Is black mulch harmful to plants? Is the dye in the mulch toxic?

June 15, 2022 Ask the Expert

Question:

Is black mulch toxic to plants?  I believe that several plants in my flower garden have been stunted or killed by the black mulch that was placed around them. If the mulch can be ruled out as the cause, I will investigate other possibilities.  Is the dye toxic?

Answer:

Liz Stanley, Horticulture Community Education Assistant  

Bark mulch is generally beneficial for trees, shrubs and perennials. It suppresses weeds, keeps soil moisture even, reduces nutrient runoff in heavy rains, and can keep temperatures moderated during heat and cold cycles.  In the spring, a mulched garden will often “awaken” earlier since the soil is warmer.

Over time, mulch improves soil by gradually adding organic matter. Shredded bark mulch can also be used in containers with annuals and vegetables to prevent the soil from drying. It also keeps flowers and fruit clean.

Mulches used improperly can have a detrimental effect if:

– They make contact with the stem, trunk or crown of the plant causing disease and rot.
– They’re applied too heavily and keep oxygen from reaching the roots. (1.5 – 2.5″ is usually plenty for perennials, 3″ for trees.)
– They’re mixed into the soil & cause nitrogen lock-up. (Soil organisms consume the carbon and in the process, take all available nitrogen from plants.)

More information in this fact sheet from UNH: Garden Mulches

Because most mulches are carbon, plants may need an additional source of nitrogen during the growing season. Look for leaves that are pale or yellow in color. If this is the case, apply a gentle dose of liquid fertilizer, or scratch some granular fertilizer in a ring around the plant under the mulch and water well. N is the first letter in most fertilizers (N-P-K). Follow label directions.

Mulch dyes are not toxic to plants. Here’s more from UMass Extension:

“Mulches come in different colors. The color does not matter to the plants, and is only for aesthetic purposes. There is no evidence that dyes used in colored mulches are toxic. However, if planning to use colored wood mulches, it is important to know the supplier and the source of wood used to make the mulch. Avoid mulches from recycled wood if it includes pressure treated wood (this mulch could be contaminated with chromated copper arsenate).