Most evergreen trees are pruned when they have finished their period of growth in late summer through fall. Pine trees, however, because of a distinct growth pattern, are best trimmed in late spring or early summer. Even then, care should be given to determine the development of the new growth, called "candles." Pines are a poor choice for non-selective forms of pruning; a poorly done trim can lead to a misshapen pine tree.
Trimming Pines for Form and Density.
Pine trees are beautiful evergreens, admired by many cultures and found around the world. They are, however, tricky to prune. Where many deciduous trees and even some non-pine evergreens will put out new growth on old wood easily, pines tend to grow from the tips out and resist putting out new growth on old wood from previous seasons. Likewise they grow from whorls at the very tip of their new growth; the whorls can either be allowed to reach maturity or be trimmed short. Either option will determine the ultimate length of the new growth for that year.
With this in mind, there is clearly only one good time for pruning a pine to achieve an effect on height, shape and density of branch and foliage: during the time of new growth.
New growth on pines are called "candles" for the simple reason that in most species they look like pale candles growing at the tips of the branches and at the very top of the tree. Sometimes the candles look like the elegant candles on a table. In earlier stages they can look as skinny and slight as the candles on a birthday cake. They are, however, easy to notice during the growth seasons. They tend to form clusters, they are of a lighter color than the older growth, the needles are shorter and the candles are not filled out into fluffy, wide-open pine needle sprays.
To control the length of the growth for the year, to create greater density of growth, and to alter the form of the tree, the candles must be trimmed. But don't simply grab your pruning sheers and start snipping. Without a bit of knowledge you can create a mess that will last for the life of the pine.
The Art of Budding
Some of the very best information on how and why to trim pine trees in particular ways can be found in writings on the art of bonsai. Because the effort to shape a miniature pine is so precise the information is superb and can be applied without as much rigor to a larger tree.
The two most applicable skills from bonsai are budding and candling. During late winter and very early spring buds are formed at the tips of pine branches, which will grow into candles. These buds look like little apple pips. The buds form in clusters and are easily identifiable. To control density of growth remove the strongest buds in areas where the tree is already very strong and built up, and where growth is weaker, remove the weaker buds leaving the strong in place. This will allow the tree to even out, keeping the density and the weight of the pine in balance.
A few weeks later, as the buds stretch out into candles, it is time to start thinking of candling, or trimming the candles of the pine. This takes two primary forms: one, complete removal, which should be done when it becomes apparent that there is too much strong growth or lopsided growth in an portion of the tree, or two, actual trimming of the candle.
The second technique offers enormous control over the shape and style of the tree. If you wish for a dense, tight tree prune the candles back in the early stages of growth, before the needles are fully formed. Cutting or pinching the tip of the candle ends the growth for that year and defines where the next year's growth will start. Depending on how long long you wish a branch to grow during this year of growth and how densely you want the needles to grow in relation to the branch, you will pinch out early or late in the growth cycle--or leave the candle untrimmed entirely.
An early trim, before the needles are fully formed, with lead to a very short, stunted branch. Needles will form at the base and the year's growth will end where you pinched out. A later trim allows the needles to form higher up the branch and presents a longer length of growth. Leaving the branch untrimmed allows the new growth to reach the maximum length nature and conditions allow.
The Top Of The Tree.
If you have ever bought a Christmas tree there is a good chance you have seen the result of poorly considered top trimming of a tree: a gnarly, knotted cluster of branches grouped around a short, dead stump, with one or more small side branches struggling to become a new central leader.
If you must top-prune a pine you need to give care to the choices you make. Cutting the candle of the top leader of the pine--the peak where a star or angel goes on a Christmas tree--will force new side branches to develop as replacement leaders. If you are dealing with a very young tree a choice to pinch the central candle can lead to a denser, brushier tree. In an older tree, however, it can lead to a sudden and disturbing burst of density in a tree that was less full overall.
Pines are beautiful trees. Pruned or unpruned they can be gems in the landscape. But if you wish to prune your pines take the time to study fully, and be sure you are acting with forethought and planning.