Crossville Chronicle, Crossville, TN

March 25, 2013

Plateau Gardening: Choosing the right weed killer

By C. Rae Hozer
Chronicle contributor

CROSSVILLE — Successful weed control hinges on choosing the right alternative for the particular plants you wish to eliminate and for conditions when and where the herbicide is to be applied.

Tennessee’s lingering cold, snow flurries, rainy and cloudy weather have stalled the typical growth spurt we expect by spring’s onset (March 20). Certain herbicides work best when plants are growing vigorously and temperatures are at least 60 degrees Fahrenheit but not over 85 degrees F. They may be less effective this year than if late-winter weather had been warmer and drier. Be sure to read the product label.

Note for gardeners who use household vinegar (5 percent acetic acid with 95 percent water) as an organic weed control and don’t have an herbicide label: Vinegar works by drying out plant tissues. Online advice suggests temps should be above 60 degrees F and that the weeds should be growing in sunlight. If conditions are not right, vinegar may not be effective.   

Know the weed types you are dealing with and their life cycles. It helps pinpoint the time of year and during which life cycle stage that nuisance variety is most vulnerable. Landscape weeds can be divided into three types: broadleaf, grasses and sedges.

Within each category weeds have one of three basic life cycles: summer annual (heat-loving, the seed-to-plant-to-seed life span lasts 12 months or less, germinates in springtime), winter annual (cold-tolerant, the seed-to-plant-to-seed lifespan lasts 12 months or less spanning 2 calendar years, germinates in late summer or in autumn) or perennial (roots and sometimes the entire plant is winter hardy, these plants persist more than two years).

There are a number of herbicide options to consider. "Post-emergence" weed control agents are applied to plant parts above ground. Some post-emergence herbicides kill top growth on contact (contact herbicides). Others (systemic herbicides) move into and within the plant to affect below ground and interior plant parts and processes. Post-emergence herbicides are most effective when used to attack unwanted plants that are little, young and not stressed by heat or cold. The objective is to get rid of weeds before they mature and produce seeds or other persistent reproductive structures. Weeds typically produce lots of seeds. Those seeds can survive most post-emergence control methods lethal to growing plants.

"Pre-emergence" herbicides affect germinating seeds. They are spread on the soil. Dry granule forms are typically activated by watering afterwards. Pre-emergence controls should be in place two or three weeks before conditions are right for seeds of the targeted species to sprout. Springtime pre-emergence herbicides are often used against crabgrass. Yellow forsythia bushes in bloom signal soil temperatures are right for grass seed germination. Corn gluten meal is an organic "weed and feed" pre-emergence product that may be used to treat crabgrass, creeping bentgrass, smart weed, dandelions, purslane, lambs quarter, barnyard grass and Bermuda grass in cool season lawns. Corn gluten keeps sprouting seeds from forming normal roots. Treated seedlings die from dehydration when soil gets dry.

Some herbicides are "non-specific" or "nonselective." They kill all vegetation they contact (desirable and undesirable, broadleaf or grass). "Specific" or "selective" herbicides target certain plant species or particular plant categories (broadleaf or grass). However, while broadleaf-specific herbicides containing 2, 4-D and dicamba won’t hurt established turf they may kill tender, young grass seedlings. Always read label directions before purchasing. When mixing and applying, follow directions precisely.

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Plateau Gardening is written by Master Gardeners for gardeners in Tennessee’s Upper Cumberland Region.  Contact UT Extension Cumberland County at P.O. Box 483, Crossville, TN 38557 (484-6743) for answers to horticulture questions, free publications and how to become a Master Gardener. Send email comments or yard and garden inquiries to Master Gardener Rae, MGardenerRae@frontiernet.net.