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AgAlert! Herbicide Contaminated Compost

Beware of compost you will be using for your upcoming growing season

Compost has traditionally been used by growers not only for supplying nutrients to the soil and plant but also due to its multiple beneficial attributes, such as balancing pH, enhancing water holding capacity, and boosting soil structure and beneficial microbial populations to improve overall soil quality for plant growth and development. Compost can hold nutrients for a longer time and deliver to plants when needed. Nutrients found in compost are released slowly as the compost decomposes, reducing nutrient loss through prevention of off-site movement. Despite all these benefits, herbicide contaminated composts can do lots of harm to plants, especially to those belonging to the family Solanaceae, which includes tomatoes, peppers and eggplants. Plant distortion due to growth regulator type herbicide is shown in Figure 1. 

Tomato plant with curled leaves due to growth regulator type herbicide.  Potato plant with curled leaves due to growth regulator type herbicide.

Figure 1. Plant distortion due to growth regulator type herbicide, shown on a tomato plant (left) and a potato plant (right)

Although growth regulator type herbicides can enter garden sites or fields through contaminated farm tools or drift, contaminated mulch, compost or manure are typically the most common causes of herbicide injury in vegetable gardens. Contaminated mulching materials typically include grass clippings from lawns treated with growth regulator type herbicides with varying degrees of soil residual activity. Several “weed and feed” fertilizers carry such herbicides in the granular form to control broad leaf weeds. When such materials are used as mulch, especially during the same growing season, susceptible vegetables usually show injury symptoms. In some cases, herbicide residues may persist in affected mulch or composts into the following growing season and cause injury to susceptible plants. Similarly, certain herbicides applied in pastures or hayfields may persist in the harvested hay or manure derived from the animal that was fed with this forage.

Composting the herbicide-treated hay, grass clippings and manure from animals fed with treated hay for extensive periods (more than 200 days) may break down most active ingredients by more than 50%. However, it would be better not to use any of these contaminated organic materials to produce finished compost for horticultural or vegetable production. Other susceptible plants include those in the legume (peas, beans, clover, etc.) and composite families (sunflower, lettuce, etc.).

How to check your compost for herbicide contamination

Simple bioassays may be performed to detect the presence of herbicide residues in suspected materials. To perform a bioassay, suspected compost material may be placed in containers and planted with sensitive species, such as beans or tomatoes, and compared to that from untreated material (control). You may see symptoms similar to the bioassay result obtained by using contaminated compost (Figure 2A) compared to non-contaminated (Figure 2B). You may get reliable results by taking a composite sample from different depths and corners of a compost stockpile. 

Snow pea plant with stunted growth and curled leaves due to growth regulator type herbicide.  Two snow pea plants in pots without any growth issues.

Figure 2. Herbicide injury symptoms on snow pea plants from the bioassay conducted with herbicide contaminated compost (left) and non-contaminated (right).

This alert impacts all West Virginia counties

Please send suspected samples to WVU Plant Diagnostic Clinic.

Get help from the WVU Plant Diagnostic Clinic


Mahfuz Rahman
WVU Extension Plant Disease Specialist 
Director, WVU Plant Diagnostic Clinic
Email Mahfuz

Trade or brand names used in this publication are for educational purposes only. The use of such product names does not imply endorsement by the WVU Extension to the exclusion of other products that may be equally suitable.