A single lavender flower.
As summer draws to a close gardeners have a raft of fall chores to complete by first frost. Among my favorite tasks is propagating perennial herbs and flowers by dividing the root balls of plants still in the ground and putting the cut-away members in flowerpots for replanting in the spring. September and October are good times to do this because the soil is still warm and moist and, as the potted plants winter over, their energy is focused on growing new roots instead of foliage.
Division works best on plants that are at least a year old. You'll need a spade to cut the earth around the plant's root ball, and to lift it whole out of the ground. Then, use a small knife or scissors to divide the plant into as many "babies" as you need. Be sure to have an assortment of pots on hand, some potting soil, and an array of plastic or wood plant markers to help identify what's in the pots. I always put a tarp down next to where I'm digging and do my re-potting on it.
It's important to make sure that you're getting enough of the plant root to re-pot or relocate. Some herbs, like lemon balm, tarragon, thyme and hyssop, are very forgiving and seem to have an innate drive to thrive; they will succeed with the tiniest bit of root and encouragement. Other plants, like lavender, are much harder to propagate and require a substantial amount of root to succeed. The "mama" lavender plant can provide one or two babies for propagation, but shouldn't be asked for more. New plants get their size and vigor from the donating plant; if the donor is meek and weak it's off spring will be similarly uninspired.
If you're re-potting plants rather than relocating them, store the pots near a light source--the window of a garage works well--and water them when you notice the soil is dry. Many of these plants, like thyme, are attractive enough to serve as houseplants, and will remind you that spring is "just around the corner."