The biggest epiphany of my horticultural career was learning about plant survival strategies. Like most gardeners, I was accustomed to classifying plants as annuals, biennials, or perennials. But dividing them into groups based on their survival strategies instead got me thinking about how they evolved to grow, which in turn helped me to cultivate them more successfully in my garden in Texas.
According to ecologist J. Philip Grime, who researched these strategies, plants have evolved certain approaches to deal with two lethal conditions in their environment: stress and disturbance.
Stress is any environmental factor that reduces plant vigor or growth. Drought, waterlogged soil, heat, cold, nutrient toxicities or deficiencies, and low or high light levels are all stressful growing conditions for plants. To continue living under these conditions, a plant would have to endure them, and if the situation persisted for too long, the plant could die.
Disturbance is any factor that damages or destroys plant tissues above or below ground. A plant can lose shoots and roots due to fire, wind, floods, herbivory, or trampling feet, like those of the wild boar that occasionally come through my backyard. Human effects from soil cultivation and mowing also count as disturbance. Losing foliage or roots in such an event would be a major setback for any plant.
| KEY CONCEPT |
Garden with survival strategies in mind
Many gardeners would like to welcome more plants into their gardens, but not everything thrives on rich soil and ample moisture. Though it may seem counterintuitive, stress and disturbance can actually be helpful tools for diversifying our gardens.
Research shows that as conditions shift from favoring competitors to being more stressful for stress-tolerators or more disturbed for ruderals, the number of different species that can grow on a site actually goes up. There is a sweet spot where growing conditions are tampered or disturbed enough to reduce the growth of their competitors, which allows stress-tolerators and ruderals to grow without the competitors completely taking over.
We can also be strategic about combining plants that have different survival strategies. If too many competitors are present, they will try to dominate each other. An abundance of stress-tolerators or ruderals will result in gaps and holes when those plants go dormant or go to seed. We need a healthy blend, similar to what occurs in wild ecosystems. For example, in early spring before competitors grow tall, it might help to have a ruderal ground cover or stress-tolerator foliage to help cover the soil’s surface and prevent weeds from germinating. In a bad year, when fewer ruderals are able to germinate, dependable stress-tolerators and competitors can fill in the gaps.
It’s good to have another tool to help us manage our gardens better. Instead of letting our designs become stagnant and repetitive, we can disturb our plantings to provide spots for new plants. We can cut back plants for rejuvenation and to create gaps for dormant species to appear too.
To encourage more plant diversity in your garden, try causing a little stress and disturbance
In the wild, increasing stress and disturbance increases plant diversity, until the stress or disturbance becomes too great and reduces the number of species that can survive in a particular location.
Here are a few strategies for creating an environment that welcomes a broader plant palette.
1. Water less and fertilize less. In a world in which water restrictions abound and fertilizer is expensive, conserving these resources can be a welcome goal.
2. Plant in a 4- to 6-inch layer of gravel or sand. Coarse grit that is low in fertility and that has few fine particles reduces weed growth and creates conditions that certain stress-tolerators need to thrive.
3. Practice shallow cultivation. You can reduce weed pressure and too many ruderals by shallowly cultivating the top few inches of soil. This disturbance helps deplete the weed seed bank.
4. Become the bison and disturb the plants. It’s OK to go into a garden bed and break up the edges of some competitors to slow their spread. This will open up new space for ruderals to pop up.
5. Try some late-spring cutbacks. Chopping back vigorous asters (Symphyotrichum spp. and cvs., Zones 4–8) and perennial sunflowers (Helianthus spp. and cvs., Zones 4–9) in spring will keep their competitive growth in check.
As you experiment with these strategies, be patient. Often with reduced fertility and water plants will grow more slowly. Prepare yourself mentally to see some reduced vigor, and be open to noticing what happens next.
| BASICS |
Three survival strategies
Plants can’t move out of the way of danger, so over countless generations they have evolved traits to help them survive environmental stresses and disturbances. It is fascinating to see how plant species around the world have responded in similar ways to these factors. Based on their response to stress and disturbance, we can classify plants into three broad categories: competitors, stress-tolerators, and ruderals.
