Growing a Three Sisters Plot
First off, a quick review of what "the Three Sisters" means- this is the shorthand term for the farming practice of growing corn, beans (pole beans) and squash all together in the same plot. This was practiced by many of the Native tribes of the Americas, where corn is originally from- bred from the wild grass teosinte which is native to Mexico.
*quick note, I took a lot of my early advice from the book Parker on the Iroquois, by Arthur C. Parker (first printed in 1968), which especially detailed how the Seneca of central New York spaced their plots out. I've made some minor tweaks but largely following the gameplan I read there. The spacing of corn hills follows an interesting pattern based on latitude...more on that later!
So...secondly, how to grow a Three Sisters plot? The principals are simple. 6 to 8 corn seeds are planted in a roughly one square foot area. Pole beans need a trellis to climb on- so they get planted with the corn- within the cornhill or barely outside it. Ideally, you pick a corn variety that still grows a vigorous stalk. Since beans harvest most of their own nitrogen from the atmosphere, they put very little demand on soil nitrogen, making them less competitive with nearby corn. Finally, squash is grown in the understory beneath the corn and beans- ideally a heavily vining type, as there are modern squash varieties bred to "bush culture" where the leaves and stalks are far less numerous. Ideally you want an old-fashioned vining type that will naturally fill out all the available space. Big squash leaves harvest sunlight before it hits the ground, reducing evaporative loss and making a moist environment that lets all the roots grow more consistently- while also shading out weeds. They both inhibit or stop weed seed germination, or directly crowd weeds that do manage to germinate.
I plant the corn in "hills"- which is an old fashioned way of saying on a grid system, with clusters of plants. Depending on soil type, you won't actually be making real hills. In overly draining soil, you will actually want to be planting the corn in minor depressions to collect water. "Hills" is just to say that you will be planting 6-8 corn seeds together in around a square foot or so of ground.
The bean seeds are sown 1 to 3 weeks after your climate's appropriate corn date. You can sow anywhere from one to three bean seeds per corn hill (though I'd say 2 plants is a more reasonable target), and you can sow as many hills as you want, depends on how much you love beans! They are the most laborious to pick and shell by hand, so don't overshoot your bean ambitions.
Roughly equidistant from each corn hill, you should plant your squash hills. These may be 1-3 seeds per hill, though you ultimately want only 1 or 2 plants per hill. If you get proper vining types, they will quickly cover most bare soil in all directions by late July.
There is a common misconception on the internet that somehow the Natives just planted these 3 species and everything worked out perfectly, with little to no weeding. I read other "Three Sisters" how-tos and it's described as a method that needs no weeding. It's just not true. I've carefully studied the actual old practices of the Haudenosaunee of New York, and they were careful, assiduous weeders. Both in older practice, and what you should be doing, you should be walking the fields at least once a week, killing absolutely any weeds you see.
By late July your field access is very difficult, due to corn height and squash foliage, but up til then, you should be frequently weeding- in some climates, this is most efficient with an old fashioned hoe meant to heap dirt up on the cornstalks. I also use a stirrup hoe and/or wheelhoe with stirrup attachment, to slice weeds frequently and quickly.
I have an old-fashioned cultivating tractor (the kind common before the chemical revolution in agriculture) that can do one direction of hilling mechanically, but it's not bad to do by hand either. For those inclined to larger plots, you should do your row spacing so that a tractor can cultivate two rows at a time. For example, my tractor's wheels are 6 feet, one inch apart, so I just call it good at one row every 3 feet.
Scraping the soil nearby kills the weeds there, and if heaped on the corn, it should bury and kill the majority of the weeds in that spot as well- two moves in one. The philosophy of old school hoeing is that several imperfect passes are better than 1 perfect pass. There are always more weed seeds that will germinate- better to strike fast, and often, and move quick, and come back another day to whack the survivors. The old timers knew the way and didn't mess around with the newest Johnny's gadget where steel has been bent in some newfangled shape and sold to you for another $40-$60. In the past, Native-made hoes were made of deer, elk, or moose shoulder blades, and the old carbon steel hoes are just metal versions of the same physics- technology that's been around thousands of years. It's quite manageable to grow a serious corn plot with hand tools. In 2021 I didn't even bother firing up the cultivating tractor- too much fun with the wheelhoe.
*2021 note- due to heavy, heavy rains constantly reviving injured weeds and germinating new ones, I focused on a stirrup-hoe style weeding, neither making hills nor depressions. I use a Valley Oak wheel hoe (best wheelhoe out there) for the long way and a regular stirrup hoe for in between. I undersowed clover, rye, and oats in early August to be an understory cover crop that will mostly winterkill (oats) and leave rye and clover to be plowed in next year). Every year, conditions must be watched and you should be ready to switch methods. Next year I will broadcast clover around July 2nd, when I put in a final wheelhoeing.
Interestingly, even more complex polycultures existed, and are still practiced in parts of the Americas. I've read that the Aztecs added, at various times, amaranth, chiles, and tomatoes into their corn plots. By understanding the interplay between species, you can come up with your own successful polyculture combinations. The principals are simple- if sunlight is landing on bare ground, weeds will show up. So fill out the space and pay attention to angles of sunlight, what species need full sun, what does well in partial shade, etc. In 2020, I was worried about vining squash leaping out of my Three Sisters plot into the potato and garlic plots on either side, so I replaced the "squash" component of the outer rows with large chile plants- one in between each corn hill. They grew tall enough to stay above what squash leaves came their way, and were far enough from corn to get their own sunlight, as heat-loving crops. Being on the border, I could visit them frequently to stay up on picking the red chiles. Following those principals, you can intercrop all kinds of things together.
You don't quite magically get the full yields of all the crops involved. My quarter acre of Three Sisters does not result in a quarter acre's worth of corn, quarter of squash, etc. Studies show that you get about 6/10ths the yield of corn and squash, which means that if you had 4 parcels to plant, and one person planted separate plots of each species (2 and 2), and another planted all four parcels to a Three Sisters style, the Three Sisters farmer would harvest 2.4 units of corn or squash for every 2 units of corn or squash of the monocrop farmer. On four plots, that means the monocropper gets 2 units and 2 units, the polyculture Three Sisters gets 2.4 and 2.4. That's not counting the beans, whose yield is a bit more difficult to express. But basically...you do grow more food per acre this way.
If you're wondering...why in the hell doesn't everybody switch to this if it's so impressive? It's simple. Three Sisters is more labor intensive. The polyculture is not easily done by machines- it's mostly hand sown, hand weeded, hand harvested. Perfect for the gardener, homesteader, or farmer whose primary consideration is space, not time.
Something to note is that the "hills" method is ingenious, for hand planting. It is far easier to chuck 7-10 seeds into a one-foot wide area, on a 6 foot by 6 foot grid, than to painstakingly plant a seed every 8-12 inches in a thin row. Rows are good *if* you own a push-seeder or a grain drill, but not the answer if you're using smaller scale tech like hoes, digging sticks, etc.