Just for perspective I've tried French, English, Spanish and a couple others. The "couple others" did well enough in huge clay pots in full sun for just 2 years. I yanked one 2 yr old out today. Totally pot bound. It was impossible to keep hydrated in the heat (Phoenix, AZ = HOT). Certainly not worth the water to grow something like that in this climate.
By FAR my best success with French, English and Spanish is in the ground on mounds of slightly amended clay and rock, full sun, regular trimming twice a year (cut back by 1/3 in a dome shape). I've had great success with this combination. My pics are in my other camera.... sorry. But they are pretty and very lush year after year.
If I try a pot again it will be huge and plastic. Lavender does not like rich soil. I think I'd try a mixture of cactus soil (non orgainic/sand), perlite and potting soil. 1/3 each. Drainage is key. Here in PHX the pot gets so hot the roots cook... unless you line the inside of the clay pot with bubble wrap or line it with styrofoam.
