Apple trees produce fruit on spurs. Spurs are stubby stems that form along longer stems. Bourse shoots are vegetative growths that do not produce fruit, but they are an important part of fruit production, and we are still not entirely sure why. [Being cousin to apples, all of this information is relevant to pears, as well.]
Apple tree anatomy
Before we learn about bourse shoots, we need a quick review of apple tree anatomy. Apple trees tend to grow long stems. If those stems grow upright, your apple tree is of the alternate bearing variety. If those stems tend to hang downward, your apple tree is a regular bearing type.
[Many commercial apple growers spray chemicals on apple trees to cause artificial fruit drop on heavy production years to encourage a bigger return bloom the following year. Return bloom refers to the blossoms that appear after the current crop is under way. This evens out annual fruit production, maintaining supply and keeping prices consistent.]
One study, published in the Journal of Horticultural Science, explains how the removal of too many bourse shoots significantly reduces return bloom.
Along those stems, whichever way they happen to grow, are little stubby growths called spurs. The majority of an apple crop is found on the ends of spurs. Each spur can produce fruit for 8 to 10 years, or more. Those spurs grow out of swollen areas, called bourses.
Not to be confused with bourses, bourse shoots are vegetative stems that emerge just below flower buds. In some cases, new spurs can suddenly shift their growth to become a bourse shoot instead of a spur. These bourse shoots feature a whorl of leaves. In the center of those leaves, a bud may form later in the season, but they tend to be less productive than spurs. This transition from vegetative growth to floral growth, called floral inflation, is believed to be caused by an abundance of sunlight and sugar. Other causes of floral inflation include fruit thinning, summer pruning, and bending upright shoots to a more horizontal orientation.
A common term among apple growers is bourse-over-bourse. This refers to bourse shoots emerging from existing bourse shoots, in a waterfall style growth pattern. Too many bourse shoots on one stem can lead to spur extinction. Spur extinction describes the point where a spur is no longer productive. If you see multiple bourse shoots on a stem, you can improve fruit production by pruning back to the innermost bourse shoot.
Pruning apple trees
Standard dormant season apple pruning involves removing all dead, diseased, or rubbing branches, as well as 15 to 20% of the previous year’s growth. Next, you should remove excessive bourse-over-bourse growth, and bourse shoots that are especially long, as they tend to be less productive than shorter or medium-length bourse shoots. Don’t remove too much, however. One study, published in the Journal of Horticultural Science, explains how the removal of too many bourse shoots significantly reduces return bloom [next year’s crop].
Bottom line: next year’s apple (or pear) crop is highly dependent on the number of leaves produced during the current year. If your apple tree has more bourse shoots, it is more likely to have more leaves, ergo, more fruit. But this is only true if those bourse shoots are spread evenly throughout the tree.
You can grow a surprising amount of food in your own yard. Ask me how!