Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted April 1997
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 lawn area what we want is essentially a large area monoculture (only a few grass species and varieties) that we cannot till or mulch to control weed development. There are several weed killers that can be sprayed on lawns or broadcast with weed-and-feed fertilizers. Each chemical has a place in lawn care and may be essential to correct a major problem, but before using them we should take the time to determine why the weed problem exists. Turf grasses tend to be very good competitors when given a proper growing environment. What is wrong with the environment that allows the weeds to grow in the lawn? Often the weeds themselves will indicate the problem and guide the correction process.
Lawn areas with sparse grass and moss growing on the soil surface may indicate several environmental problems. Heavy shade in the area may be weakening the grass and providing a more desirable situation for the moss growth. If shade is not the problem, moss often is a first indicator that the soil is infertile and too acidic for the grass species. Poor drainage, possibly due to soil compaction, also favors the moss.
Chickweed, ground ivy, and violets are other plants that tolerate areas too shady for good grass growth. When the soil also contains excessive moisture, heal-all and horsetail are common indicators of poor drainage and poor aeration. Without the heavy shade, nutsedge (a grass-like plant but with triangular stem) indicates a high moisture level. On the other hand, pussy-toes and yarrow indicate the soil is too dry for the grass and of low fertility. The same is true for red sorrel, which also indicates very acidic soil. At moderate moisture levels, very infertile and acidic soils are indicated by orange hawkweed and oxeye daisy, especially when both are present together.
Some lawns weeds are not indicator plants but are only opportunists. They grow in a wide range of environments; all they need is a space to germinate in a weakened turf. Dandelion is a prime example of this type of weed.
Turf specialists always say that the best weed control method for a lawn is to keep the grass healthy. This means to maintain an environment that favors turf competition against the weed plants.
With the correct grass type for the area under proper soil conditioning, fertilization, watering, and mowing, the lawn environment will be unsuitable for the indicator plants and the community of grass plants will be dense enough to exclude opportunistic weeds. If weeds are present in the lawn, use them to "read" the lawn environment before reaching for the weed killer.
(Adapted from "What Plants Can Tell Us," by Alan R. McDaniel, Extension Vocational Horticulturist, in The Virginia Gardener Newsletter, Volume 3, Number 8.)
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