I'm having good results using cheeseweed (malva parviflora, little mallow) as a green manure. I'm working with a 10x15 patch of dirt behind the shop where I work in zone 9; soil's mechanically-compacted and clayey.
After last winter's rains, the mallows came up profusely in every spot I hadn't yet cultivated. Since I scraped them off at the roots and left them in place, I'm seeing dandelion, clover, and sorrel in their place, among several others I've yet to identify. Soil that wouldn't start beans this time last year now starts pretty much any seed I throw at it.
There's also benefit to leaving some mallows to flower, as the immature seedheads attract wasps.
Anyway, hope this helps!