This is my first year with moonflowers. I have two planted on my deck in Miracle grow potting mix. They seemed to wilt in the full hot sun, so I had to move them to a shadier place, and I water them frequently. Now they receive about two hours of full sun a day, and they are thriving. The two that I planted in a flower garden that receive about 6 hours of full sun are ragged and unhappy. The blooms are smaller and fewer. I hope this helps. There is a forum for moonflowers, or Datura on the garden web, they will be able to answer any questions.
Colleen

If this is the giant white datura, they would indeed need a lot of sun. I allow a few go to seed each fall to come up when and where they want to. A few end up where I can leave them and they reward me with lovely 6" flowers. But I did find that they can't be crowded and don't like shade. Some were transplanted to a potato patch to test if potato beetles indeed prefer datura. (They don't, at least not this variety!) If the plants managed to stay ahead of the potatoes, they are now huge and blooming nightly. If they couldn't stay ahead, they simply stalled and became overwhelmed by the potato vines.

Martin

I've grown them for maybe 5 years now. Starting them in the spring, either inside or directly sown, doesn't work for me as well as if I just leave the seeds on the ground in fall. If I leave seeds on the ground in the fall, they seem to be much hardier.
They are on the south side of the house and get sun up to 3pm, do very well there. Like Martin says, I just thin out what I don't want. They need plenty of room.