Figure 1. Extensive colony of Canada goldenrod (Solidago canadensis).

 

When observing a meadow, a fallow field, or a forest edge, one might think that plants grow wherever they please. In reality, they follow well-established rules — social rules, even. Some keep to themselves, others gather in tight groups, while others take up all the space they can get.

This “social life” of plants was summarized by Hansen and Stahl in what they call the sociability index, introduced in 1993 in their publication Perennials and their Garden Habitats. A simple yet highly useful tool when creating a sustainable landscape, designing a seed mix, or simply understanding why some plants expand quickly.

What is the sociability index?

The idea is simple: every plant, in nature, has its own way of occupying space. Some individuals grow isolated (level I), others in small groups (level II), others in small patches (level III), in large patches (level IV), or even in extensive colonies (level V).

It is a valuable indicator that helps us understand how a plant will behave in a landscape. Will it stay where it is planted? Spread gently? Or assert itself and dominate the space?

Figure 2. The 5 sociability levels. (Hansen and Stahl, 1993).

Why do some plants grow alone while others grow in groups?

Several reasons have been identified to explain these behaviours.

Solitary plants (level I) are often more demanding species: they germinate under very specific conditions, have limited propagation capacity, or are easily dominated by neighbouring plants. They are beautiful and unique, but not the type to take over.

In contrast, plants in levels IV and V have very effective propagation mechanisms: spreading rhizomes, stolons, abundant seed production, or the ability to take advantage of bare soil after a disturbance. They colonize, expand, and eventually form wide patches. They are not “invasive” by default, but they are highly competitive.

Between these extremes, levels II and III represent “moderately social” plants: they group together, but without seeking to occupy all available land. They tolerate competition, but competition also keeps them in check. In many cases, these are the plants most useful for stabilizing a landscape.

Figure 3. Flowering meadow composed of species with different sociability levels.

Examples of native species in Quebec

To make the concept of sociability more concrete, here are a few native Quebec species and their typical behaviour.

• Level I: Cardinal flower (Lobelia cardinalis), Black cohosh (Actaea racemosa)

• Level II: Anise hyssop (Agastache foeniculum), Lance-leaved tickseed (Coreopsis lanceolata)

• Level III: Zigzag goldenrod (Solidago flexicaulis), Wild bergamot (Monarda fistulosa)

• Level IV: White panicled aster (Symphyotrichum lanceolatum), Canada wild rye (Elymus canadensis)

• Level V: Canada goldenrod (Solidago canadensis), Sallow sedge (Carex lurida)

How to use the sociability index to create an effective seed mix

This is where the index becomes a powerful tool. Creating a seed mix or a native planting is not only about aesthetics. It is also about the relationships between species.

A good strategy is to think in three layers:

1. The matrix (level II–III): These are the plants that do most of the work: occupying the soil, preventing weeds from entering, and stabilizing the whole system. This includes sedges, small grasses, asters, and more discreet goldenrods. They are the foundations of the landscape.

2. The accents (level I): These are solitary plants that do not take up much space but bring style, colour, and aesthetic diversity.

3. The service species (level IV–V): These are plants to use sparingly, but they can help stabilize a slope, a bank, or a ditch, or even compete with an invasive species. Everything depends on the goal of the project.

In summary

When used well, the sociability index makes it possible to create landscapes whose canopy closes quickly, that require almost no weeding once established, and that become self-balancing over time. When we better understand how plants behave, we stop placing them in conditions where they fail or become unmanageable. Instead of placing plants randomly, we create coherent, resilient, beautiful, and living communities. This is exactly what we observe in natural environments: a dynamic coexistence where every species has a role.

Sources

George, B. 2021. Perennial Plant Sociability Metrics: Tools to Interpret Designed Plant Communities for Public Gardens [bachelor’s project, Cornell University].

Hansen, R. and Stahl, F. 1993. Perennials and their Garden Habitats. Cambridge University Press.

Prairie Up. Design Workshop Pocket Guide. PDF.

Rainer, T. and West, C. 2015. Planting in a Post-Wild World: Designing Plant Communities for Resilient Landscapes. Timber Press.