Plants with a Purpose, Designing Perennial Polycultures — a Presentation

Richard Freeman


In this brief presentation, I overview a process for designing perennial polycultures.

First, I’ll begin by discussing some key concepts and principles pertinent to designing these garden polycultures.

Next, I’ll outline the process of designing polycultures.

Finally, I’ll conclude by answering your questions. Please hold your questions until the question/answer session. We’ll have plenty of time to discuss these topics further then.

1. Important Concepts and Principles

Understanding a few important concepts and principles is key to designing effective, productive polycultures.

1.1. Ecosystem

A key concept is the Ecosystem. In the most general terms, an ecosystem is “the environment” – the physical, chemical and biological world around us. For research or management, we define boundaries based on our goals. Any garden zone or farm zone can be an ecosystem, depending on your perspective.

1.2. Ecology

Ecology is another important concept. Ecology is the study of ecosystems and ecosystem components and how they function and relate through time, with special emphasis on energy flows.

1.3. Biodiversity

Another fundamental concept is biodiversity, short for biological diversity. Biodiversity describes the diversity of the organisms in an ecosystem. Ecologists describe three types of biodiversity, including diversity of composition, diversity of structure and diversity of function.

Biodiversity is key to effective horticulture, even at a small scale. The most robust, resilient ecosystems exhibit high diversity in function, structure and composition. With the smallest organisms – nematodes, microbes, etc. – more species representation is better than less. The vast majority of the smaller organisms are beneficial or neutral to gardens and crops. A biodiverse ecosystem with high species representation will usually deter the small minority of pest species that are present and keep damage within a management comfort zone.

The term biodiversity refers to one of more of the following analytical categories.

1.3.1. Composition

The term composition refers to the species and higher taxonomic groups that are present in an ecosystem. Taxa, short for taxonomic groups, include individual species, genera and families, all the way to kingdoms and domains. The term composition also encompasses the population patterns of the taxa that are present. So, compositional diversity refers to the diversity of species, families, etc. in an ecosystem.

1.3.2. Structure

The term structure describes the spatial and temporal distribution of organisms and taxa in terms of volume, mass and shape. So, structural diversity refers to diversity of organisms and taxa in physical, spatial terms.

1.3.3. Function

The term function describes the biological processes at work. Functions contribute to net productivity in an ecosystem. More specifically, functions refer to the effects of organisms on each other and how these effects determine productivity. One species can affect another species or it can affect an entire family and so on. Likewise, a family can affect another species or another family, and so on.

Furthermore, organisms can affect other organisms in various ways, among them behavior, physiology and/or metabolism. In combination, these effects, or functions, determine the net productivity of the entire ecosystem.

So, functional diversity refers to diversity of functions that contribute to productivity in the ecosystem.

1.4. Ecological Dynamics: Succession

In ecology and ecological design, succession is equivalent to the maturation of an ecosystem. In this process, shade tolerant plants gradually supplant sun-loving plants.

During succession, three patterns emerge that are beneficial for horticulture:

1.4.1. Increase in long-term soil organic matter

Long-term soil organic matter increases due to three factors: Increased perennial root mass

Live root mass is a major component of soil organic matter. It varies widely between ecosystems. As a percentage of total dry plant matter, it can range from 21% (boreal and temperate forests) to 49% (shrublands) and higher (xeric and desert ecotypes). Accumulation of dead root mass

Dead root mass is also a major component of soil organic matter. As large plants die, their dead root mass increases long-term organic matter and contributes resources to the plants that emerge to replace them. Increasingly intensive microbial networking

During succession, microbial network generally increases in biodiversity and biomass until the ecosystem is mature (characterized by old, large trees), when microbial diversity can decrease while networking (resource transfer) and mass increase — implying a stability in nutrient exchange.

1.4.2. Increase in microbial networking and resource transport and exchange.

Microbial networking, which increases soil organic matter, also provides its own benefit, by networking links organisms into resource transactions and optimizing resource use. Additionally, microbial networking decreases resource loss from the ecosystem.

1.4.3. Increase in biodiversity towards the maximum possible diversity on a site

Increases in plant diversity and perennial biomass drives diversity of other organisms, which depend upon plants.

An ecosystem will proceed through succession until one of two states emerge. Climax stage

In one state, ecosystems tend towards a climax stage. In a climax ecosystem, the most shade-tolerant plants that can grow on the site’s conditions dominate plant composition. Disturbance

In the other state, disturbance reduces vegetation and increases sunlight to the forest floor. Thus, after a major disturbance, pioneer species emerge. Pioneer species require full sun for vigor and they can tolerate soils with relatively less organic material. Examples of disturbances include fire, flood, logging and harvesting.

1.5. Rhizosphere

Another important concept is the rhizosphere. The rhizosphere is the soil environment immediately surrounding the root. In a rhizosphere, physical, chemical and biological properties determine the vigor and survival of the plant.

1.6. Soil Food Web (SFW)

The soil food web is a community of interdependent organisms making up the soil ecology.

