Not many shrubs bloom madly in September but Lespedeza thunbergii and Vitex agnus-castus do. I’ve never grown either and you don’t see them all that often around Seattle which is a pity because they pack a lot of flower power at eye height and above at a time when most flowers are closer to knee level. The two plants are totally different in form. Vitex is upright, practically military in its bearing – but showing a little softness around the edges, fanning out like a big bouquet, whereas the Lespedeza slouches – but it’s an exceedingly graceful slouch.
Lespedeza thunbergii, aka bush clover, is a member of the legume family which means it is able to fix nitrogen for itself in the soil – a nice trick. In his Manual of Woody Landscape Plants, Dirr refers to L. thunbergii as the “beauty among the beasts of this genus.” In some parts of the country it is a perennial, the woody parts dying back each year and new growth sprouting from the crown. In other areas, it is a deciduous subshrub. I expect treating it like you do many of the fuchsias is the best bet around here. Leave it standing all winter and then cut it back to 6-8″ in late winter. I expect a fair number of the branch ends may die even if the bulk of the branch remain viable so this hard cut back may make it look tidier without a lot of fiddly removal of dead wood. Plus, it’s a big sprawling shrub 5′-8’H x 5-10’W so this pruning may help keep it on the smaller side. Lespedeza thunbergii has a zillion purply-pink pea flowers held in racemes and bluish green, fairly scanty trifoliate leaves. If you can’t find them at your local nursery, they are readily available via mail order.
Vitex agnus-castus (chaste tree) looks like a stately Buddleia (butterfly bush). (The leaves are totally different but you pretty much never notice either when they aren’t in bloom.) The branches are upright and so are the flower inflorescences which come in shades of purple, or white. Typically, Vitex reaches 8-10′ by as wide but can progress to small tree status. It hales from the Mediterranean and is fairly drought tolerant but is a popular plant in the southeast so clearly it can take humid as well as dry heat. The ones I’ve seen flower well here (perhaps there are others that don’t – I’m sure I wouldn’t notice a Vitex without its flowers). Flowers bloom on this year’s growth – I wonder if it gets pollarded in the South as happens to so many crape myrtles?
Which to pick? Slouchy, pinkish, sortof low profile and maybe elegant or maybe not? Or upright, bold and purple (or white)? I expect the bloom time of Lespedeza extends later into the fall than the Vitex. I’ll try to keep an eye on that. Here’s a link to a Lespedeza being more elegant, just so you can see that they CAN do it.