Goutweed proving persistent? Here’s how to eradicate the pesky weed
“I read an article you wrote about goutweed. I am in the process of eradicating it from a church garden and have a question I cannot seem to find an answer to. Last August, we smothered the goutweed (cut it close, removed the cuttings, covered it with plastic, and then covered that with mulch).
Lifting up the mulch and plastic 13 months later, I see a spaghetti of pale rhizomes and roots. Are these dead or alive? I am afraid to pluck or rake them out for fear the rhizomes are still alive and will regenerate, or worse, splinter and regenerate.”
— Catherine Marquis, Park Ridge
Goutweed (Aegopodium podagraria) is a very aggressive invasive plant that forms dense patches and will choke out desirable plants if not controlled in home gardens and native plant communities.
I have been trying to get rid of a couple of patches of this plant in my garden by spraying them with an herbicide. Two years later, after multiple herbicide applications, there are still occasional sprouts of goutweed here and there that need to be removed.
The rhizomes of goutweed are long, white and branching, so what you are describing under the plastic could be living goutweed rhizomes. Dead roots would tend to be more brown and dried-out looking and starting to decay after 13 months of cover.
The roots you are seeing could also be from adjacent trees and shrubs, with the roots developing just under the plastic, where there will be more moisture and oxygen. Knowing how tenacious this plant is, I would assume that the rhizomes and roots you are seeing are likely living goutweed.
Killing out goutweed by covering it will work and is best done in spring at the time of leaf-out, which depletes it of its carbohydrate reserves. Cover the patch with black plastic once the leaves have started emerging and leave covered for the growing season.
One advantage of not covering the plastic sheeting is the solar heating under the plastic will help kill any undesirable plants under it. The black plastic covering, though, is an aesthetic detriment to the garden’s appearance.
Waiting until the plants are fully leafed-out and cutting them back to ground level in spring, and then covering with plastic, will have more negative impact on the goutweed. Waiting until later in summer, as you did in August, can be less effective, as the goutweed would have built up more food reserves in the roots by that time of year. You did take the right steps to clear out the goutweed.
Go ahead and clean out any of the rhizomes and roots you are seeing under the black plastic. I would throw this debris in the trash and not risk getting this plant started in a compost pile.
It’s OK to leave uncovered for the winter and then observe what happens in spring. Watch closely for the goutweed to return and, if it does, let it fully leaf out, mow it down, and then re-cover with the black plastic again for the summer. A light layer of mulch on the bed will look nice and not slow down the growth of any living goutweed in the spring.
If there is just a sprout here and there in May, then you can consider starting to replant the garden, providing you have the capacity to monitor the bed and quickly remove any goutweed that restarts. I would take more time to be sure of the eradication of the goutweed if the future plantings were going to be perennials and/or groundcovers, which will be more difficult to keep the goutweed out of.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.
