I am guessing that you are referring to standard Lavender? With a stem and then the bush part at the top? Treat just like any other Lavender. Prune after flowering around mid late August back to the denser growth below the elongated flower stems. Don't go too far back into older wood though as they won't thank you for it!

Ps the actual height of a tree starts at 15ft (5m) so unlikely to be really classed as a tree I suspect ;-)