The basic idea: Fruit trees manufacture two main types of sugars: cellulose or fructose. Cellulose is a structural sugar meaning that it is the woody portion of the plant, used simply to give the tree its ability to stand and hold itself up. Fructose is a metabolic sugar meaning that it is the edible sugar in the fruit of the tree that we eat.
The object in pruning fruit trees is to force the tree into investing more of its energy into the manufacture of fructose instead of cellulose; that is, we want the tree to produce lots of big juicy fruits instead of lots of woody growth such as new branches. Pruning fruit trees removes existing wood from the tree thus decreasing the trees options to invest in more woody growth. With fewer cellulose or woody growth options left after a thorough pruning, the tree’s other alternative is to invest more in fructose or the fruit. This is precisely our desire.
The details: The fascinating thing about trees (or plants in general, for that matter) is their ability to take water, carbon dioxide, and sunlight to make carbohydrates or sugars. Carbon dioxide in the air is literally transformed into sugar. Chemically speaking, there are hundreds of different kinds of sugars and not all carbohydrates are for food. For the purposes of this article, there are two main types of sugars that I will be focusing on: cellulose and fructose.
Cellulose is a structural sugar. Unlike us, plants don’t have bones or any kind of skeletal structure to help them hold themselves up. Instead they use the sugar cellulose which is the main chemical component of wood. It’s amazing to think that the physical strength of wood is created by nothing more than a mere mass of cellulose sugar! When a tree or a bush grows tall and wide the plant is converting its harvested carbon dioxide into cellulose for the manufacture of all that new woody growth. In fruit trees, however, too much woody growth is not desirable because we want the tree to produce the kind of sugar that we can eat: fructose.
Fructose is a metabolic sugar; that is to say, it’s a sugar we can eat. Fructose is the sugar found in the fruit produced by fruit trees and is what gives the fruit its intense sweet taste that we enjoy so much. When a fruit tree matures it starts to transition, spending less energy on woody growth and more of its energy on producing offspring or seeds which is where the fruit comes from (‘seed’ and ‘fruit’ are actually, botanically speaking, very different but I won’t get into the specifics here). Even though a mature tree converts less of its harvested carbon dioxide into cellulose development and more into fructose development (woody growth vs. fruit growth), the tree will still continue to grow branches and get taller and wider. Owing to the fact that we are more interested in the fruit of the tree than its wood, our goal in pruning is to encourage the tree to produce more fruit and less wood.
Pruning fruit trees removes the already existing wood from the tree thereby decreasing the tree’s options of investing in more woody growth. Cellulose and fructose production are two different processes that are essentiall in competition for the tree’s harvested carbon dioxide. Carbon dioxide is only a mere .04% of the atmosphere and is therefore a limiting factor to the tree’s manufacture of carbohydrates. Removing woody growth quite literally forces a tree into favoring fructose production over cellulose production: this is exactly what we want. A fruit tree that is well pruned will produce vastly superior quality and quantity of fruit than a fruit tree that has not been pruned in a long time or as they say “has gone to wood.” Notice the picture to the right: I pruned this apple tree quite heavily and the yield in fruit is excellent. What’s even more impressive is that these apples still have a month to go before harvest and they already look fantastic.
The question then becomes how much do we prune? The general rule is this: if, when you are done pruning the tree, it still looks nice you have probably not pruned enough. If you are serious about excellent fruit quality the pruning will need to be more severe than most people think, especially on peach trees. Unlike other temperate fruit trees, peaches produce fruit on first year branches. This means in order to get the maximum amount of fruit on a peach tree you need the tree to make lots of first-year, or new, branches. Hence, pruning quite severely on a peach tree forces lots of new branches and with it lots of high quality fruit. The other temperate fruit trees don’t need to be pruned back as severely as does the peach, but they should, in general, be pruned back fairly hard.
Where do I start and how do I do this?The first easy thing that should be done is to cut out all dead or diseased wood. Observe the outward appearance of the branches of your tree. Even a complete novice can quickly discern a healthy looking branch from a dead or diseased one. Dead branches are dry and brittle, alive branches are flexible. Diseased wood is often sunken or cankered in appearance with black flesh or other off-color demarcations. Remove all dead wood and cut diseased wood out at least a few inches (~7.5 cm) below the apparent infection.
When is the best time to prune?The best time to prune fruit trees, and most other plants, is when they are dormant, which is during the winter time. Late winter is best, but any time during dormancy or the winter will do.