Killing Bald Cypress Knees with Herbicide: What to Expect

If you're trying to eliminate a bald cypress (Taxodium distichum) by treating its knees (pneumatophores), understand that this method will kill the entire tree—and it takes time.

How It Works

Cypress knees are connected to the root system. Applying concentrated glyphosate (Roundup) to fresh cuts on the knees allows the herbicide to translocate through the roots to the whole tree.

Application Method

1. Cut each knee cleanly with a saw.
2. Immediately paint the cut surface with undiluted glyphosate using a disposable brush.
3. For faster results, also cut a frill (overlapping cuts) around the trunk base and apply herbicide there.

Timeline

• Small trees (under 10 ft): 4–8 weeks for full kill.
• Large mature trees: Several months to a year.
• Spring application (active sap flow) works fastest.

Important Caveats

• Glyphosate is non-selective—avoid contact with desirable plants.
• Dead roots will decay in place, potentially leaving voids.
• Knees may continue appearing from untreated roots for a season.
• Check local regulations—some wetlands protections restrict cypress removal.

If you only want to remove knees without killing the tree, that's not possible—they're integral to the root system. Consider simply mowing them off at ground level if they're a lawn nuisance.