Killing weeds

Contact herbicides cause cell death only where the chemical is applied.

As a teenager, weed control around the yard and farm was my responsibility. To save time, I used broad-spectrum herbicides to reduce the amount of work I had to do with the string trimmer. Like many habitual herbicide users, I thought that if a little herbicide was good, a lot was better; therefore I would double the recommended amount of the active ingredient, expecting it to work better.

Many homeowners, landscapers, and farmers use this same approach to manage weeds, which ends up wasting money, negatively impacting the environment and ultimately breaking the law.

All chemical companies are required by law to provide a label for their products. The label outlines the approved uses for the product, including research-based recommended rates and the best application techniques to achieve the desired results, as well as the environmental, health and safety precautions.

This process often takes millions of dollars and years of research to complete. Once the product has been approved for use, the declarative statements on the label become public record of the law, and it becomes the responsibility of the consumer to read and follow the label instructions.

Herbicides are classified by either pre-emergent or post-emergent activity.

Pre-emergent herbicides are applied before weeds germinate and control weeds when they first begin to sprout. These pre-emergent herbicides must get at least a half inch of rain or irrigation water to penetrate the soil profile. They are active in the soil for a short period of time before they degrade and are no longer able to control germinating weeds; therefore timing is of the essence when applying pre-emergent herbicides. These herbicides are nonselective, which means that they will control most germinating seeds, but typically work best on grasses.

Post-emergent herbicides are effective once weeds have germinated, therefore are able to control actively growing weeds. They work in one of two ways, either systemically or on contact.

Glyphosate is one of the most common active ingredients in herbicides on the market, and is sold under several trade names. It is absorbed into the leaves of the plant and is translocated throughout the plant's vascular system.

Other active ingredients such as diquat are considered contact herbicides, and work by injuring vegetative materials on contact. It is very important to know the active ingredient of an herbicide as well as the mode of action before choosing a product.

For any herbicide to work optimally, good coverage is essential. A light mist of tiny, individual droplets will remain in contact with leaf surfaces longer, allowing the active ingredient to be better absorbed.

A great analogy is to consider the way dew droplets cling to the surface of a waxed car. Actually, the fastest way to dry the car would be to spray the car with more water to remove the droplets. The sheeting action of the water from the hose forces the tiny droplets from the dew to stick together and run off.

Waxy leaf surfaces will react the same way when too much herbicide is applied. When so much herbicide is applied that it runs off the leaf surface, product is wasted and may prevent good control. Mixing products and applying them at labeled rates will control weeds faster and is more cost effective than following the "more is better" philosophy.

Below are a few tips to get the most efficient use out of the herbicides:

Correctly identifying the weed and its life cycle will help determine the best herbicide to use. Consult the Clemson University Home & Garden Center for factsheets on many of the common landscape and garden weeds and the recommended herbicides.

Read the product labels and remember the label is the law.

Follow recommended rates when mixing and apply only until the plant is damp or lightly misted; do not spray until runoff occurs.

Spray weeds that are actively growing for better control.

If you still have questions, contact the Clemson Extension Master Gardener office for free weed identification, label interpretation, and non-chemical weed control strategies.

On Sept. 20 and 21, Magnolia Plantation and Gardens, along with Clemson Extension Tri-County Master Gardeners, will hold the seventh annual Autumn on the Ashley Open Air Craft Fair. Artists and demonstrators throughout the Lowcountry gather to share their work with visitors. The festival and parking is free to the public.

The Lowcountry Chapter of the S.C. Native Plant Society will hold a native plant sale at 9 a.m.-noon Sept. 27 in the parking lot of Charles Towne Landing, 1500 Old Towne Road, Charleston. Admission to the plant sale is free. The sale will include colorful perennials, trees, shrubs, ferns, native grasses and edibles. Native plant list and prices to be posted on the SCNPS website a few weeks before the sale. Cash, check or credit card accepted. Contact Colette DeGarady for more information:, 843-937-8807 ext.15 or visit our website at