(News article for April 17, 2020)

I’ve spent some time over the last couple of weekends weeding my parents’ landscape beds. Using a hoe, it didn’t take long to get them looking better.

Once I hoed up the weeds that were there, I spread pine straw. 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 inches. Newspaper or cardboard can be placed under the mulch for an additional layer of protection.

We sometimes see mulch piled up at the bases of tree trunks (“volcano mulching”), but this is not the best idea. Mulch piled against a trunk can keep the trunk moist and contribute to disease problems.Avoid piling mulch against trunks.

Natural mulch will decompose over time, so you’ll need to refresh it each year to maintain a depth that will block sunlight from reaching the soil surface.

I occasionally get questions about using herbicides in landscape beds, but there are not a lot of herbicide options for killing weeds that are already present in landscape beds. Many of the herbicides that would kill the weeds would also injure the plants that we want and so are not labeled for use in landscape beds.

There are, however, pre-emergence herbicide options. Pre-emergence herbicides prevent germination of weed seeds rather killing weeds that are already there.

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 Weed Preventer), dithiopyr (e.g., Hi-Yield Turf & Ornamental Weed & Grass Stopper Containing Dimension), and the combination of benefin and oryzalin (e.g., Amaze Weed and Grass Preventer₂). Make sure that whatever product you choose is labeled for use around the types of plants that you have, and follow label directions when using it.

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.

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., Fertilome Decimate Weed & Grass Killer) can be used in this manner. Some glyphosate products can be used along the edges of beds, as well, 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.

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.

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

If you have questions, please let me know.

Contact Mary Helen Ferguson.

Weed management in landscape beds at the Hammond Research Station relies heavily on pine straw mulch. (Photo by M.H. Ferguson)