LSU AgCenter's Weekly Message

Make Landscape Bed Weed Management Easier for Yourself

Mulching is one of the best tools in the toolbox for managing weeds in landscape beds. It helps block light, which is needed for germination of some seeds and growth of plants, from reaching the soil surface.

A number of different materials can be used for mulch, such as pine bark, leaf litter, and compost, as well as pine straw. Spread mulch to a depth of about three to four inches.

We sometimes see mulch piled up at the bases of tree trunks (“volcano mulching”), but this is not recommended. Mulch piled against a trunk can keep it moist and contribute to plant disease problems. Avoid placing mulch immediately next to plants.

Natural mulch will decompose over time, so you’ll likely need to refresh it each year, or more frequently, to maintain a depth that will block sunlight from reaching the soil surface and adequately suppress weed growth.

I occasionally get questions about using herbicides in landscape beds. We do have some options but need to be careful to avoid injuring the plants that we want to keep.

Because most landscape ornamentals are broadleaf plants rather than grasses, if your weeds are true grasses, that makes things easier. Some herbicides containing sethoxydim (e.g., Bonide GrassBeater II, Fertilome Over-The-Top II Grass Killer, Hi-Yield Grass Killer, Monterey Grass Getter, Segment II), clethodim, fluazifop-p-butyl (e.g., Fusilade II, Ornamec 170 Grass Herbicide, Ortho Grass B Gon Garden Grass Killer), or fenoxaprop can be used around and over the top of many non-grass ornamental plants.

Where sedges are a problem, some halosulfuron-methyl and imazaquin products can be used around certain ornamentals.

There are herbicides that can be used to kill already-germinated weeds growing along the edges of landscape beds. Some with the active ingredient glufosinate ammonium (e.g., Bonide KleenUp High Efficiency Weed & Grass Killer Concentrate, BioAdvanced Weed and Grass Killer Super Concentrate, Fertilome Decimate Weed & Grass Killer, Finale XL T&O, Martin’s Nullify A/P) can be used in this manner. There are also products containing ammonium nonanoate, pelargonic acid, or a combination of caprylic acid and capric acid that can be used along the edges of beds.

Some glyphosate products can be used along the edges of beds, too, but these need to be used with great care. Glyphosate is systemic, and if it drifts onto desirable plants, it can injure or kill them, too.

This leaves us with a question about what herbicide options exist for broadleaf weeds that grow within landscape beds. For these, there are pre-emergence herbicides. Pre-emergence herbicides prevent germination of weed seeds rather killing weeds that are already there. (Many of these prevent germination of grassy weed seeds, too.)

Some active ingredients to look for in pre-emergence herbicides for landscape beds include trifluralin (e.g., Hi-Yield Herbicide Granules Containing Treflan or Preen Garden Weed Preventer), dithiopyr (e.g., Hi-Yield Turf & Ornamental Weed & Grass Stopper Containing Dimension), and the combination of trifluralin and isoxaben (e.g., Preen Extended Control Weed Preventer).

Many pre-emergence herbicides need either water (from rainfall or irrigation) or incorporation (e.g., with a rake or tiller) to be effective. Read the product label to find out what needs to be done.

To prevent germination of cool-season annual weeds, a pre-emergence herbicide can be put out around the second half of September. For warm-season weeds, it can be put out beginning around mid-February. Sometimes, an additional application is needed during the season to maintain a barrier to seed germination.

Pre-emergence herbicides won’t prevent the presence all weeds. Perennial weeds come back from roots as well as from seed. It’s best to use pre-emergence herbicides in combination with other weed management approaches, like mulching.

In any case, if you plan to apply an herbicide, make sure that whatever product you choose is labeled for use around the types of plants that you have, and follow label directions.

If you do end up with some weeds that need to be removed by hand – and most of us will – keep in mind that a hoe is primarily a tool for scraping, not digging. When you turn over soil, you’re likely to bring buried weed seeds to the surface, where they may germinate. If you just scrape the surface of the soil enough to uproot small weeds, you can minimize the number of weed seeds brought to the surface.

Finally, remember that it’s important to manage weeds before they flower and go to seed, to minimize future weed problems.

The third class in the gardening and lawn care series will take place next Tuesday, September 23, at 6 PM at the Lee Memorial Forest Main Lodge (21139 Lee Memorial Drive, Franklinton). Please join us if you’re interested! Call or email me to register or for more information. The LSU AgCenter and LSU provide equal opportunities in programs and employment.

Let me know if you have questions.

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Dr. Mary Helen Ferguson is an Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu or 985-277-1850 (Hammond) or 985-839-7855 (Franklinton).