Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted November 1997
When something is not right in a plant's growing environment, the plant usually will tell us by displaying some sign that can help determine what the problem is. For example, drooping leaves and a grayish coloration might indicate a drought condition; lower leaves of a garden plant that yellow and drop off suggest a nitrogen deficiency; and leaves that are mottled, speckled, or crinkled may be the result of a virus infection. These signs, or symptoms, can be very helpful in diagnosing plant ailments due to insects, diseases, and nutritional or other cultural disorders.
Healthy plants provide information about the environment as well. When a plant is adapted to a particular environment, it will grow well and reproduce successfully, thereby creating a community of that species. If the plant is not adapted, young plants will grow poorly and will succumb to competition from the better adapted plants nearby. Several factors are involved in this adaptation, including light (too little or too much), water (too little or too much), temperature (winter and summer), soil (type, depth, pH, organic and nutrient content), and periodic damaging agents (pests, fire, flood, wind).
In nature, plant communities are not of individual species but include several species that are adapted to the environment of the area. Agriculture including home gardening and landscaping changes the plant community from what would normally exist in an area. Plants that could not grow before due to competition from the original vegetation may now germinate and develop. If the new plants are of the crop we want to grow, all is fine, but we know that most of the plants that germinate will be "weeds."
Many plants become weeds because they are heavy competitors with crop plants. They can grow faster and are very effective in their reproduction. Classic examples of this are purslane, lambsquarter, and common dock in the garden. These plants were probably few or absent in the original vegetation, but when the garden soil was first tilled, they spread rapidly but only to the edge of the garden. These plants can compete with other plants in the "disturbed" soil of the garden, but they cannot successfully invade the plant community in the surrounding "undisturbed" soil. In this way the dock, lambsquarter, and purslane are indicators of a certain environment (a disturbed soil area) that is different from the environment of nearby areas.
This example of what we would call "indicator plants" may seem a little foolish since a garden soil is obviously disturbed by the tillage. However, if we should stop using the garden site for several years, the initial change in the soil environment that allowed establishment of these plants would still have some influence, allowing them to persist. As the soil assumes a less disturbed state, other plants would become established and compete successfully. The succulent purslane probably would be the first to die out; next would be the lambsquarter, especially if the area is mowed or grazed. The dock, however, could hold its position successfully for many years due to its low growth habit and perennial life cycle. It would be the longest term indicator that the soil was disturbed.
Using indicator plants to "read" the environment is a common practice, whether or not we recognize we are doing it. Probably the most common use of an indicator plant is with "moss grows on the north side of a tree" as this shaded side of the tree trunk is cooler and moister - the kind of environment needed for survival of the moss.
In the home garden and landscape our hands, a hoe, and mulches are our principal methods of dealing with weeds. This can be successful because the garden and flower and shrub beds are smaller and usually not a monoculture. However, where moss, dandelions, or ground ivy invade the garden, soil pH and fertility may not be optimal and the soil should be tested.
(Originally published as "What Plants Can Tell Us," by Alan R. McDaniel, Extension Vocational Horticulturist, Virginia Tech, in The Virginia Gardener Newsletter, Volume 3, Number 8.)