Perennial herbs may take time
Starting them in pots will help prevent the seeds
from being disturbed or forgotten in the garden.
Some need a period of moist cold before sprouting, and these
should be planted in very early spring or even in the fall. Most herbs
will be more potent and more winter-hardy if given excellent drainage.
If you have heavy soil or live in a moist climate, consider planting on a
mound, terrace, raised bed, herb spiral, or among stones. Adding gravel
(not sand) to the top couple of inches of soil can help also. Shelter from winds can also increase winter survival.
Perennials should generally be divided or cut back every three years to get the best growth. How to know which?
If the plant has made a clump, sprouting from several points in the ground, you should divide it. Pry the clump apart into 3 or 4 pieces, each with some roots and some sprouts. Chives, Mexican Tarragon, Skullcap, and mints fall in this group.
If there is a single stem like a trunk arising from the ground and then branching, you will want to cut the plant back. The time to do this is immediately after blooming (within a month). Use scissors or shears to cut each branch so that the entire plant has been trimmed into a smaller, roughly ball-shaped, shrub. Make sure you do not cut back as far as the totally woody, bark-covered stems. You should leave at least an inch of growth that is flexible, and has little leaves, buds or scales along the stem where new branches will emerge. Lavender, Thyme, Sage, and Rosemary are all small shrubs that should be cut back in this way.
What if your plant has neither a shrublike shape nor a clump of sprouts? Then leave it alone! Some plants like Lovage, Grindelia, and Lomatium will just go on getting larger without needing any care.
See The Medicinal Herb Grower in our book section for excellent advice on growing herbs and indeed all sorts or perennials, trees, and shrubs from seed.