Post 2 in our pruning series focuses on wood-hardy perennials and shrubs, and things get a little trickier. In general, for flowering perennials and shrubs, a good rule of thumb is to ONLY prune right after the plant blooms. Again, it’s not foolproof, but it’s a pretty good general rule. This is because most of these plants bloom each spring on new wood they created the previous growing season. Lilac is a great example of this – if you prune a Lilac in the spring before it blooms, you’ll see no blooms that year.
People often ask about pruning Clematis. The first thing to keep in mind that if you’ve found a good spot for a Clematis and it’s growing well, there’s very little risk that you’ll hurt your Clematis by pruning it. While picky about location, once you find the right location, Clematis are pretty hard to hurt. By the same token, ignoring them completely and letting them twine and vine untouched for years will delight them. They simply don’t care much whether you prune them or not.
That said, there are three general types of Clematis, and they each have slightly different pruning requirements (if you do choose to prune). Without going into detail, I generally just apply the same “prune after the bloom” logic, and it will generally work. Early in the spring, it’s common for people to worry about their Clematis, as it will look woody and dead. Be patient, and soon you’ll see new growth appearing on those woody old vines, followed by the profusion of green and bloom that make you love the plant so much. Soon after the green appears is a good time to gently look through the bundle of plant, and prune out any pieces that are obviously dead, though there will be fewer than you think!
While Lilac is the woody shrub people most often have questions about pruning, it is also the one that is most flexible in terms of how you can prune it. While it can be “trimmed” back after pruning, it’s characteristics are most favorably displayed by pruning it several times a year to resemble a tree more than a shrub. If you do this, then it’s good to get down into the base of the shrub early in the spring (before blooming) and cut out all the suckers coming up from the ground. Then the energy of the root will go into the bloom above. Following the bloom, start to shape the shrub with monthly prunings throughout the growing season, taking care after the first pruning to leave plenty of this year’s growth upon which next year’s flowers will occur.
There are a number of woody shrubs that people typically “trim back” in order to control how big they get. Forsythia and Viburnum would be examples. This pruning should occur right after flowering, and don’t be shy about cutting them back really hard. They might look bad initially, but within a few weeks you’re likely to be happy with the results.