- Plant the herb you want to control in a container garden if possible. This takes care of any roots or underground stems that can grown into a new plant. Use beautiful or unusual pots, or simply bury your pot right into the ground and mulch over the top. For especially vigorous growth (mint, for instance) leave the top edge of the pot above the soil level, so you can see if any tendrils are escaping.
- Prune or pinch regularly. Doing this will keep your invasive herbs from going to flower, and keep them from producing seeds, resulting in next year's problem. Once an herb has self-seeded, the work to control it goes up exponentially.
- Mulching deeply is a great way to control vigorous growers. Mulching properly is a two-step process. First, cover the ground with a thick layer of overlapping newspapers. Then, cover the papers with at least 4 inches of organic mulch. Any weeds underneath won't be able to germinate, and any seeds that escape to germinate in the mulch can simply be pulled up.
- Finally, hand weeding is the most work, but also the most rewarding. Every day, check how your garden is growing. I highly recommend twice a day, to monitor properly. Check for weed seeds germinating, flower buds, weakened or damaged herbs, and other changes in your garden. Keeping ahead of the invasive herbs by plucking a tiny sprout or blossom every night helps prevent having a front lawn full of spearmint.
Enjoy every herb - even the ones that want to be in control. Careful watch, and a few simple tasks each day, will keep your herbs growing where you want them to. That really is all that makes a plant into an herb and not a weed, right?