by Heather Smith Thomas
To successfully control an undesirable plant, learn how it grows and reproduces. Mowing or spraying at the wrong time of year may be wasted effort and expense. You must attack it when it is most vulnerable.
Burdock is a perfect example, according to Dr. Don Morishita, University of Idaho Professor of Weed Science and Extension Specialist. Burdock is a native of Eurasia, brought to North America by seed burrs stuck to imported animals. It puts forth clusters of round burrs that adhere to the hair coat of cattle and other animals.
Burdock blooms in late summer, producing a composite seedhead that matures by mid-August in southern areas and late fall in northern climates. When burrs are ripe they release hundreds of microscopic barbed slivers. If these get into an eye they cause severe irritation—especially if caught under an eyelid where they continually scrape the eyeball every time the animal blinks.
Burdock can be controlled by chopping/mowing before it’s mature enough to make seeds. It can also be controlled with herbicides. Morishita says several broad-leaf herbicides will kill burdock, if applied properly. Burdock is a biennial; it lives for two growing seasons. The first year, it doesn’t bloom; it grows leaves and accumulates food reserves in its roots, like a carrot (also a biennial).
The second year it grows a long taproot and tall stalk, producing flowers and burrs. This exhausts food reserves in the root; the plant dies after burrs are mature. After the stalk comes up, it is harder to kill with herbicides because it’s sending food up from the roots instead of down.
Burdock plants are easiest to kill in early spring or in late fall. The first-year plant stays in rosette stage that first summer (circular cluster of leaves, no tall stalk). “Apply spray when the plant is putting food into the root, since you have to get herbicide into the root to kill the plant. A broad-leaf herbicide like 2,4-D is a translocated herbicide that can move down into the root. If you spray early in spring you generally kill new young sprouts and last year’s rosettes (plants that are trying to create more food reserves in their root to complete second year growth and make burrs). You must spray very early to get the second year plant. After the stalk comes up it is harder to kill. If you spray in the fall you are killing this year’s rosettes–plants that would mature and create burrs next year,” he says.
“Fall is a good time to spray and kill young plants that are storing food reserves for next year’s growth. The first hint of cold weather is a trigger to send food to the roots. By contrast, in spring the second year plant is taking food from its roots to produce leaves and make the big push for a tall stalk and blooms. The food is going up, and it’s harder to get the herbicide down into the root.” Food reserves in the root are lowest when the plant starts to bloom.
When using herbicides to kill burdock, don’t overdo it. “If you use too much, it quickly kills the leaves and doesn’t get down into the taproot. The root survives, to regrow. You want a slower kill so leaves survive long enough to transfer the herbicide down into the root, to kill the whole plant. Use recommended rate (2 to 3% solution of 2,4-D which is about .5 to 3.75 ounces per gallon of water) and spray plants lightly–until they are barely wet but not dripping,” he explains.
He also cautions against using anything other than broadleaf herbicides. “Burdock is a bare-ground plant; it doesn’t grow well where there’s grass cover or competing plants. Don’t use Roundup. It kills everything–including grass that inhibits regrowth of burdock.” He usually recommends 2,4-D for burdock but other herbicides like Milestone and Redeem R&P work very well, too.
Chopping/mowing is effective, but you must do it at the right time or the plant regrows from the root. The best time is after the stalk is budding but before burrs are ripe. At that point the food reserves are so low in the root that it cannot regrow, says Morishita.
Burdock patches may take several years of diligent control to eradicate, since the seeds can live a long time. There may be viable seed in the ground that will sprout and grow. Keep checking those patches, says Morishita, and get rid of new plants that grow up from old seeds.