Weeds are plants—sometimes, plants with value—that are in the wrong place. In a natural, uncultivated setting there are no weeds. Ralph Waldo Emerson once wrote that a weed is “ a plant whose virtues have not yet been discovered.” The natural tendency of plants is to use every available space on the soil’s surface. If sun, water, and nutrients are available in a space where little or no competition for those resources exists, then there will soon be a thriving plant. Nature provides this opportunity to protect the soil from erosion and to provide the cooling and moisture preserving effects of shade. Many so-called weeds are cultivated and sold as valuable medicinal herbs or ornamental plants. An expensive installation of a ground cover that creeps into a lawn becomes a weed at the garden’s border. Perennials that propagate underground and spread into turf become weeds no matter how rare or beautiful they may be.
What makes a plant a weed is the intolerance of the turf manager. Most weeds cannot survive constant and low mowing so there are relatively few that invade turf. Zero tolerance on weeds is a tough stance even with an arsenal of herbicides. Part of developing an ecological maintenance program may be learning to live with a few weeds.
Here are some conditions that contribute to turf weed infestations.
Herbicides may eradicate most weeds, but they do not change the conditions that gave weeds their competitive edge. Additionally, some herbicides can increase pest insect activity by suppressing predators, stressing plants, or both. Research from Cornell University shows that applications of certain herbicides can also increase the severity of some disease symptoms.
Most weeds play an important role in the broad scheme of things. Dandelions, for example, are very beneficial plants. Their deep roots return leached nutrients to the surface and are large producers of organic matter. Earthworm populations thrive in their vicinity and the plant does not compete with turf for consequential amounts of nutrients, water, or light. Clover is another beneficial plant that has gained undeserved notoriety. Clover is a legume that can fix free nitrogen from the atmosphere and share it with turfgrass. Clover roots are extensive and contribute a significant amount of resources to soil organisms. These plants are also extremely drought resistant and can stay green long after turfgrass has gone dormant.
Often, effective weed control in turf is as simple as raising the height of cut. This action enables the turf to be more competitive by using the sun’s energy for itself and blocking it from fueling unwanted species of plants. For every eighth of an inch that a mower is raised, there is a 30 percent increase in leaf surface area. That increase causes a relative increase in photosynthesis, which feeds a larger and healthier root system and a larger, healthier population of organisms in the rhizosphere. The lateral roots (rhizomes) of some turf varieties often develop new plants that further thicken the stand, providing even more competition. Spring is an important time to mow higher, as the plant is in its reproductive stage, creating new tillers, stolons and rhizomes. Raising the height of cut in the spring can control many weeds all season long. Turf roots can produce allelopathic chemicals capable of suppressing weed seed germination. The greater the root system, the more allelochemicals can be produced. Additionally, the lush growth typically observed in spring is the plants way of processing nutrients into proteins, carbohydrates, and other compounds via photosynthesis. Mowing at an extremely low height of cut (HOC) inhibits this process and can lead to a host of other problems.