Killing rhizome grass

Asked April 12, 2020, 1:03 PM EDT

I have a tall growing rhizome grass that started in a small patch, it came in with a quince I planted, and it has been spreading over the years. I’ve pulled it up every year but there is now too much. It’s invaded a 5x5 patch of lilies of the valley and more. How don’t get rid of this and can I save the lilies?

Multnomah County Oregon

1 Response

Thank you for choosing Ask an Expert for help with your weed control problem. Grasses that spread by rhizomes (underground stems) are quite difficult to control. This one is most likely quackgrass, common in our area.

Rhizomes not only spread the plant to new place, they also store energy the plant uses to grow. So when you pull the tops off the plant, these rhizomes have enough energy to produce tops and roots again. Plus, if you leave pieces of the rhizome in the ground they just start over. No wonder it’s so difficult to get control!

The trick is to keep the plants from replenishing that energy. You can pull off or cut down the tops whenever they show their faces or cover the area to keep them from absorbing sun energy. This can starve them over time.

Weed killers, such as those containing glyphosate, can be sprayed on the leaves and will be take down to the rhizomes and kill them.

The problem is that this has invaded your lily bed. You might consider transplanting the lilies for a season while you deal with the quackgrass. Do what control you can over the summer, and move the bulbs in the fall. At that point you can spray or cover the area, working to get rid of the quackgrass over the following year.

Good luck and good gardening,