Plant guilds are a permaculture concept that embraces the natural patterns of biodiversity and layers to create thriving ecosystems on a micro scale. Plant guilds consist of many different species that coexist happily together. Each species serves different functions, and together they create a mutually beneficial environment for all of the species in the guild.
Guilds can serve many purposes and facilitate many different natural functions. Some guilds are known because they have been observed in nature, and others have been devised by permaculture designers who have observed that some plants thrive in the company of other plants.
Plant guilds are a similar concept to what is called "companion planting," but guilds take into account several natural patterns and typically perform more functions that just providing edible food for humans. There is not a finite number of guilds. New guilds are regularly identified by observation and experimentation. With a general understanding of this basic permaculture concept, you can create new plant guilds in your own garden or food forest.
Plant guilds are planned in layers, much like food forests. The exact definition of the different layers varies by source, but there is some general consensus. As with food forests, a complete guild should include a canopy, and understory, and an herbaceous layer along the ground.
A more complete guide is offered by Midwest Permaculture's Bryce Ruddock, who defines the seven layers of a plant guild as overstory, understory, shrub, herbaceous, groundcover, vining, and root. Plant guilds do not need to include each of these layers, and individual plants may be considered part of one layer or another, depending on circumstances. These layers are only a guideline to help understand the concept.
Each layer adds to the biological diversity of the entire guild, and each species brings its own features and functions to the planting. Plant guilds are typically designed with one or more functions in mind, and a well-designed guild will likely include many of the functions discussed below.
The functions performed by plant guilds might be beneficial to humans, to the native wildlife, to the plants in the guild themselves, or to the ecosystem as a whole. There is no limit to the number of functions a strong guild can perform. When planning guilds, you should normally seek to include plants that serve as many functions as possible.
Examples of functions that serve humans are to provide food, medicine, fuel, and raw materials. Examples of functions that serve wildlife include providing food and shelter. Functions that serve the plants themselves include nitrogen-fixing plants, mulches and groundcovers, and plants that mine water and nutrients from deep in the ground.
A guild can be designed to serve human functions, wildlife functions, plant functions, or all three. The best guild is typically that which serves the most functions for the greatest number of stakeholders. The only requirement for a plant to be included in a guild is that it serves some function and will coexist happily with the other plants in the guild.
Some plant guilds have become well-known as they have been documented and discussed by various permaculture communities around the world. These guilds have been proven to perform well and serve many functions. But these are only a few examples, and there is literally no limit to the number of guilds you might conceive of or observe in nature.
Fruit tree guilds are very popular among permaculturists because of their usefulness in planning food forests. A typical fruit tree guild might include full-size fruit trees like persimmons or loquats in the overstory, semi-dwarf fruit trees like plums or peaches in the understory, berry-producing shrubs like goji berry or blackberries, perennial vegetables like artichoke and asparagus, edible groundcovers like strawberries and thyme, and edible root crops like garlic and bunching onions.
An example of a popular guild that provides functions to birds includes juniper in the overstory, hollies in the understory, berry-producing shrubs, lantana in the herbaceous layer, and pigeonberry or some other berry-producing groundcover. A guild planted in this way can provide edible food and comfortable shelter to birds of all kinds.
If you'd like to learn more about plant guilds, you might consider taking a permaculture design certification course. Those courses are by far the best way to gain a thorough understanding of all of the principles and techniques that go into planning a guild.
If you're looking for a quick reference, check out the book Integrated Forest Gardening by Wayne Weiseman and Daniel Halsey. Alternatively, you can get some training online from Toby Hemenway in his Food Forest Design class.