Few plants can top lavender (Lavandula spp.) at providing fragrance in a home garden. There are several different types, including English lavender (Lavandula angustifolia) and Spanish lavender (Lavandula stoechas), which grow in U.S. Department of Agriculture plant hardiness zones 5 through 9 and 7 through 9, respectively. Whatever type you grow, you can easily increase the size of your planting, but older plants don't respond well to root division. Instead, use other propagation methods for these plants, such as layering and taking cuttings.
Lavender plants become shrubby and woody when mature, so the best approach to propagate an older plant is to make cuttings, allowing them to root and develop into new young plants. Most lavenders bloom in early to late summer; the best time to make cuttings is immediately after flowering is done, when the plant isn't putting energy into making new flower buds.
Use sharp shears or a pruning knife to cut sections from leafy stems that didn't produce flowers, making sharp, slanted cuts. Disinfect the blade by wiping it in rubbing alcohol between cuts to prevent spread of plant disease. Cut off any leaves from the bottom half of each cutting, and place the cutting into moist potting soil or vermiculite in a pot or tray. Lavender cuttings root readily without rooting hormone.
Once you've made cuttings, label them with the name of the parent variety and keep them misted and their soil or mix lightly moist. Lavender cuttings usually root in about three weeks; test for roots by gently tugging on the cuttings, feeling for slight resistance. Place each rooted cutting into a 2- or 4-inch pot filled with moist potting soil and keep the pots in a sunny spot, watering regularly until the young plants have well-developed root systems. This usually takes a few weeks.
Choose a sunny, well-drained spot for the new plants when they're ready to transplant and, if you live where winters are cold, mulch them in the fall with straw or evergreen boughs to help keep roots warm.
You can also make new plants from a mature lavender plant with a method called layering, which allows a stem to remain attached to the mother plant while roots develop. In the spring, choose a healthy, nonflowering stem near the outside of the plant and bend it down toward the ground.
Snip off all the leaves from a section of stem that's 8 to 12 inches from the growing tip, leaving foliage intact on the rest of the tip. Moisten the leafless section of the stem, and sprinkle rooting hormone on the area. Use a bent piece of wire to pin this portion of the stem into a 4-inch deep hole in the soil; keep the leafy end of the stem unburied. When the stem is well-rooted, cut it free and plant into a pot or directly into the ground.
Don't propagate lavender plants by saving seed because some types produce sterile seed, while others have seed that doesn't develop into plants identical to the parent. It's also not a good idea to divide an older, woody plant by separating its roots manually to make two or more divisions -- a process that might produce new plants that don't thrive.
But if the lavender plant is young and vigorous, it's possible to divide it successfully using two garden forks to separate the plant's fibrous roots system. Moisten the soil first and keep as much soil around the roots as possible. Water the divisions well once they're relocated, and keep soil evenly moist for several weeks after the move.