Killer Compost

Is your garden in danger of killer compost?

By Kate Burkes

Benton County Master Gardener Newsletter April 2024

In the last twenty years, a new threat has appeared in home gardens: persistent herbicides. Persistent herbicides are chemicals commonly used in commercial agriculture to control the growth of weeds in hay and in some grain crops that produce straw. These herbicides are “persistent” because they do not break down quickly in the environment and can remain active in soil for up to three years. 

How does this contamination happen? Hay treated with a persistent herbicide can pass through an animal’s digestive system and into their manure without breaking down.  If the manure is added to compost, the herbicide will persist even through the composting process.  This compost can then carry the herbicides into the garden, causing unintended contamination.

Another possible means of contamination is from straw - many home gardeners use straw as mulch or may add animal bedding containing straw to their home compost piles.  Soil mixes or other fertilizer products containing compost or manure may also be a problem.

Certain plants are more susceptible to persistent herbicides than others, including tomatoes, eggplant, peppers, potatoes, beans, peas, carrots, and greens. Many flowers including dahlias, marigolds, sunflowers and some roses may also be affected, along with fruits such as strawberries and grapes.

Symptoms of herbicide damage may include stunted growth, leaf curling, yellowing or browning of leaves, and overall poor plant vigor.  Note that not all herbicide damage is from persistent herbicides in the soil but could be from herbicide drift (carried on the wind) from other sources.

The persistence of herbicides in the soil can pose a risk to subsequent plants grown in the same area. Even if the contaminated compost or manure was applied in previous seasons, residual herbicide activity may still affect the growth of new crops planted in the affected soil.

Unfortunately, there is no easy lab test for persistent herbicides. Many sources of manure or compost may not know if their material is contaminated. So what can you do to protect your garden from killer compost?

·       If you can, make your own compost from materials that you know are not contaminated with persistent herbicides, such as kitchen waste, unsprayed garden or yard waste, leaves, etc.

·       If you purchase compost, ask if they can certify that contaminated materials were not included, or that the compost has been sampled (see bioassay below) to check for herbicides.

·       Look for sources of compost that use food waste or other products that don’t contain manure or straw.  Note that some mushroom farms do use straw as a substrate for growing.

If you are not sure of the source materials, do a bioassay before adding it to your garden.  Collect a few samples of the compost and plant a bean seed or a tomato seedling in it.  Place the pots in a sunny spot and keep them watered. Watch the plants for about three weeks (after germination) then evaluate plant growth for signs of herbicide damage. More complete instructions can be found on the NC State Extension website.

By being aware and proactive, you can safeguard the health and productivity of your garden and avoid the heartache of killer compost!

More Information:

Herbicide Carryover in Hay, Manure, Compost, and Grass Clippings | North Carolina State Extension Service