You will make things far easier on yourself if you start with a nursery-grown plant. Rosemary can take years to fill in, so expect to pay more for a mature plant than for a small rosemary start.
If you would like to start your own rosemary plant, the best option is to start with a cutting. Rosemary seed can be difficult to germinate and often do not grow true to their parent. It's much faster to start with a cutting, and you will be sure of what type of plant you will get.
To propagate with cuttings, snip about a 2-inch cutting from the soft, new growth of an established plant. Remove the leaves from the bottom inch, then dip the cut tip into a rooting hormone and place the dipped end into a container of dampened, sterile seed starting mix that drains well.
Ornamental and Edible Sweet Bay: An Ideal Container Plant
Place the container in a warm spot with indirect sunlight. Mist the cuttings daily and make sure the soil does not dry out. In about 2 to 3 weeks, test for root growth by very gently tugging on the cuttings. Once the cuttings have roots, transplant them into individual pots about 3 to 4 inches in diameter. Pinch off the very top of the cutting to encourage it to develop branches.
If you're growing rosemary in containers, move your pots back outdoors once all danger of frost has passed. As with most potted plants, the soil in your rosemary pot will degenerate through watering and root growth. Re-pot at least once a year. Spring is the best time to re-pot your rosemary, but it should be fine no matter what time of year you get to it.
When the rosemary plant puts out considerable growth or looks like it just can't get enough water, it has outgrown its pot and needs to be transplanted into a larger one. If you want to maintain the size of your rosemary plant, root prune it by slicing off a couple of inches of the roots from the bottom and sides of the root ball and replanting in the same pot. Be sure to trim some of the top at the same time to lessen the workload of the roots and the stress placed upon the trimmed plant.