I've got a three sisters plot maturing (first official day of autumn tomorrow
)
I read a lot about it first, and a big red flag was failures begause people liked the idea, but didn't get the 'why' of the system.
Supposedly
Native Americans generally used it as a 'plant and walk away' type thing, leaving produce to mature and dry in place. So winter squash, drying corn, drying beans rather than zucchini, sweetcorn and snap beans.
I also read lots about planting succesively as people mention, but since I was trying to use
some of the original gardener's techniques, and I was going on holiday
I just put everything in at the same time, which is apparently traditional, but I didn't plant on mounds, which is not at all traditional! It's a
very small area, prone to drying out and I avoid going up if possible. I put seed straight into the (very amended) soil, corn about a foot apart with beans close.
I'd suggest planting less in each spot, especially squash as the plants usually get huge. I planted 3 corn and took out the 2 weaker ones, 3 beans and two pumpkins in the entire plot, one on each end
I planted giant sunflower seeds too which make excellent bean-poles.
I've got marina di chiogga pumkin, a traditional 'ironbark' storage squash, anasazi beans, an ancient mezoamerican drying bean and 'black Navajo' corn, which can be eaten fresh, but is more a dent/flour corn. I've had a pretty average time with corn in the past, but this has 3-5 decent ears
per plant.
Can you put in a leguminous cover-crop first and cut it down before planting? As you've pointed out, maintaining OMs a big issues in sand! Can you get grass clippings? I've found they make a good mulch on my three sisters plot.