Trimming White Pine Trees

(Question)

I have about 75 white pine trees on my property which I’ve been trimming every July in order to keep the branches close and the tree a good visual screen. Your statement on your website says: “Pruning the candles can used to shape a pine and keep it dense and within bounds. Completely breaking off the candle will discourage further elongation at that point. Shortening the candle, decreases the distance at which the next whorl of branches develops and makes the tree denser. Snapping the end off with your fingers rather than with pruning shears will avoid cutting off any expanding leaves which would then brown at their tips….White pines can also be pruned to control growth while the tree is still dormant and just before growth begins.” Because of that last sentence, I did not do any trimming of my trees last summer since I would prefer to do the trimming in the winter or early spring.

My question is: If I trim back some of last year’s new growth in February or March, will the cut ends of the branches develop new growth areas in late spring when the new growth normally occurs? In other words, will the tree respond in a similar way whether I trim it in July or trim it in February or March?

 

(Answer)

One of the key statements in our earlier response to your question, White pine trimming, is “There are two opportunities to prune your white pine (Pinus spp.): while the pine is still dormant and again when the ‘candles’ which are the expanding new growth have appeared…”

It is important to distinguish between pruning tree branches and pruning/trimming candles.  Agriculture and Agri-Food Canada’s “Pruning trees and shrubs”  makes this clear, in stating that pruning a tree during dormant season (e.g., late winter/early spring) will help the tree focus on strong shoots that grow rapidly – especially on the most severely pruned limbs. That being said, when the pine tree is in the “candle stage”, it can be pruned before the needles begin to unfold, which is around mid-June (dates will vary according to where you live, among other factors; references I consulted indicate that this period is usually from early June through early July). The candles can be pinched off or trimmed by 1/3 to 2/3 to limit the tree’s yearly growth.

Trimming the candles will encourage a denser growth, since by the end of that growing season, new terminal buds will have formed at the cut ends.  Next year’s growth will spread from these buds, creating another “whorl” of branches.  So….if you trim last year’s growth in February/March (which would likely include cutting off the new terminal buds), this would affect the current year’s growth (the “whorl”) that would have emerged from those terminal buds.   The overall effect would be that the growth would be less dense than if you were to continue to trim the candles in early July.  Accordingly, I suggest that if you are happy with the overall look of your trees, you continue to “trim” the growth as you have been doing.  If, on the other hand, you want a less dense look, pruning the terminal buds from last year’s growth in February/March would be fine – this would result in fewer “whorls”.

Should full branches require pruning, this should be done around now (February-March), as suggested by some experts, although others suggest that if branches are  small, you can prune them any time of the year.

Here are some helpful references:

  • The Virginia Cooperative Extension’s “A guide to successful pruning, pruning evergreen trees”,   states that candles of whorl-branched conifers (this includes pines) should be pinched back from mid-to late-spring. They caution the gardener to avoid pruning evergreen trees from late summer to early autumn, as this could stimulate new growth that would be damaged/killed by the cold weather (this is a good general point about when NOT to prune).
  • In our earlier response, we provided you with a link to a University of Idaho publication, “How to prune coniferous evergreen trees”. It is recommended that pruning be done in late winter to early spring, and not in later spring to summer, when the live branches are growing. If you do prune during the growing season, you are more likely to damage the bark, and the tree would be increased risk for fungal infection.
  • One very interesting publication, by David Funk (of the US Department of Agriculture – Forest Service) in 1961 “Pruning white pine: a literature review” states (on p. 4) that some experts have found that the best times to prune are from December through February, but also notes that other experts feel that when pruning smaller limbs, the season you prune is not important. He goes on to advise that pruning early is the most satisfactory, and provides a good explanation as to why this minimizes risk of fungal infection. (There is no need to download this document – simply click on the page to move to the next one)

Finally, as Toronto master gardeners, we generally respond to questions from gardeners in or around Toronto, or our province of Ontario. While we are happy to answer your questions, for future reference, you may wish to contact Master Gardener groups closer to your home.  For example,  see the Michigan State University Extension – Berrien County:  near the top of the page is a telephone number for those with home gardening questions. Although you do not live in Wayne County, the Master Gardeners of Western Wayne County has a very helpful website, where you can pose any gardening questions, either online or via telephon.