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A Guide for Homeowners in the Piedmont Region of NC
Weed control is an essential part of gardening. Weeds reduce aesthetic value of lawns and landscapes. In addition, weeds compete with desired plantings for nutrients, water, and sunlight, and can harbor pests, such as insects, disease-causing pathogens, and animals. Some weeds, such as poison ivy, pose health and safety concerns. Weeds should be managed using an integrated approach. Eradication is an unrealistic goal, but weeds can be controlled to an acceptable level. Integrated weed management includes several means of weed control. These may include preventative measures, assessment of the weed populations and the necessity for taking action to control weeds, cultural control measures, and judicious use of herbicides.
Weed Identification
Properly identifying weeds allows gardeners to choose more effective means of controlling them. Plants are easiest to identify when they are in flower. However, preventing weeds from flowering is key to managing weed populations in the landscape. Learning to recognize the vegetative characteristics and unique characteristics of weeds allows for more effective control. Weeds can be separated into two broad categories. Monocots are plants whose seedlings have one cotyledon (seed leaf). Typically, these plants are recognizable by their long, narrow leaf blades with parallel veins. Grasses, rushes, and sedges are all examples of monocots. Dicots are plants whose seedlings have two cotyledons. Dicot weeds are often referred to as broadleaf weeds. Broadleaf weeds are highly variable, but generally can be identified by leaves that are broad with netted veins.
General Weed Prevention in Landscape Beds
Sanitation and prevention are important for controlling weeds in landscape beds. As much as possible, gardeners should use soil, amendments, and plant material free of weeds and weed seed. Mulching landscape beds with two to three inches of organic mulch inhibits the germination of annual weeds and is an excellent way to reduce weed pressure. Proper spacing of desired plants to fill the garden bed will provide competition for sunlight and can help reduce weed pressure. Weeds that do grow in the landscape should be removed before flowering. When pulling weeds by hand, gardeners should try to remove the roots as much as possible. Light scraping of the soil surface with a hoe is a good method of control for seedling weeds. Germination of weeds is often triggered by soil disturbance. Renovation and other activity in garden beds can lead to the germination of weeds. Gardeners may choose to till the soil several weeks before planting to allow weed seeds to germinate. Weeds should be killed or removed before planting. Chemical control of weeds in landscape beds can be more difficult than in turf, as most landscape plants are dicots and will be damaged by herbicides that target broadleaf weeds.
Herbicides
The use of chemicals for weed control should always be the last resort in an integrated weed management plan. Herbicides can be classified by their mode of action and how they are used. When using herbicides, always read and follow the label instructions. Be sure to wear the proper personal protective equipment. For a list of herbicides approved for use in the landscape, see the North Carolina Agricultural Chemicals Manual. Homeowners may prefer to hire a licensed professional to perform pesticide applications.
Pre-emergent herbicides prevent germination of seeds. Pre-emergent herbicides are not selective and will also kill seeds of grasses or desired plants in the landscape. Pre-emergent herbicides are most effective on shallow-seeded plants and do not kill existing plants or dormant seeds. Pre-emergents applied in late summer and early fall control winter annual weeds, while applications in early spring control summer annual weeds. Most pre-emergent herbicides require rainfall or irrigation to activate the herbicide.
Post-emergent herbicides are applied to the foliage of weeds that have already germinated. Post-emergent herbicides can be contact or systemic herbicides. Contact herbicides kill only the part of the plant that comes into contact with the herbicide. Adequate spray coverage is necessary for controlling weeds with a contact herbicide. Systemic herbicides are absorbed through the foliage of a plant and are translocated through the vascular system of plants. Thus, systemic herbicides can also kill the underground portion of plants. The majority of post-emergent herbicides are systemic herbicides.
Post-emergent herbicides can also be classified by their selectivity.
Selective herbicides control certain plant species without serious impact on the growth of others. For instance, some herbicides can be used in lawns to control broadleaf weeds without harming the grass. Other herbicides do the opposite and can be used to control grassy weeds in flower beds.
Nonselective herbicides damage all plant species. These should be used to control all plants in a certain area. Nonselective herbicides can be used to spot-treat weeds, but gardeners should exercise care to avoid damaging non-target plants.