Every year at this time, the number one question from visitors to my corner of Katy is "WHAT is that tree????????!!!!!!!!!!!!!!!!!!", followed by "Where can I get one?"
This is Vitex agnus-castus 'Montrose Purple', aka Chaste Tree, Texas Lilac or Lavender Tree. 'Montrose Purple' and 'Shoal Creek' seem to be the best varieties for this area: the bloom spikes are larger and deeper in hue than the species. Most of the independent nurseries in the greater Houston area should carry them.
Vitex are very easy to grow. Plant them in full sun in well-drained soil and give them room to spread. They can be grown as large shrubs, or limbed up and shaped into a small tree, as this one has been. They make lovely multi-trunked trees, and the HG has long been unhappy with me that I didn't shape ours that way. Vitex are fast growing and established plants purchased from a nursery should bloom for you their first year. Be aware that they do reseed (seedlings are easy to pull). They are deciduous trees and will lose their leaves before new growth emerges in spring. The leaves are very fragrant, with a sharp aroma. If you're prone to allergies, the leaves may cause problems for you as they have for me: mild irritation of the eyes and sinuses. Removing the spent blooms/seed pods can encourage another flush of bloom but in a tree of this size, that can be a difficult task! This year I chose not to prune off the spent bloom tips and I've been pleasantly surprised in how little difference it made to the lushness of the blooms.