Few trees planted for Christmas trees will attain the quality necessary to be sold in the consumer market without help from the grower. Most consumers want a tree that is uniformly dense, has a straight trunk and has a stem that is both straight and the correct diameter range to fit into a holding stand. Individual trees often develop problem tops, and if this is not corrected during the year the problem occurs, the tree will not have the desired straight trunk at harvest time.
Problems with tops can be caused by insect damage to the intended terminal leader, breakage by birds perching on new growth that is not sufficiently strong, mistakes made in previous years’ shearing, among other reasons.
Some trees, through poor genes, just never get the message that they should grow. In the latter case, these trees should be removed before time is wasted on them year after year.
In order to properly correct problem tops, it is important to have an understanding of how a tree grows to determine its response to corrective treatment. Essentially, what we’re after is one leader in each whorl of branches that continues in a straight line with the lower part of the trunk, with no offsets (often called “doglegs”). Such a trait is called apical dominance, or the ability of the topmost terminal shoot to dominate all other branches on the tree. This dominance is determined by hormones produced by all actively growing buds, but in progressively samller amounts on lower branches, making a tree is taller than it is wide.
When trees are sheared to attain the proper shape and density, the top leader is cut at the desired length of approximately 12” at a 45 degree angle. This encourages a new bud to develop in a higher position, thus producing more of the growth inhibiting hormones and a greater chance of one strong leader the next year. Often two or more buds develop near the same position on the cut stem, resulting in two or more leaders the following year. Treatment for this is to select the straightest and cut the others out entirely. If the leaders are divergent, the strongest may be straightened by tying it to the other. The supporting leader should be cut off just above the tie. Plastic flagging is the preferred tie material because it will not girdle the stems, and will eventually fall off when no longer needed.
After the top is corrected, the entire tree is sheared diagonally in a straight line from the top to the outermost tips of the bottom branches. This will shorten the lateral shoots of the top whorl to the proper length., which should be approximately ½ the length of the leader. Remember that when the top leader has been cut to the appropriate length, the buds on all laterals of the top whorl must also be removed to avoid one assuming an off center leadership role the ensuing year.
Frequently, because of insect or disease incidence, or mechanical injury, the tree will have no terminal leader. In this case, one of the stronger lateral shoots of the top whorl must be brought up and tied. This can be done successfully so long as the selected branch is left longer than all the others in the top.
Corrective top work can take a little time, but will often result in greatly upgrading a tree from a cull to one that is profitable.