Pruning by Numbers
Written by Alan Haigh, All rights reserved. Reprint permission required. firstname.lastname@example.org
The basis of this formula I am about to describe is employed by many commercial apple and pear growers in Washington state and elsewhere. It may have been devised to provide simple guidance to minimally trained pruning crews called on to train hundreds of acres of free-standing fruit trees. A free-standing tree is defined as one at least 40% the size of standard that requires no support at maturity at most sites.
These growers instruct their workers to remove any branch that exceeds 1/3 the diameter of the trunk at its point of attachment to the trunk. Until a tree begins to bear fruit all other branches are left on the tree except rubbing or broken branches. Permanent scaffolds are not selected until trees begin to bear fruit.
For many varieties of apples and pears, this is all that is required to bring a tree into productivity as quickly and efficiently as possible. Of course, there are some exceptions which will be discussed later. What is less known is the idea of applying the 1/3 rule (or at least at least a ½ rule) not just to scaffolds but to secondary and often even to tertiary branches. In other words, maintain a central leader on each scaffold and remove every secondary branch that exceeds 1/3 to ½ the diameter of the scaffold at the secondary branches point of attachment to the scaffold.
A big part of the theory behind this involves the “economy of wood”. Using ratios as a guideline for removing crowding wood encourages the development of a tree that uses the least amount of structural wood possible to hold the largest amount of small, fruitful, well light exposed wood. This may be a confusing concept when read off the page, but as you prune your trees it will gradually make sense over time.
You can use this formula on apples, pears, peaches, plums, apricots, cherries and even mulberries. You may elect to train peaches and most other stone fruit to a three or four scaffold open-center after about 3 years. Even if you do, selecting scaffolds from only “moderately vigorous wood” creates scaffolds that are dependably strong, precocious (early fruiting) and a relatively compact tree. Incidentally, in my experience, three scaffolds are usually all that are needed to produce the easiest to manage and most productive open center tree.
It is commonly recommended that apples and pears be maintained as central leaders because in studies it has been shown to use space and light most efficiently for these species. Think of a central leader tree as two or three open-center trees growing on top of each other with each subsequent tree being about half the size as the one beneath it. Incidentally, in a home orchard where absolute efficiency isn’t essential, you can train your apples and pears as open center trees, if you prefer. It is a less complicated form to maintain.
Note the expression “moderately vigorous wood”. It is very important to understand the relationship between a branch’s diameter and its vigor. The relative diameter of a scaffold branch at its point of attachment to the trunk strongly influences its access to sap. Think of a branch like a hose- the wider the diameter of a hose, the more water that can pass through it at any given pressure.
Because the pressure that pulls sap through a scaffold is provided by evaporation from leaves and the resultant capillary pull, the number of leaves and their relative exposure to the sun is the other determining factor of a scaffold’s access to sap.
So how does access to sap affect a scaffold’s relative vigor? Well, access to sap is the same as access to the roots and the roots provide water and most of the essential minerals for plant growth. This means that a scaffold with a smaller diameter stem functions in the manner of a dwarf fruit tree while a larger diameter stem functions more like a standard. The smaller diameter stem tends to store relatively more of its carbohydrates and sends less to the roots creating a cycle of fruit productivity at the expense of vegetative vigor.
If you are patient and give it enough room, an excessively vigorous scaffold will eventually become fruitful. But if you try to keep it in bounds and aggressively cut it back, the branch will tend to remain vegetative, probably partially because of a greater ratio of nitrogen to carbohydrate. The nitrogen is brought up from the roots and when you prune a branch the remaining buds have access to more of it and everything else brought up from the soil.
Speaking of the ratio of nitrogen to carbohydrate, it is important to realize that a branch’s access to carbohydrate is strongly influenced by its relative access to sunlight. This means that in order for a branch to be fruitful it needs adequate light. However, the carbohydrate-nitrogen relationship is a little tricky because greater exposure to sun also draws more sap.
As promised, here are some notes about possible exceptions to the 1/3 rule. First, some varieties benefit from an early selection of permanent scaffolds. This would mean removing all but three or four branches of each tier of a central leader tree by the second year of their selection instead of just leaving all properly ratioed branches until the onset of fruiting. The 1/3 rule would still apply to the selected scaffolds.
In my experience, Fuji is slow to come into bearing if there are too many branches on the tree, even branches of low relative diameter. For a twiggy, hyper-vegetative variety like Fuji, it may be useful to eliminate all but the permanent scaffolds as soon as possible to help early development of flower buds. Fuji tends to excessively shade itself, apparently causing a tendency towards extended juvenility when grown on free-standing root stock.
Lanky varieties like Ginger Gold and Yellow Delicious may also benefit from early scaffold selection to encourage greater secondary branching. They may also benefit from heading back these scaffolds by about 30% of annual growth to make scaffolds stouter. If such heading cuts are made, come back during the growing season to retrain the scaffold to a central leader by pinching back competing buds near the heading cut and allowing the straightest one to lead. You can also wait until mid-spring to make these cuts after the tree has leafed out. This will quiet the fierce vegetative response at the point of the cut. As trees get older and have filled their allotted space it is best to prune scaffolds back to a secondary branch when heading back.
Because they are determined to fruit early and profusely, varieties such as Empire, Arkansas Black, Braeburn and Goldrush (as well as many pear varieties, especially Asians) will perform well with relatively larger diameter scaffolds. Scaffolds up to half the diameter of the trunk are fine in such cases, as long as the unions are strong, with no inverted bark. This larger ratio can also work for the scaffolds of open-center trees but may result in a less compact, later fruiting tree. It may also lead to a weaker union of branch to trunk. It will not delay peach productivity, however.
