Lavender tree? Can you be more specific - are we talking about Lavender (which isn't a tree), or something else?

Lavender should flower freely every year, so perhaps we are not talking about the common lavender?
If you want something different that flowers with wonderfully fragrant foliage, get some Lavandula multifida (oregano perfumed lavender) seed (Chilterns used to supply it) and it grows easily and quickly from seed, flowering in a few weeks. By the end of the summer it makes a large plant covered in flowers. Collect the seed for future years, as it is too tender to survive the winters. Seed you collect yourself germinates very easily.