North American Lobelia siphilitica (ht 60 - 90 cm) is an unusual, clump-forming perennial relative of the bedding Lobelia that is grown for spring displays. It is sometimes known as the blue cardinal flower and it blooms in late summer and early autumn with stout spires clustered in the recognisable hooded form of other Lobelia plants, in colours of white or blues. There are other sorts of perennial Lobelia but this is the only one that has steadfastly stayed with me. It self-seeds occasionally, so new plants pop up here and there. It likes a sunny, moist position. Mine sited itself next to Salvia 'Wendy's Wish' and I enjoy the combination of the milky blue Lobelia with the rhubarb colour of the Salvia flowers. It is best to divide the clumps every few years in early spring, into soil amended with compost and fertiliser. Otherwise, it will eventually die out.
Postscript: My plant did eventually die out! They are not long-lived in the Sydney climate but I enjoyed mine whilst it was with me.