If you struggled with controlling perennial weeds in your lawn and garden beds this spring and summer without complete success, here’s some good news: Fall is the time to attack those weedy pests to get better results!
So why are some perennial weeds so tough to kill? Perennials are able to store nutrient reserves in their roots. Although the leaves and stems may be damaged or even killed by herbicides, perennial weeds have enough energy stored in the roots to regrow a new set of leaves and stems—sometimes repeatedly—until the nutrient reserves run out.
However, like most plants that go dormant in winter, perennial weeds begin to move energy resources from the leaves and stems to the roots and crown in fall. As the nutrients flow from leaf to root, any herbicides you apply will go along for the ride, killing the plant at its core. In spring, this process is reversed, with nutrients flowing from the roots to the leaves and stems, keeping herbicides from being fully effective; while you may cause some damage, the reserves are there to survive and regrow.
Choose an herbicide appropriate for the weeds you want to control, especially if you choose to use selective herbicides which will kill only certain plants that will be listed on the label. If a weed is not listed you may get only partial control, or none at all. Grassy weeds and broadleaf weeds require different selective herbicides to control them—if you have both types you will need two separate products.
Non-selective systemic herbicides (the best known is Roundup®) pretty much kill whatever green plant they touch, although multiple applications may still be needed for tough perennial weeds.
The key is persistence—if the weeds are not killed with one application, you will need to repeat applications as frequently as the label allows to get acceptable control of your perennial weeds.
What about non-chemical or organic controls for perennial weeds? They can work but may take several seasons to fully get control. As noted before, unless the roots and crown of the weeds are killed/destroyed, regrowth is likely. Repeated application of botanical oil or vinegar based herbicides is usually necessary. If hand weeding or digging, complete removal of the roots is essential, as many perennial weeds can regrow from small pieces of root remaining in the ground.