Less vigorous, more fruitful varieties may also benefit from an increased number of scaffolds, particularly if secondary and tertiary branching is sparse, which is the case with spurry varieties of apples (Goldrush, Arkansas Black), and many pears.
When pruning mature trees, the ratio rule is very useful in deciding which of two crowding branches to remove. When oversized wood is removed in favor of wood with a better ratio to the branch it is attached to you are creating a more efficient and fruitful tree.
That completes my explanation on the use of ratios to guide the training of many types of fruit trees. I continue to use the general principles here in maintenance pruning as well. I don’t automatically remove every secondary and tertiary or even quatiary branch that violates my ratio rule, but when branching becomes excessively crowded it is primarily this principle that determines which branches are removed. I won’t leave a huge empty spot in the canopy to enforce this rule, however. Replacement shoots sometimes need to be pulled and trained to grow where you need them (you can even tape them to an oversized branch you plan to remove later).
DISTANCE BETWEEN TIERS Another number that might come in handy is the distance between tiers on a central leader tree. On apple trees growing on rootstocks with vigor equivalent to M7 to EMLA 111 you need about four feet between the first and second tier and three feet between the second and third. Each successive tier should extend about half the distance from the trunk as the previous tier and the branches should have about half the diameter of branches in previous tier. This will assure that higher branches are not excessively shading lower ones. This accomplishes the efficient Christmas tree shape mentioned earlier.
Because the higher you go on the tree the better the access to the sun the upper scaffolds will gradually outgrow the lower ones and, depending on variety, may stop being fruitful if you keep trying to prune them to the desired length. If you let them grow out as far as they need to be fruitful they will shade the lower scaffolds and your Christmas tree shape will be destroyed.
Therefore, It is necessary to replace upper scaffolds every few years when the diameter gets too large. You should plan a couple years in advance for this and always leave some low vigor wood to form eventual replacements. For spurry varieties this is less of an issue.
THE HIGHWAY ANALOGY OF BRANCH STRUCTURE Consciousness of ratios will, in itself, lead to the development of a more efficient tree structure but it may also be useful to think of wood as a transport structure similar to a highway system (although functionally, it more resembles a power grid). The scaffolds are like a 12 lane super highway while secondary branches are your 4 lane highways and tertiary your 2 lane roads. You can call the smallest wood down to spurs as your alleys and driveways.
Highways are expensive to build and maintain just as wood is expensive (energy-wise) to build and maintain. The least amount of asphalt or wood required to satisfy the needs of transport the better. Another ratio issue is about access to territory for each scaffold. If the first tier has three scaffolds then each scaffold should have dominion over one third of the circle that the tier encompasses. The scaffold should ideally be trained in a reasonably straight line from the trunk, dividing its territory (1/3 of the canopy “circle” in a 3 scaffold tier) down the middle, or 60 degrees on each side.
In other words, if you looked at the tier from above as a two dimensional circle, each branch should occupy one third of that circle- like a pie cut into 3 equal pieces. Before you prune you should study the scaffolds to determine where the border lines are between scaffolds- where wood from one is encroaching on another’s space. When two smaller branches are crowding each other this will guide you to which one to remove- it will be the one attached to the encroaching scaffold.
So the three or four scaffolds of each tier should have approximately the same amount of small wood connected to them. This will happen just by respecting each scaffolds allotted space.
For secondary branches it’s a little more difficult to determine how much tertiary wood should be on it but generally the amount of smaller wood on a branch should coincide with its relative diameter. As trees grow, secondary and tertiary branches will often become over-sized with too little space available to grow enough small fruiting wood to justify the branches access to sap. By guiding the growth with the removal of this kind of wood, the vigor balance of a tree is maintained.
This completes my mathematical explanations of training fruit trees. Unfortunately it still falls short of providing a complete road map. Following these guidelines, you will certainly wind up with a reasonably open tree that has access to adequate light throughout the canopy; however, the issue of exactly how open the tree needs to be is not completely addressed.
I’ve always been fond of the old time appleman’s explanation that an apple tree should be open enough that you can through a cat through it. PETA members, please forgive me, but I find the image humorous- just don’t try it at home.
Exactly how open a tree should be will depend on a lot of factors. In more southern areas too much exposure may cause sunburn. Trees in relatively shady sites or further north will benefit from being pruned more openly and will also often benefit from substantial summer pruning of annual wood to allow sun to reach the leaves that serve the fruit.
What I’ve tried to cover in this article is essentially a general explanation of structural training and maintenance of fruit trees. An extremely important issue not addressed so far is any explanation of the detailed pruning required to best manage the fruiting wood.
This subject can get quite complicated as each species and many varieties within species have their own particular fruiting habit- peaches only bear (and leaf) on wood from the previous year (1-year wood)- many apples and pears bear their best fruit on 2-year wood and/or the tips of shoots formed the previous year. Some species and varieties within species bear all or most of their fruit on older, knobby and short pieces of wood called spurs which require renewal pruning.
Because of the wide range of factors involved in making the best decisions in this kind of detail pruning I will save this general subject for another article. To master the art of pruning you must prune and observe the results and then adjust based on the information your trees provide you. Be courageous and remember that most of us have a strong predisposition to remove too little wood.
Here’s a link to another guide “Training and Pruning Apple and Pear Trees” published by the American Society for Horticultural Science which you can download.