When is the best time to prune holly shrubs? To answer that question, we must first turn to botany. Secondly, your own personal tastes also play a role in this task. I explain these factors below.
Botanical Factors: Holly Dormant in Winter, Blooms on Old Wood
First and foremost, I need to relate two tidbits of botanical knowledge, along with the implications of each. Weigh these two factors first, and then you can make a more informed decision about when to prune your holly.
Spoiler alert: there isn't one single right answer, because your decision will depend, in part, on your own personal circumstances.
Many plants, including holly, undergo a dormant period during the winter. While it is not necessarily ideal to prune them during this time period (and while it is surely acceptable to prune some hollies in mid-summer, say), we can say this much with certainty: that it is, at least, safe (in terms of plant health) to prune them at this time. Let me explain:
When you winter-prune a shrub that blooms on old wood, in doing so you are potentially cutting back on the number of flowers that will appear on the plant during the following growing season. However, the trade-off is sometimes (depending on the particular plant, the region you live in, and other factors) a worthwhile one, because you are choosing a time to prune that is safe for the plant (and plant health trumps other considerations).
At other times of the year, pruning may leave the plant vulnerable to diseases or spur new growth that will be damaged by frost.
Holly is, in fact, one of the shrubs that bloom on old wood. Consequently, any floral buds (whether on a male or female holly shrub) located on the branches you prune off in winter are buds that will not open during the upcoming year, resulting in reduced pollination and fewer holly berries (this is less of a consideration for those types of holly grown mainly for their foliage, rather than their berries).
We typically prune flowering shrubs that bloom on old wood in spring just after the blooming period is over. But holly is a bit different, since we are not growing it for its floral display. Nevertheless, if safeguarding berry production is a priority for you, you could certainly wait until your holly has flowered and begun to form berries before pruning (you could then prune off just those portions of the branches that do not have any berries on them). Just remember not to prune too late in the summer, because you could generate new growth that would only be killed in the first frosts of fall.
Speaking of the berries, it is time to move on to how your personal taste influences the timing of pruning holly bushes (at least for those grown for their berries).
A Matter of Taste: Enjoy Holly Berries Indoors or Outdoors?
Part of the fun of growing certain kinds of holly shrubs is in admiring the holly berries. If you have such a shrub and it is covered in berries come December, would you rather leave the bush intact during the holiday season or bring some branches indoors to decorate your home?
While the general answer as to when to prune holly may be "winter" (see above), we can get more specific based on your answer to that question.
Some people prune holly in early winter, since they wish to bring the trimmed stems -- with their holly berries -- inside for the holidays, to enjoy in the house. Others prune holly later in winter, because they prefer their display of holly berries outside. That part of it is up to you, since, either way, you will be pruning at a safe time, when the shrubs are dormant.
Conclusion: While there is more than one "best" time to prune holly shrubs, I, personally prefer December. The way I figure it, it is desirable to bring some berries into my house for the holidays, so I might as well kill two birds with one stone and get the task of pruning off my plate at this time. The reminder that over-zealous pruning will rob me of berries for next year will serve to restrain me from getting carried away in my pruning.
How to Prune Holly: Some Specific Examples
Now that I have given you some choices regarding when to prune holly shrubs, let's turn to the question of how to do so. First of all, use bypass pruners, not the anvil style. Secondly, you should always remove dead, damaged, or diseased branches whenever you find them, and it is generally advisable to cut off a branch that is rubbing against another branch or jutting out in such a way that it spoils the overall appearance of the bush. But beyond that, holly is too diverse to give a general answer to this question. So let me tell you how I handle the pruning on a few distinct types of holly that I, myself grow.
- 'Blue Princess' holly: Blue Princess is one of the Meserve hollies. It is a shrub with red berries and dark-green, prickly leaves. Of all my hollies, I have found this one the most bothersome to prune. You need to get in there and prune this type while it is still young in order to give it a sustainable shape that you can then maintain as the bush ages. If you skip pruning for a few years while the plant is still young, you may end up with an unmanageable plant later on, one whose shape you are never quite satisfied with. Once you do attain a desirable shape on this shrub, it is best to prune sparingly (ideally, you have selected a space for it that is sufficiently large to accommodate it at its mature size, so that you are not forced to "shoe-horn" it in by pruning). Simply tip-prune it to maintain shape wherever possible. If you do need to cut further down a branch, be sure to make your cut right above a leaf bud. If, on the other hand, you are forced to perform a drastic pruning on this bush (because you have neglected it for years and/or have not chosen a sufficiently spacious spot for it), be prepared for it to look unsightly for a long time.
- 'Hetz' Japanese holly: Hetz is that type of holly that you have perhaps mistaken for a boxwood shrub while driving through your neighborhood and inspecting your neighbors' landscapes from a distance. Very different from Blue Princess, it has tiny leaves that are not prickly; nor is it grown primarily for its berries. Also unlike Blue Princess, it is very easy to prune. In fact, I shape mine as I shape my boxwoods: with a battery-powered hedger. I have not had any disease problems with mine, so I let it grow as thickly as it wants to. But if you do have disease problems with yours, you might want to open up the inside some by making thinning cuts; this promotes healthy air circulation.
- 'Sky Pencil' holly: Sky Pencil is another Japanese type, but it is valued for its oddly narrow plant form. And, like Hetz, it is easy to prune. Depending on your needs and desires, you may not even have to prune it much at all. I do prune mine to sculpt this "column of green" according to my tastes. But pruning should be minimal, unless your bush suffers severe damage from snow and ice build-up. The latter can be avoided by wrapping twine (or even bungee cords) around the shrub to pull the branches in tightly toward the plant's center.
- Inkberry holly: Inkberry is another no-fuss kind of holly (again, something grown as a foliage plant, with berries being just a bonus; the berries are black). That quality is precisely why you see mass plantings of it in public places: it is easy to maintain. Prune simply to thin, shape, and control its size.
- Holly trees: Holly also comes in tree form. American holly and Nellie Stevens holly are examples. You can shape a holly tree to your tastes by pruning, and some people also cut off some lower branches to expose more of the trunk. If you do not want your tree to get overly tall, you can prune back the leader every winter. Be sure to make your cut right above a branch with some good, vigorous green growth on it, because this is your new "end point."
- Winterberry holly: Unlike the kinds of holly presented above, winterberry is not evergreen. In fact, it is so different from the more typical hollies that we might as well be discussing another plant altogether, especially when it comes to pruning. While drastic pruning is generally a bad idea for evergreen hollies, it is quite acceptable for winterberry, which responds well to rejuvenation-style pruning, as you would perform on lilacs. Remove up to 1/3 of the stems each year, targeting the oldest, thickest ones (and, as always, removing dead branches).
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