By: Sandy Feather ©2011
Penn State Extension
Q. When is the proper time to prune blueberries (Vaccinium corymbosum)? The home we purchased has an established planting of blueberry bushes, but it is badly overgrown.
A. It is best to prune blueberries sometime in March, while they are still dormant, especially if you need to prune severely to renovate overgrown shrubs. The goal of pruning blueberries is to remove older, less productive canes and encourage the growth of new, fruitful ones. It is easier on the shrubs to spread the renovation out over two or three years, particularly if they are very old and overgrown. To do so, simply remove one-third to one-half the tallest, thickest canes at ground level.
The blueberries should respond by producing new shoots that will become young fruit-producing wood. Thin the new shoots by removing those that are weak and spindly and those that tend to lie on the ground. Retain those that are vigorous. and well spaced in the shrub.
Avoid shearing or pruning too much on the tips of the canes (known as heading back). This will result in a dense tangle of twigs at the ends of the canes. This growth blocks the sun from the interior of the shrub, resulting in less fruit production. Fruit production is improved by keeping the center of the shrub open to sunlight as much as possible. It is fine to shorten or remove a few the twigs at the tips of the canes to create space for fruit or improve air circulation. Just try to keep it to a minimum.
Once the renovation is complete, the goal of pruning is to achieve a mix of ages in the canes. The ideal blueberry shrub will have 10 - 15 canes, with two or three canes each of one-, two-, three- and four-year-old canes. Of course, this is the ideal and many plants never grow the way some textbook says they should. Blueberry canes have a productive lifespan of five years. After that time, they are less productive and should be removed to allow the growth of new shoots that will produce fruit in following years.
Mature blueberries should produce three to five new shoots annually. If yours do not respond to pruning by producing a sufficient number of new shoots, have your soil tested to be sure that the pH of the soil is between 4.5 and 5.0, and that nutrients such as phosphate, potash, calcium and magnesium are in the optimum range. Unless corrections are necessary for pH and/or deficiencies in those nutrients, nitrogen is the only nutrient required on a yearly basis to promote fruit production.
It is probably safe to assume that your plants have not been fertilized for a while, so start this spring when we are passed danger of a hard freeze. Avoid fertilizing after early July, because fertilization forces new growth that may not harden off completely before winter arrives.
A typical nitrogen recommendation for mature blueberries (over six years old) is one-half pound of ammonium sulfate (21-0-0) per shrub. In addition to supplying nitrogen in the ammonium form preferred by blueberries, ammonium sulfate helps keeps the pH in the acidic range they require for optimum growth and production. Organic gardeners can substitute cottonseed meal (6-2.5-1.7), feather meal (13-0-0) or corn gluten (9-0-0).
In addition to pH and fertility, be sure that the physical condition of the soil is appropriate for growing blueberries. They are best in an evenly moist, yet well-drained soil high in organic matter. It is good practice to mulch them to help maintain soil moisture, moderate soil temperature and keep weeds down. It also protects their canes from injury by lawn mowers and weed whackers. Three or four inches of well-rotted sawdust make an ideal mulch for blueberries. Avoid fresh sawdust as it can burn tender stems and rob the soil of available nitrogen, which creates a nitrogen deficiency for the shrubs.