Stacking functions is a permaculture concept that is at the root of every successful permaculture design. In the simplest terms, stacking functions means that every element in a design performs more than one function. In permaculture design, the ideal is to have each individual element perform as many separate functions as possible.
One example of stacking functions is incorporating chickens into a compost pile. In a traditional setup, the compost pile serves two functions - it disposes of organic waste, and it creates fertile compost to be used in the garden. In a traditional setup, chickens serve one function - they provide food in the form of eggs (and maybe chicken).
By introducing chickens into the compost pile you can effectively increase the functions served by both elements - the compost pile and the chickens. Let's take a closer look at how this works...
When chickens are turned loose on a compost pile, a few things happen. You used to need to turn your pile over periodically to mix up the compost and improve aeration. Now, the chickens turn your pile over constantly. Function stacked. Before, you might have searched for nitrogen sources to break down large masses of carbon like piles of dead leaves. Now, the chickens are constantly adding nitrogen to the pile as they deposit their droppings. Function stacked. You might have occasionally noticed infestations of roaches or other critters in your compost when it got out of balance. No longer, the chickens will eat anything and everything that walks around in your compost pile. Function stacked.
And the functions served by the compost pile have increased as well. It now serves as a food source for your flock. Function stacked. And it also serves as an instant, work-free chicken poop cleaner-upper. Function stacked.
By taking two separate elements that performed a total of three functions and combining them, we now have two separate elements that perform at least eight different functions between them.
Stacking functions is a concept that presents infinite possibility to the designer. And, once you get the hang of it, it's hard to stop. Every permaculture designer becomes something of a professional function-stacker.
When you take a compost pile and move it inside a greenhouse to heat the area in winter, you're stacking functions. The compost pile does everything it used to do, and now it also helps to heat the greenhouse. When you take a rain barrel and move it inside a greenhouse to serve as a thermal mass, you're stacking functions. The rain barrel does everything it used to do, and now it also serves to heat the greenhouse.
When you plant a plant that serves many purposes, you're stacking functions. A yaupon holly provides tea and medicine for people, berries for birds, and habitat for insects and wildlife. A rose bush provides tea, cut flowers, and nutritional supplements for people; mulch for the ground or compost; and forage for pollinating insects. When you select plants, select plants that serve as many functions as possible, and you will be stacking functions in your garden.
When you install a wood-burning stove or rocket mass heater to replace a fossil fuel-powered furnace, you're stacking functions. Your old furnace only heated the house. Your new stove heats the house, disposes of excess wood, and provides you with ash that can be used in soap, fertilizer, and medicine.
If you want to learn more about stacking functions, you might consider taking a permaculture design certification course. These courses will teach you everything you need to know, and by the end of the course you won't be able to stop finding ways to stack functions all around your home and landscape.
Scott Pittman's online Permaculture Design Class is a good option for anyone who can't make the financial or time commitment to take a full PDC course. And Bill Mollison's Permaculture: A Designer's Manual is a good option for people who prefer to read, rather than watch.