Indicator plants: They're the coal-mine canaries of the garden, plants whose presence and condition give at-a-glance clues to specific soil or site conditions.
They let gardeners know whether or not to "sweeten" the soil with lime for the peppers, to add fertilizer for the cukes, to add organics and even sand to improve tilth and drainage for the perennials or to give up trying to grow grass by the too-shady north fence.
Though our rural ancestors might have been able to read them, our predominantly urban selves, for whom street sense seems more immediately crucial than nature sense, usually pass by these little botanical signposts unaware. But farmers still scan them regularly for clues to what the land and its crops need.
"Using indicator plants to 'read' the environment is a common practice," writes Diane Relf, extension specialist in environmental horticulture for the Virginia Extension Service at Virginia Polytechnic Institute and State University in Blacksburg.
"Sheep sorrel [Rumex acetosella] tells you the ground is acid," says Earl Chance of Georgetown, who has farmed in Kent County for seven decades. "You need to add lime."
Of course, even pavement-hardened urbanites know that moss grows on the north side of a tree, which provides the shady, moister environment moss needs. But few also know that moss on the ground hints that the soil there may have "issues."
"In general, mosses do better in acidic soil and poorly drained or compacted soil," says Dave Clement, director of the University of Maryland Cooperative Extension Service Center and regional specialist in plant pathology. Mosses often indicate lack of fertility, too.
Trying to plant a lawn? Bear in mind that chickweed (Stellaria media), ground ivy (Glechoma hederacea) and violets (Viola) spring up in areas usually too shady for good turf growth. Self-heal, also known as heal-all or woundwort (Prunella vulgaris), and horsetail (Equisetum arvense) are indicators of poor drainage and poor aeration. On the other hand, pussytoes (Antennaria dioica) and yarrow (Achillea) indicate the soil is often dry and probably none too fertile.
"At moderate moisture levels, very infertile and acidic soils are [also] indicated by orange hawkweed [Hieracium aurantiacum] and oxeye daisy [Chrysanthemum leucanthemum] especially when both are present together," Relf writes.
In addition to hinting at soil conditions, some plants can be used as early-warning systems.
"Petunias and broad beans give you a quick indication of INSV [impatiens necrotic spot virus] and TSWV [tomato spotted wilt virus]," says Clement. "It takes other plants longer to develop symptoms, whereas these plants will develop a necrotic lesion right away."
Petunias show distinctive small brown to black spots on their leaves when infective thrips feed on them. Noninfective thrips leave whitish feeding scars. Other plants act just like the canaries that keeled over in mines just early enough for the miners to scramble out alive. For example, most lichens are particularly sensitive to environmental pollution, and turn up their toes when there's too much sulfur dioxide in the air.
Some plants can indicate the history of a piece of ground.
"Vinca or wild onions growing in the woods is a good indicator that the place has been grazed in the last 100 years," says Jonathan McKnight, associate director of habitat conservation for the Maryland Department of Natural Resources in Annapolis. "And huge dying Virginia pines almost always tell you you're on an old field. So when you find a tulip poplar woods with a few ancient loblollies in it, you can guess it was a tobacco farm years ago."
Plants can have legal impact, too. The Maryland Department of Environment uses a range of wetland indicator plants to identify wetlands for protection. Among them are common marsh marigold (Caltha palustris), swamp mallow (Hibiscus moschuetos) and green bulrush.
The type of vegetation in an area also may signal groundwater proximity. For example, some plants (called phreatophytes) have long taproots and can extract water directly from an underlying aquifer. They include salt cedar (Tamarisk), willow (Salix), salt grass (Distichlis spicata), cottonwood (Populus deltoides) and cloverlike alfalfa (Medicago sativa).
The military's "Multi-Service Procedures for Well Drilling Operations" manual (Field Manual 5-484) discusses using vegetation in combat areas to find potential potable groundwater sources.
And finally, plant distribution, appearance and growth anomalies can help in prospecting for ore deposits (called geo-botanical prospecting) while biogeochemical prospecting analyzes a plant's chemical content for clues to the minerals in the ground on which it lives. For example, some wild asters readily suck up selenium, so testing the plants helps to gauge selenium levels in soils.
Maryland Cooperative Extension
University of MD
www.hgic.umd.edu / pubs / online / misc1.pdf
Maryland Department of Natural Resources
580 Taylor Ave.
Annapolis, MD 21401
Environmental Concern Inc.
P.O. Box P
201 Boundary Lane
St. Michaels, MD 21663
12610 Eveland Road
Ridgely, MD 21660
Chesapeake Natives Nursery
Sara A. Tangren
326 Boyd Ave. #2
Takoma Park, MD 20912