If you have shrubs in your yard that have become too large for their space, are misshapen, or that fail to bloom, they might need some pruning.
As you may have guessed, these are three of the main reasons why a person might want to prune shrubs. In addition, if there are branches that rub together or are growing downward, pruning would help the shrub to grow more healthfully.
If you decide to try your hand at pruning your shrubs, proper timing is important. You can prune dead, diseased, or damaged branches at any time. However, other than that, you have to pay attention to timing. Early spring bloomers such as lilac and the early spirea species should be pruned right after they are done blooming. The reason for this is that they bloom on “old wood,” so they get ready for next year’s bloom during the current growing season.
Shrubs that bloom later, toward fall, should be pruned in late winter/early spring, before the new growth begins — probably March here in northern Minnesota — because they bloom on “new wood” that has grown during the summer before they bloom. Shrubs like arborvitae continue to grow all summer and can be sheared any time through mid-summer. Arborvitae are sheared rather than pruned as I will be talking about next, because they are not multi-stemmed shrubs.
There are various pruning techniques, and which one you should undertake is dependent on your goal for pruning. As stated above, you can cut out dead, diseased, or damaged branches any time. If your shrub is just too full and you want better air circulation, you can thin by choosing branches or twigs to cut back their length to a side branch, or a quarter inch above an outward facing bud. This will result in a more open shrub. You can also choose to head back a shrub to improve its overall shape by cutting outer growth.
Two other, more drastic, kinds of pruning may be needed when a shrub is very overgrown; these pruning techniques should be completed only on shrubs that have multiple stems. Renovation or renewal pruning is done over a period of a few years. You would cut off one-third of the thickest stems to about 4-6 inches above the ground. Over the next couple years, you continue to cut about one-third of the thickest stems each year. This will give the shrub new growth. In general, pruning should not be done to more than one-third of a plant’s growth in one year.
If a shrub is VERY overgrown, possibly never pruned or has just run away on you, you may opt for rejuvenation pruning. This means cutting the whole thing down close to the ground, while the shrub is dormant. I have done this with good results with lilac and potentilla shrubs.
Keep in mind: If you do a fairly significant pruning, you may not have blooms for a year or two following your actions. The plant’s energy will go into replacing its greenery rather than blooming.
For further information on pruning shrubs, see the University of Minnesota Extension website extension.umn.edu/planting-and-growing-guides/pruning-trees-and-shrubs.