Pinching and cutting both seem a little brutal, but in the perennial garden, these are things you really do need to do to keep your plants looking their best and to optimize the number of blooms you will get.
Pinching is something you'll want to do regularly to keep your perennials looking their best. Pinching means removing the growing tip of the stem. What this does is divert the plants' energy from growing taller, and forces it to put its energy into growing side shoots, which results in a bushier, fuller plant. The only drawback to pinching is that it may delay bloom time, resulting in blooms a week or two later than you would have gotten them before you pinched. The upside is that you'll probably get more blooms, so it pays off. One thing to keep in mind is that you'll have to keep this bloom delay in mind for late bloomers such as asters and mums. You will want to stop pinching them by early summer to allow them to produce plenty of flower buds for fall blooms.
When pinching, you'll usually remove the top inch or two of stem. It is best done with pruners, though very tender growth can be pinched off with your fingers.
Cutting Perennials Back
Cutting back is another way to keep your perennials looking tidier and encouraging them to produce healthy new growth. It differs from pinching back in that it is done after the plant blooms, and it's different from deadheading in that you're often taking off quite a bit more than just the spent flower heads.
To cut back perennials, use hand pruners to cut back each stem to just above a leaf or bud. You will want to remove one-third to one-half of the stem's length. You can also cut back to reduce pest or disease issues; just trim off any damaged sections, all the way to the ground if you must, and the new growth will most often be healthy and free of the pests that plagued the old growth. This works very well for getting rid of persistent powdery mildew. The plant probably won't bloom again during the season after such a drastic pruning, but at least you'll be looking at nice, healthy foliage instead of diseased foliage.
Certain clump-forming perennials may send up very thick crops of stems as they mature, and these stems may start to crowd and shade each other out. The end result is that the plant flowers sparsely or not at all and diseases may result due to poor air circulation through the plant. Thinning is the way to combat this issue.
Thinning perennials means removing about half of the crowded stems. To do this, simply prune the weakest of the plants' stems all the way back to the ground. The remaining, stronger stems, will be more vigorous and less disease prone.
However, if you'd like to take it a step further, thick clumps of perennials can often be divided. This is best done in early spring before the plants get too large, or in fall after you've cut them back during your fall clean-up. Simply dig up the plant, and divide the clump, making sure that each section you've divided the plant into has both a bit of root and some crown sections. Then, simply plant the divisions around your garden. You've not only made your plant healthier but earned more plants, as well!
While it seems mean at first, cutting back and pinching your perennials are important methods to growing a beautiful, healthy perennial garden.