In the soil food web, plants transform sunlight and nutrients into plant tissue and fluids (or solutions), which are energy-rich food for a multitude of organisms. Plants exude fluids into the rhizosphere in the form of root exudates. These protein- and sugar-rich solutions provide a major food source for soil organisms. They commonly contain enzymes, hormones, proteins, glyoproteins, sugars, starches, minerals, and other useful food resources.

Among the organisms that feed off root exudates are symbiotic bacteria and fungi. These organisms, in turn, provide the plants with minerals and compounds, for example plant growth-promoting hormones. Often, bacteria and/or fungi live within the plant’s tissue in a symbiotic relationship.

Other organisms that feed off exudates and plant tissues are pathogenic, or disease-causing organisms.

In turn, other microorganisms, the saprophytes, feed off dead plant and animal tissue.

All these micro-organisms, in turn, become food for protozoa, nematodes, other bacteria and fungi and a multitude of other predators. These predator organisms release nutrients as they digest their prey. In turn, these organisms become food for yet other organisms – more nematodes, micro-arthropods, worms, tardigrades and a multitude of others. All these organisms release nutrients as they digest their prey.

The soil food web is key to maintaining robust nutrient-cycling and to averting pest problems. Furthermore, it constitutes a huge portion of the soil organic matter and is responsible for soil aggregation, fundamental to soil health. A biodiverse soil food web builds on itself by establishing the conditions for increased productivity. Productivity is measured in terms of living tissue and/or agricultural crops. Soil food webs become more complex, massive and robust as ecosystems mature.

1.7. Limiting factors

For selecting plants, an important principle is limiting factors. Limiting factors are environmental factors that limit plant growth or survival on a specific site. The concept of limiting factors includes two components: the law of minimums and the law of tolerances.

1.7.1. The law of minimums

The law of minimums states that of all available mineral nutrients in a plant’s rhizosphere, the one most nearly depleted is most likely to limit plant growth. This nutrient is the limiting factor. For example, consider a garden plant that has access to plenty of every nutrient besides potassium. When the plant depletes available potassium, it will be unable to continue growing. According to the law of minimums, potassium is the limiting factor for this plant on this site at this time.

However, the law of minimums has two caveats. First, it only applies to an ecosystem under a “steady” state, in which available nutrient levels are not rapidly changing. So, it will have more relevance to an orchard or food forest than to a plowed and fertilized garden or farm field. Second, depletion or surplus of one nutrient can alter a plant’s requirements for another nutrient.

1.7.2. The law of tolerances

The law of tolerances states that an excess or deficiency of an environmental factor can limit plant growth or survival. Environmental factors can include nutrient availability or other factors such as temperature, moisture and light. Each species or variety has a “range of tolerances” for each important factor.

However, five caveats qualify the law of tolerances:

1) An organism’s range of tolerance can vary for different factors.

2) A deficient or excess level of one factor can affect the tolerance for another factor. For example, high temperatures can increase the minimal tolerance level for water on a given site. Likewise, for example, a deficiency in calcium can limit uptake of phosphorus, making existing soil reserves less available.

3) A sub-optimal factor can mask other sub-optimal factors.

4) During reproductive stages, ranges of tolerance narrow substantially.

5) Species and variations with wide ranges of tolerance have wide geographical distributions.

1.8. Edible Forest Gardens

Edible forest gardens is the name of a design framework within permaculture design that focuses on plant ecology as a model for design of productive ecosystems that produce a surplus for consumption. Forest gardens design follows a pattern-to-detail process, starting with permaculture zones and progressing through food forests then patches and then to detailed polycultures.

1.8.1. Food forests

A food forest is a landscape zone designed to grow food and to meet auxiliary goals. Food forests are dominated by trees in the top layer and shrubs, lianas and herbs in sub-layers. In addition to growing food, a food forest can help restore soil structures, attract natural pest-predators, provide aesthetic refuge and provide other benefits. A food forest often includes annual crops growing within the perennial stand structure.

1.8.2. Food forest patches

A food forest patch is a sub-unit of a food forest. Patches meet specific management goals or reflect distinct environmental conditions. For example slope, aspect or soil type can determine the shape of a patch within a food forest.

1.8.3. Polycultures

A polyculture is a distinct group of three or more plant species growing within a patch. Plant species and variety selections depend on your goals for the larger patches and on plant growing requirements.

1.9. Plant Habit

Another important concept includes Plant Habit. Plant habit, also known as growing habit, can refer to a species’ or variety’s general shape or “architecture” as it develops through growth stages. Plant habits are descriptive, for example, the general categories of trees, shrubs, herbs and liana (woody vine).

Plant habit can also refer to crown form and even to root form. Form pertains to the the shape of the mature plants, above- and below-ground. Form types are also descriptive. Examples for tree forms include evergreen or standard. Examples for herb forms include clumping, running or matting. Crown form and habit can include dozens of descriptors, depending on the author. Both are important for designing polycultures.

Section 2. Designing a Polyculture

Designing the polyculture starts with identifying your own goals and translating them into spatial, dynamic ecological design. The following steps describe this process.

