The use of dwarfing rootstocks is the primary means utilized to affect tree size. Apples are the fruit crop that most commonly utilizes dwarfing rootstock. Size range from the very dwarfing rootstocks such as M.27 and P.16 to nearly standard size rootstocks such as MM.111 and MM.106. In pears there is not nearly the range in size controlling rootstocks the majority of which are only +/- 20% the size of seedling. Recently dwarfing rootstocks for cherries have been introduced from Germany. There are also dwarfing rootstocks for peaches and plums although within the U.S. they are not very common commercially outside of California. In Europe the interspecific hybrid GF 677 is common for peaches.
While dwarfing rootstocks are the most common practice for controlling size in fruit trees, there are many other methods or techniques used to control the size and vigor of trees. Any practice increasing the harvest index or proportion of biomass going into fruits rather than wood or leaves will tend to reduce the size (total volume or biomass) and vigor (rate of growth) of trees. The following are some of the most common techniques for controlling tree size.
Genetically dwarfed cultivars
For example ‘Wijcik’ cultivar is a genetic dwarf McIntosh. The use of spur vs. shoot bearing sports of ‘Delicious,’ is a common practice in commercial orchards. Diploid cultivars are usually smaller than triploid cultivars. A new set of cultivars is being developed in Germany. They are descendents of the Wijick and are called the CATS cultivars. CATS stands for Columnar Apple Tree System.
Plant Growth Regulators
Treatment of trees with synthetic hormones to reduce lateral budbreak (auxins), or decrease internode elongation (Alar, Cultar, Apogee, and other GA antagonists). Increasing lateral shoot development at the expense of terminal shoot elongation through the use of such materials as Promalin, benzyladenine, and other cytokinin analogs.
Witholding Water or Nutrients
Mild drought stress greatly reduces cellular expansion and shoot elongation, since these are directly dependent upon hydrostatic pressure (cell turgor). For example. DeJong and others in CA and TX, using precisely timed drought stress to manipulate fruit size and control peach shoot growth. This is commonly referred to as deficit irrigation. Mild nitrogen stress reduces shoot growth before other visible symptoms are apparent. However, there is a fine line between controlling vigor and weakening trees. And it is possible that stress applied in one year might increase shoot growth the following year or can lead to a crop reduction and subsequent excessive growth. Lower levels of K, Ca, P, Mg, B, Cu and Zn will also reduce tree vigor
Orchard floor management can decrease tree vigor. Weeds compete with trees for water and nutrients. The most competitive timing is when the trees are young and during the first 4 to 6 weeks after bloom
Cool or otherwise marginal sites and/or climates
‘McIntosh’ does best in northernmost areas with shorter growing seasons, where its excess vigor is less problematic. Conversely cultivars like Pink Lady and Granny Smith when grown in warmer climates may produce trees that are excessively vigorous. Site effects are also important. Replant sites produce trees with lower vigor than virgin sites. Soil types can be ranked and relative tree size or vigor predicted based upon soil N release, depth, water retention and seasonal table.
Fruiting as a dwarfing process for trees
The most cost effective means of controlling tree size is by maintaining an adequate crop load each year. C. Forshey: once remarked “A good crop of apples is the best growth regulator ever developed.” Annual crops, evenly set throughout the tree and carried to maturity, will reduce the growth and vigor of fruit trees.
Branch positioning to 60-90 degree angles by spreading, tying, or trellising, etc. The further a shoot is away from vertical the less vigorous it will be.
Pruning is always a dwarfing process. However excessive heading cuts will stimulate growth locally at the site of the cut. Minimal pruning and the use of more thinning cuts will help reduce tree growth. Dormant pruning stimulates current season growth near the cut, and this new growth will not bear for one (peach) or two/three (others) more years for apples, pears and cherries.
Root pruning with chisels 4 to 6 feet from trees, late spring around bloom time or early summer will reduce shoot growth. Deep tillage or cultivation for weed control can have the same affect. The downside of these methods results in reduced fruit size.
Images by Rob Crassweller, Penn State University
Girdling, ringing, etc.
Cuts through bark and phloem during late fall or early spring will temporarily disrupt nutrient and water movement up and down the vascular system. These cuts heal over within a month or two. Some more drastic measures include the use of chain saws on larger trees. The advantage of these methods are less reduction in fruit size and quality than root pruning.
|Images by Rob Crassweller, Penn State University|
Twisting or cracking branches
Fracturing wood on vigorous upright branches may result in a similar growth reduction as occurs with girdling. Some commercial orchardists have made extensive use of weights, spreaders, and tape to tie down shoots. Their use has resulted in reduced terminal growth of shoots, more lateral budbreak and promotion of fruit buds.
|Image by Rob Crassweller, Penn State University|
Root restricting containers
In the early 1980’s extensive research was conducted with growing peaches in subterranean bags. The bags were made of either fabric or plastic. Some commercial growers utilize the bag method for growing sweet cherries.
Planting density and design
Utilizing orchard designs affecting root competition for essential soil resources. As tree planting density increases, soil volume water available to each tree decreases. Thus increased planting density reduces tree vigor. Planting trees at an angle such as in V-trellises or in a Marchand type trellis reduces shoot extension because the trees are not vertical.
The practices outlined above are not mutually exclusive, and can be integrated into overall management programs which effectively limit tree size and vigor, maximize light interception and soil resource utilization in the orchard, reduce the time and expense of pruning out unnecessary wood from the trees.