What will cutting my plants back actually do?
Trimming will encourage them to branch at a faster pace than normal and will yield a fuller, more shapely plant.
How does it work?
This is the coolest part. Ready?
The growing tip of a plant is called the shoot apical meristem (mehr-ih-stem). In some plants, this meristem will release hormones that flow down the stem and inhibit side branches from forming. By cutting it off, the flow of hormones is interrupted and branches can form. In turn, the growing tips on each new branch will release the hormones, and cutting these will allow side branches to form on the side branches. And so on, indefinitely (or as long as you keep trimming).
Where on the plant should I cut?
I usually count up from the bottom of the stem and cut just above the second set of leaves. (If you’re cutting on side branches, leave at least one set of leaves on the branch.) This is enough to photosynthesize and make food for the plant while it’s putting out those new branches. Never cut a stem so that it has no leaves– if it can’t photosynthesize the plant will die before it can make new leaves.
When cutting, cut just above the leaf node, or where the leaves meet the stem. Not too far up– the extra stem can, in certain conditions, rot and take the rest of the stem with it. (Marjoram, thou art a heartless wench. This is such a problem with sweet marjoram, at least indoors.) If that happens, cut the whole stem off before the infection spreads. At the same time, if you cut too close to the leaves they may not branch. Leave about 1/16 to 1/8 of an inch of stem above the leaves, as a general rule.
I can’t do it!!
Yes, you can. Don’t feel bad– you’re basically doing what a deer, rabbit, or other herbivore would in nature, and the plant has evolved to deal with the situation. Use really sharp, clean shears. It’s basically amputation, so you want a nice clean cut instead of a nasty jagged infected one. Dull shears make plants cry. Your plant might look a little bare or shabby right after, but it won’t stay that way– give it a week or two and the results will be very gratifying.
When should I prune my plant?
As a general rule, it’s easier (and better for the plant, in terms of healing) to cut back green, softer growth. Many herbs will become woody as they mature, and when you cut back into woody stem it is much harder (sometimes impossible) for them to push new leaf buds through, which can kill the plant. Train your plant while it’s young and flexible, then maintain it, and you’ll have a great-looking plant.
If your plant is already quite bushy, you don’t have to be as precise and cut every single stem one at a time. (It would take ages…) The stems will, for the most part, have leaves at about the same intervals and if you cut evenly you can cut many stems at once with identical results.
It’s good to trim most herbs after flowering; it’s called dead-heading, and will direct the plant’s energy back into the leaves (instead of seed production) and promote healthy foliage, which will allow the plant to store up more energy for the winter or for another cycle of flowering. If your plant is done flowering and looks kind of drab and crappy, then trim it. Some flowers can be snapped off by hand, others will need to be cut with shears. Always, always prune your lavender after it’s done flowering.
I’ve gone through several of my herb books, and there’s some debate over whether herbs should be cut back in the fall or very early in the spring. My advice, and what I do in my own garden, is to prune everything in August or September. It’s still before any major frost so it gives the plant a little recovery time (cutting back in cold, wet conditions can encourage disease and rot), and the plant starts the spring with a clean, trim shape. If the weather gets nasty, just wait it out and do your pruning early in the spring. I always inspect my plants as soon as they start putting out fresh growth and remove any branches that have died over the winter and correct anything that wasn’t cut short enough in the fall. If you wait too late in the spring, you can delay flowering or branches can become woody and harder to cut back.
Plants that you should definitely cut back for better results:
Lavender, sage, mints, catmint, and oregano will form terrific bushy plants with a little maintenance. Mints can be cut back quite ruthlessly. Lavender responds so favorably to pruning that it’s not even funny. The Genus Lavandula states, “…As a general rule the harder lavenders are pruned, the longer they will live.”
Pineapple, Melon, and Tangerine sage all have a tendency to get leggy in a very short period of time, though Pineapple is the easiest to manage. Tangerine and Melon have a tendency to lose their lower leaves, which leaves you with several inches of bare, unsightly stem. The good news is that if they get away from you, you can cut them back in stages to get branches back in these areas. Leave several sets of leaves on the stem, but cut as much off as possible. When new branches/leaves have formed further down the stem, cut it back again. Repeat until you’re satisfied with how the plant looks, and keep side branches maintained for a shapely plant.
Thyme stems are rather slender but can become woody quite quickly, which means that young thyme can be cut back more rigorously than mature plants; if you have a mature thyme plant that has fallen open in the center, try layering dirt on top of any bare woody stems (weigh them down with rocks if you have to). The branches will root, and you can get some new growth going on the softer stems to fill in the gaps.
Scented geraniums: Some varieties get leggier than others. Branches can get heavy and cause the plant to fall over, which can damage it and make it harder to water if you’re keeping it in a pot. ‘Concolor Lace’ stays rather compact but will benefit from the occasional trim. The pink rose-scented, ‘Attar of Roses’, and ‘Sweet Mimosa’ varieties will be most in need of trimming.
A lot of herbs have leaves arranged in opposite pairs, meaning that on the stem two leaves emerge from the same node. Scented geraniums have alternating leaves, which means that at one node a single leaf will emerge on the left, and at the next node the leaf will be on the right. Here’s how that affects their branching:
It’s also good with geraniums to remove older, larger leaves if they’re crowding developing leaves. Once you’re satisfied with the shape, cease cutting so that the plant can flower. Remove spent flower stems when they’ve finished blooming.
Whew. If you have any questions about any of that, do ask. I’ve been working on this for a week, sorry for the delay and double sorry if there’s anything I’ve missed!