PLANTS THAT ARE COMPETITORS
Rule when stress and disturbance are low
• Has a tall, wide, or large-leafed habit
• Grows rapidly upward or outward
• Forms large, expanding clumps
• Spreads by rhizomes
A plant that has the ability to grow tall like Joe Pye weed (Eutrochium purpureum, Zones 4–9), to grow wide like obedient plant (Physostegia virginiana, Zones 3–9), or to produce large leaves like hosta (Hosta spp. and cvs., Zones 3–9) will often be able to shade out the other plants around it. Competitors thrive in ideal conditions. Many of them grow in large clumps and have roots or rhizomes that prevent nearby plants from getting water, light, or nutrients. They are very sensitive to stress and disturbance, but as long as those are minimal, these plants will dominate in the ecosystem and in our gardens. Just think of how quickly mint (Mentha spp. and cvs., Zones 4–8) takes over a garden bed, or how aggressive rough goldenrod (Solidago rugosa, Zones 4–9) is in a field.
PLANTS THAT ARE STRESS-TOLERATORS
Take tough growing conditions in stride
• Often takes multiple years (three or more) from seed to flowering
• Forms some type of storage organ,
• May store water and resources in stems or leaves
• Can go dormant for several months of the year
• May have dense, branching growth with small leaves
|Plants that originated in stressful habitats with limited water, light, or nutrients have adaptations that help them survive when times are tough. Often their growth rate is slow, and it may take these plants years to go from seed to flower, as is the case with daffodils (Narcissus spp. and cvs., Zones 3–9) or trillium (Trillium spp. and cvs., Zones 4–9). Such a plant survives by storing nutrients and water in its tissues and may form some type of storage organ like a bulb or a corm that can go dormant when conditions are too harsh. It will then start to grow again when conditions improve the following season. Other stress-tolerators, such as heath (Erica spp. and cvs., Zones 5–8), exhibit dense, shrubby growth, an adaptation that reduces their exposure to taxing environmental conditions.
PLANTS THAT ARE RUDERALS
Pop up when the odds are favorable for growth
• Is short-lived, often annual or biennial
• Produces copious amounts of flowers and seed
• Frequently blooms en masse in a garden setting
• Self-sows or volunteers in beds
|With the introduction of disturbance, ruderals begin to dominate. The word “ruderal” comes from the Latin rudus, meaning “rubble,” and these plants are often found in areas of heavy disturbance, such as mudslides, cliff faces, roadside grit, or sandbars. Ruderals are short-lived and produce copious amounts of seed, because their environment is so unstable that they can’t count on returning year after year. Their mode of survival is to produce plenty of seed and pop up when conditions are good, as do the superblooms of California poppy (Eschscholzia californica, annual) or the glorious spring displays of Texas bluebonnet (Lupinus texensis, annual). Sometimes ruderals such as spider flower (Cleome hassleriana, annual) become too plentiful in the garden because of their heavy seed production. Many ruderals are annuals, but short-lived perennials like blanket flower (Gaillardia spp. and cvs., Zones 3–10) also fall into this category.
| HOW TO |
Use Grime’s Triangle as a Sorting Tool
How do you know if a given plant is a competitor, a stress-tolerator, or a ruderal? One of the fun activities I do with my students is to play “pin the plant on the triangle.”
On the chalkboard, I draw a diagram known as Grime’s triangle, labeling the three corners of the triangle with the three survival strategies. One side of the triangle represents increasing stress, and the other side represents increasing disturbance.
Students then research the characteristics of various plants and use this information to plot where each plant occurs on the triangle. Each of the three categories has a trade-off. Many competitors don’t tolerate stress or disturbance very well. Stress-tolerators may not be as successful when resources are plentiful and stress is low. And if competitors and stress-tolerators claim most of the available resources, ruderals may not have a chance to emerge.
Often a species isn’t purely one category but a blend of two, or even all three. Butterfly weed (Asclepias tuberosa, Zones 3–9), for example, has a storage organ root like a stress-tolerator but produces copious seed and flowers within a year or two of germination like a ruderal. Trees and shrubs generally tend to be on the competitor and stress-tolerator sides of the triangle.
Jared Barnes, Ph.D., is an associate professor of horticulture at Stephen F. Austin State University in Nacogdoches, Texas.