I don't know about the soil, but I cut mine back to the ground after the bloom at the beginning of fall. Mine are blooming now; they just started, though. I am coastal, if that helps.

Mine was very leggy and the bloom was stingy. Maybe that's how they are.
Renee

They can be cut back hard, but doing so this late in the year will affect flowering. They can get rangy if given too much water or not enough full sun, and ordinarily would be found in leaner or more gravelly soil conditions.

They are hard to get rid of. The roots go very deep and they grow back from the roots. The hummers like it so I leave it.

Mine receive little water and will stay more compact than others that receive more water. Also, mine bloom more heavily with less water. I'm inland near Fresno, and grow both z. latifolia and a local native variety that I grew from seeds.

It helps to pinch them periodically to encourage branching - most flower heavily and the flowers weigh down the branches. The different cultivars do flower at different times, Bart O'Brien wrote an excellent article on Epilobiums about a trial done at Rancho Santa Ana Botanical Garden.

Here is a link that might be useful: Enjoying Zauschnerias