2.1. State your goals for the polyculture.

Practical examples include the following:

1) Producing berries for market.

2) Improving soil productivity.

3) Creating a beautiful backdrop.

4) Supporting ecological pest management.

2.2. Assess the site

for soil type, sun exposure, temperature ranges, moisture, wind, access, aesthetics, privacy, noise, dust, hazards and toxins.

2.3. Draw a basic black-and-white map

of the site and make photocopies. Include the outlines of landscape elements and buildings more permanent than your garden. Also include relevant areas such as soil types, sunny and shady spots, frost pockets, etc.

2.4. Write a plant list

in table form, with rows and columns. For each plant, the plant list should include the plant name and some basic plant properties that are helpful for designing polycultures. The plant list can start short – for example, 10 or more plant species that are perennial to your site or region. Once started, you can build on this list over time.

Some important plant attributes relevant to designing polycultures include:

1) Binomial name – Genus and Species. For example, Taxicum officinale.

2) Common names are optional, but helpful. For example, dandelion.

3) Major plant habits are important.

4) Crown forms are also important for designing polycultures.

5) Crown dimensions of mature plants, especially height & horizontal diameter, are important attributes, as well. I prefer to state them in terms of feet and fractions of feet.

6) Root forms are descriptive, and often difficult to determine because they grow underground and disturbing them alters their shape. Examples of root form include fibrous roots, tap roots and rhizomes.

7) Root dimensions (height and width), also difficult to measure, are also important for design. I prefer to describe them in feet.

8) Soil pH tolerances are important.

9) Soil texture tolerances are also important. Examples include sandy, silty or clayey.

10) Soil drainage tolerances are important.

11) Food benefits, for example, berries, pomes or vegetables, are an important property.

12) Ecological benefits are also important. Examples of ecological benefits include promoting pest enemies or improving soil (by fixing nitrogen, uplifting nutrients, increasing SOM or promoting aggregation).

2.5. Identify a location for the polyculture

Identify a location for the polyculture, preferably in a patch within a food forest and draw it on the map.

2.6. Assess the location

Assess the location in terms of factors for plant growth. Consider the requirements of the mature plant. Relevant conditions include soil type, sun exposure, moisture, temperature profile, wind exposure, nutrient profile, etc. A soil test is invaluable.

2.7. Select a plant species

From the plant list, select a species for the first plant. I prefer to designate an anchor plant. The anchor plant will be the largest-sized plant in the polyculture so it will create more shade than the others. Also,the root system of this plant will play a large role in the soil ecology of the polyculture and will determine the ability of other species to flourish.

Depending on your objectives, some anchor selections might bear fruit, while others might contribute to soil nitrogen fixation or some other benefit. Choose a species whose needs match conditions of the location. For example, a goumi shrub can yield food and help fix nitrogen into the soil.

However, one can select the first plant species based on any preference that makes sense rather than identifying an anchor plant.

2.8. Estimate the form, height and diameter of the mature plant.

2.9. Assess suitability

Assess the suitability of the location in reference to the specific requirements of the mature anchor plant.

2.10. Assess potential impacts

Determine if and how the full-grown plant will impact other existing plants, landscape elements, buildings and infrastructure. Are effects acceptable?

2.11. Choose the center placement of the plant.

2.12. Draw center of plant on map

On the map, identify the location of the plant’s center and the estimated horizontal outline of the mature crown.

2.13. Select second plant species

Select the second plant species, based on your polyculture goals. For example, rhubarb produces food and will benefit from the nitrogen of the goumi shrub.

2.14. Repeat steps 9-13

Repeat steps 9-13. Consider root properties as well as crown properties and rule out potential conflicts.

2.15. Choose the third plant species

Choose the third plant species based on your objectives. For example, you might pick strawberry plants, which go well with rhubarb in pie and expand rapidly under the right conditions.

2.16. Repeat steps 9-13.

2.17. Consider a fourth selection

Consider a fourth selection if room allows. For example, a perennial allium like elephant garlic grows well with rhubarb, strawberry and goumi plants, and it draws earthworms, for an immense ecological benefit.

2.18. Select a perennial ground cover

If selecting plants that grow or spread slowly so that bare ground will be present, select a species or two for perennial cover plants. The polyculture plants will eventually supplant the cover plants. For example, clover and blue fescue can hold a place against weeds and build soil while the polyculture plants establish themselves.

Section 3. Working with Succession and Disturbance

Species that have evolved predominantly in late-successional forested ecosystems will be shade-tolerant. Generally, they will prefer moist soil that is high in organic matter. These species will struggle on unsheltered sites that have been severely disturbed.

For challenging sites, starting with pioneer species will be preferable, since they thrive in full sun and harsh soil conditions. After a few years, as anchor species grow and provide shade and contribute to a reserve of soil organic matter, the gardener can replace pioneer species with shade-loving plants accustomed to late-successional conditions.


In addition to the benefits I have described, perennial polycultures can add significant value to a small farm. Importantly, diverse perennial plant selections and harvests can prevent the calamity of a single-crop failure, and they can offset harvest for a more continuous harvest and market offering.

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