Pruning is essential to maintaining a productive planting of highbush blueberries over time. Yet many blueberry growers fail to prune in a timely fashion, while other growers, or their workers, prune the plants without fully understanding what they are trying to accomplish. The following information summarizes the why and how of blueberry pruning, adapted from articles by blueberry experts in the Northeast: Dr. Gary Pavlis of Rugters University and Dr. Marvin Pritts of Cornell University.

Why prune? Pruning maintains the vigor and yield of blueberry bushes, helps manage insects and diseases, promotes larger fruit of higher quality, and shapes bushes so they are easier to harvest. One reason that pruning is often overlooked is that its benefits are relatively long-term – you don’t see them right away. Another reason is that a plant may look like it’s been pruned pretty well, and it may produce a satisfactory crop – but it’s hard to know that better pruning would have improved yields if there is nothing to compare it to.

How blueberry plants grow and make fruit.  In the beginning, a cane emerges from the base, or crown, of the plant. As the cane grows it produces two kinds of buds: vegetative buds that become new branches, or laterals, and fruit buds that become flowers. The flower buds are plump and rounded and they’re larger than the pointed vegetative buds. Flower buds are located near the end of new branches, while the vegetative buds are located further down the shoot. Both types of buds are only produced on new growth, so one-year old “wood” is the source of all fruit, as well as new lateral branches, on a bush.

As canes get older, they get thicker at their base and their buds are produced further and further away from the crown. Over the years the new laterals produced on a given cane decrease in diameter, and are prone to being “twiggy.” These old, weak branches produce fewer, smaller fruit than branches on younger canes. Older canes also compete with newer canes for space and light. So you can see that it’s important to remove older canes in order to maintain fruit production and quality as well as to allow new canes to develop for future production.

Pruning young bushes. Highbush blueberry plants can live for many decades; but early in their life, they don’t need much pruning. For the first two years flower buds should be removed either by rubbing them off or by cutting off the tips of shoots.  Starting in the third year, remove any twisted or low-growing canes, and if more than two new canes were produced the previous year, remove all but the two healthiest, down to the ground. In subsequent years continue to remove all but 2 or 3 of last season's canes, so when the plant has reached full size and is about 8 years old, it should have 10 to 20 canes of all different ages. Since different varieties produce different numbers of canes they will also differ in how much pruning they require.

Pruning mature bushes. Older bushes should not have a lot of old canes, and that’s where growers often run into trouble – by leaving canes in place too long.  Once canes get to be six to eight years old their productivity declines, and they need more leaves to support fruit growth than they did when they were young. They will also have branched many times over the years, so their new growth will be relatively thin and weak, as explained above. If you’re not sure of the ages of your canes, a rule of thumb is to remove canes larger than one inch in diameter; they’re usually gray with lichen growing on them. If you have fallen behind in your pruning you may need to remove several of these dinosaurs per plant to open up space for younger canes. In general, up to 20% of the older wood can be removed from a bush without adverse effects on yield. Berry numbers will be reduced, but larger fruit on younger canes will compensate for this. However, if pruning has been seriously neglected it may be necessary to remove quite a few old canes and suffer a short term yield reduction for the sake of future growth.

Which canes to remove? Start by removing any dead, injured or damaged canes. Canes with diseases or insects such as cankers and scales should be taken out. Then, I find it helpful to remove those old canes to get them out of the way so I can better see the shape of the plant as I continue to prune. The next step is to remove canes that are in the wrong place: they may be sticking out too far into the alley, or growing down too low to be harvested easily. If two canes are rubbing each other one of them should be removed. If the plant is very dense in the middle, take out a cane or two to open up the canopy so light and air can get in. When whole canes be sure to make the cut as close to the crown as possible. Do not leave stubs that can become a source of disease inoculum.
When to prune. Early spring is the best time to prune blueberries, while they are still dormant. That’s when you can identify winter-injury and remove it. Waiting until spring to prune, instead of doing it in the fall, also allows time for the plants to move carbohydrates that may be in the canes down into the roots and crowns.

How often to prune. Annual pruning is needed to achieve consistent fruit production and highest yields. If you only prune every few years that will encourage a flush of young canes to grow the year after pruning and these will age together, becoming unproductive at the same time. Irregular pruning promotes erratic yields from year to year, and it leads to tall bushes because having an excess number of canes causes them to “stretch” as they compete for light. Moderate pruning every year spreads out the age of canes, keeps their numbers in check, and allows bushes to fulfill their productive potential.

Summary. Proper pruning takes time and thus costs money, but it’s a worthwhile investment. By pruning every year the time and cost is spread evenly throughout the life of the planting. You could say pruning is expensive, but will cost you more if it isn't done well. 

An excellent short video with Dr. David Handley, University of Maine Cooperative Extension, shows you how to prune a blueberry bush and clearly explains what you are trying to accomplish when pruning, see: http://umaine.edu/gardening/videos/how-to-prune-blueberry-bushes/.