a limb or trunk of a seedling tree.
There are several methods of grafting and budding; in all the objective is to bring the cambium layers of the stock and scion together and to hold them tightly while the graft union forms.
Buds on the scionwood and budwood (material to be used as the scion or bud source) must be dormant when the grafting or budding is done. For whip and cleft grafts the operation is done with freshly collected scionwood onto dormant stocks. Bark grafting is done after growth has started in the spring; for this purpose scionwood is stored at about 0°C until it is used.
It is essential, for all grafts, that the graft union be held together tightly and secured by tying or wrapping using string or rubber bands (whip graft), wedging (cleft graft), or even nailing (bark graft). The fresh graft is sealed with grafting wax to prevent drying of the graft union before it heals.
Whip graft: the stock and scion are of more-or-less equal diameters. Whip grafting is often used in root grafts where scion wood is grafted to a piece of root. Illustration of whip grafting.
Cleft graft: scionwood ¼ to ½ inch in diameter is inserted into stubs of stock that are 3 to 4 inches in diameter. Usually done in the late winter, early spring before growth resumes.
Bark graft: Used for species that are difficult to graft. Done in the spring after growth has resumed and the bark separates easily from the wood.
T-budding is the most common method for propagating fruit trees. In T-budding, a T-shaped cut is made in the stock. Buds (taken from budsticks or budwood) are inserted under the bark of small seedling stock plants a few inches above ground level. The buds are inserted and tied in place with budding rubbers. After growth starts the tops of the seedling rootstocks are cut off. T-budding is usually done in the late summer. T-budding illustrated.
Patch-budding is used for thicker barked trees, especially walnut and pecan. Here a patch of bark is removed and a same-sized patch with the bud is inserted in its place. Patch budding is normally done during the growing season when the bark separates readily from the wood along the cambial layer.
If you have one peach tree in your yard and want to extend its fruit-bearing season, you can bud or graft one or more variety of peach on it. You can bud several varieties of peach on a young tree or graft two or three additional varieties onto an older tree or add a pollinating variety to a tree by grafting.
Within a limited amount of space, you can grow several varieties of fruit on a few trees. However you can usually grow only like kinds of fruit on the same tree. There are a few exceptions to this rule. For example, you can grow plums, apricots, almonds, nectarines, and peaches on peach seedling roots, but the growth habitat of each differs, so it is difficult to manage these different species on the same tree. The safest method is to put varieties of like fruits (species) on one tree and varieties of another on a separate tree.
Rootstocks are a means to propagate clonal cultivars. Many species and cultivars cannot be propagated on their own roots. The only means of clonal propagation is by grafting or budding onto rootstocks of the same or related species. Rootstocks offer an opportunity to adapt a given tree to additional environmental factors as well, and, in many cases, trees on carefully selected rootstocks, can be grown in sites where it would otherwise be impossible or nonproductive.
Rootstocks provide resistance or protection against soil-borne organisms that are pathogens or pests. More common than insect-resistance (i.e. phylloxera) is resistance to soil-borne pathogens, especially pathogenic fungi and some bacteria. These include organisms that cause oak root rot, stem and crown rots, wilt diseases and crown gall. Another important class of soil pest is nematodes, microscopic worm-like organisms (not related to earthworms), which can be highly damaging to a great range of plant hosts. Peach trees are highly susceptible to some species of nematodes; ‘Nemagard’ rootstocks for peach are resistant and allow peach trees to grow in nematode-infested soils.
Virus-resistance is an issue for citrus. Sour orange, once widely used as a rootstock for sweet oranges, is no longer used in California because it is susceptible to tristeza virus (quick decline). Appropriate rootstocks are used for local conditions. If, for example, oak-root fungus is not a problem in an area, there is no advantage to using an oak-root fungus-resistant rootstock.
Other factors involved in the selection of rootstocks.
Controlling tree size: Ultimate tree size can be controlled by rootstock in many species. Apple is the best example. A complete series of apple rootstocks exist which regulate the size of the tree from the most dwarfing to quite large. Dwarfing rootstocks are used in many other species: pear, cherry, plum, peach, citrus and others.
Adaptation to unfavorable soil conditions: Rootstocks may be tolerant of poorly drained, heavy clay or saline soil conditions.
Resistance to low winter temperatures: Some species, especially apple and citrus, survive cold winters better on some rootstocks than others.