The lavender plant does best in direct sun, spaced so that air can flow around the plants. The soil should be slightly alkaline and fast draining. (Sometimes it helps to mix a little sand in with the soil.) L. dentata and L. stoechas the Spanish and French varieties are tender and considered frost sensitive.
To plant lavender spread the roots and gently place into the hole. Fill in gently.
Prune lavender in the spring or in the fall after harvesting.
Harvest lavender just after the sun has dried the dew in the morning. As the sun grows hotter the esential oils evaporate from the buds. Lavender flowers that will be dried should be picked with no more than 3 open buds and no fewer than 2.
Propagation
Mature plants may be divided in the fall. Alternately, 3 to 4 inch non-flowering shoots may be picked and rooted in moist sandy soil, in the shade.
References:
http://www.purplehazelavender.com/growing/
http://www.lavenderfrog.com/Growing.html
http://www.lavenderfarms.com/info/growing.htm
http://txtx.essortment.com/growinglavender_rgxc.htm
http://www.chicago-botanic.org/PPLavender.html