Timing, tips on killing weeds Care needed when using herbicides

Poor application technique of herbicide resulted in damaged turf.

Recently, a study identified nature as a way of coping with stress. Hiking, gardening or regularly walking outdoors significantly improves our happiness.

This explains our investment in gardens.

But weeds can disrupt that respite from the daily grind. Pulling weeds isn’t always enough; sometimes other means of control are necessary. However, that doesn’t guarantee things will get better.

The Scotts Bonus S product was in the news recently for some unintended side effects after application to centipede lawns. Scotts is a nationwide company whose brands are marketed to the homeowner as easy to use and effective. Their products can be found in most large garden centers. Scotts Bonus S is popular because it is a weed-and-feed product.

Weed-and-feed products are a combination of fertilizer and herbicide. When the herbicide is a pre-emergent, it has to be applied before weed seeds germinate. In the Lowcountry, a pre-emergent herbicide should be applied on Valentine’s Day to control summer crabgrass.

One of the problems with this application is that time of the year is too early for nitrogen fertilizer. Nitrogen should not be applied until early May when turf is fully out of winter dormancy and the threat of late frost has fully passed.

A pre-emergent herbicide such as Crabgrass Preventer or Dimension should be applied on Valentine’s Day, not a weed-and-feed product. Scotts Bonus S, however, can be applied in early May for extended weed control.

Keep in mind, pre-emergent herbicides are not required. In fact, they can have a negative impact on turf, especially when overapplied. Healthy, dense lawns frequently outcompete most weeds.

If pre-emergent herbicide is deemed necessary, then it needs to be applied at the correct rate and time of the year. However, the problem many homeowners experienced with Scotts Bonus S this past spring was no fault of their own. Scotts Bonus S has inadvertently killed centipedegrass.

The culprit is the new active ingredient metsulfuron. In the past, atrazine was the active ingredient. It was safe to use on centipedegrass and St. Augustine, although it is toxic to bermudagrass. This had the advantage of controlling bermudagrass in centipede lawns, but granular atrazine can have unintended negative impacts. Because the chemical moves easily through the soil, atrazine can potentially contaminate groundwater as well as harm trees. This is especially true when applied beneath the canopy. Some tree species, in particular young ones, can be stunted or killed.

Metsulfuron, the new ingredient in Scotts Bonus S, was intended to replace atrazine and avoid these potential side effects. Unfortunately, centipedegrass did not tolerate the metsulfuron as was expected. However, Scotts has recognized this result and will reimburse homeowners for the cost of resodding a centipede lawn damaged by Scotts Bonus S.

Homeowners need to supply a receipt of purchase, two sodding bids from landscape contractors, and a photo of the damaged lawn. Customer service should be contacted first at 877-309-9560.

Some landscape contractors have resodded with a different species to avoid any potential of residual harm. Alternatives include zoysia where the lawn gets more than five hours of sunlight. Less than that may require St. Augustine, but a landscape contractor should be consulted.

There are many natural herbicides that are safe to use in the garden, but they are primarily nonselective weed killers that are good for use in planting beds.

Herbicidal soaps are potassium salts of fatty acids and approved for organic gardening. Vinegar can be used as an herbicide, but not household vinegar.

Herbicidal vinegar contains 20 percent acetic acid that can burn skin and injure eyes, so be cautious.

Essential oil herbicides contain eugenol that disrupts cell membranes, but the dead weeds smell delicious.

Corn gluten is an organic fertilizer with reported pre-emergent activity. The herbicidal results are not as effective as synthetic products. It should be used primarily as a nitrogen fertilizer and any weed control considered a bonus.

Tony Bertauski is a horticulture instructor at Trident Technical College. To give feedback, email him at tony.bertauski@tridenttech.edu.