Starting lavender from seed can be a difficult process. Most success in growing new lavender plants happens with by the process of propagation of stems cuttings. When a cutting is made of a lavender stem, a callus forms at the cut and cells near the callus work together to form roots if proper conditions are present.
Choose a stem that is leafy (it will be without buds, as lavender buds form on tall, leafless stems). Make sure that each cutting is 2 - 4 inches long and has 2 or 3 leaves growing along it. Make a cut 1/4 inch below a leaf node and then pull off all the leaves that are at the nodes that will below the surface of the potting mixture. Sometimes rooting hormone can be used to ensure root stimulation. I don't care for this method, as the rooting hormone comes with all sorts of safety clauses for humans (do not touch, get on skin, etc.). Since lavender starts so well without it, I prefer to do without. Once your stem is prepared, poke a hole in the potting mixture and then insert the cutting. Make sure the soil is moist. Commercial potting soil can be used or you can make a mixture of 1/2 vermiculite and 1/2 sphagnum peat moss. Be sure your container has excellent drainage (remember, lavender does not like wet feet). The container can be covered with a plastic bag tent or container to help maintain humidity, but be careful that the little lavender doesn't drown in moisture! I'd play this by ear, checking frequently to see if the tenting is really necessary. The little plants should be in a warm area with indirect light so they are not baked in sunshine. Rooting will take several weeks to take hold. After a few weeks, gentle tugging will reveal a resistance. This lets you know that rooting has started and you can soon transplant into a larger container or garden bed. Generally, lavender plants do better in a garden bed than they do potted containers. Along with wet feet, lavender doesn't seem to like containers very well.
The lavender and greenhouse pictures
were taken at Robin's Lavender Farm