Morning Glories would be my first gut response, because they grow just about anywhere, even in pretty shitty soil, and they grow fast with good coverage. Just be careful because they can choke out other plants since their roots are super aggressive and grow unbelievably quickly. Luckily they aren't a danger to trees since they die off annually, so they don't stay long enough to hurt them. Also, once you get them established, they are phenomenal at self seeding. I would suggest a mix of Morning Glories and Moon Blooms, that way you have some open during the day and some open at night.
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