I've (re-)started working my way through Gaia's Garden: A Guide to Home-Scale Permaculture (notes) by Toby Hemenway in preparation for the gardening season. I just now got to the chapter that I was waiting for, the reason I was looking for a book about permaculture: complementary plants.
Plants as complex systems is an appealing idea. I don't think about them like that. In my mind, without thinking about them very deeply, they're just single-themed entities. A tomato plant makes tomatoes. A flowering plant has flowers. Weeds are a nuisance. Lettuce has edible leaves. And so on. Whatever main feature the thing has, that is its only feature—in my mind, at least.
I first came across the idea of permaculture while reading Seeing Like a State (notes) by James C. Scott. In Chapter 8, "Taming Nature: An Agriculture of Legibility and Simplicity", he screeds about agriculture. ("If the logic of actual farming is one of an inventive, practiced response to a highly variable environment, the logic of scientific agriculture is, by contrast, one of adapting the environment as much as possible to its centralizing and standardizing formulas.") But he also tours briefly through forms of agriculture that are alternatives to the only kind I know—the long, long, well-ordered rows of single-cropped corn and soybeans set in forever-long flat fields of glaciated Illinois soil.
The image of permaculture, on the other hand was messy, riotous. Aesthetically—from a distance—it was unordered, unkempt, uncontrolled. But the underlying logic made sense: if you conceive of each plant as being a system with more than a single-output, then each of those other outputs—the leaves it drops as mulch, the shade it throws, the wind it blocks, the rain it collects or blocks, the chemicals it produces around its roots, the nutrients it processes from the soil, the bees it attracts, and on and on—become an input for other plants, animals, and on and on. Designing the system to a human aesthetic of well-ordered rows breaks the network of inputs and outputs.
The city desk of a newspaper, a rabbit's intestines, or the interior of an aircraft engine may certainly look messy, but each one reflects, sometimes brilliantly, an order related to the function it performs. In such instances the apparent surface disarray obscures a more profound logic.—James C. Scott, Seeing Like a State
That's where my head is right now. First it was acquiring a raft of seeds for their single outputs that I wanted. Next is figuring out if any of them are complementary in some way so I can think of how to plant them. Then, if there is time, I'd like to fill in the gaps: what other plants fit in the network of inputs and outputs and produce an "optimal system" (whatever that might be), or at least a good system.