The daylily is a hardy perennial plant with an unstoppable tuberous root called a rhizome. Though many gardeners relish the beautiful orange and yellow blooms of the daylily, the plant can turn into a wild nuisance and spread into unwanted areas. Digging up a patch of wild daylilies---or ditch lilies---is back breaking work and may not be enough to kill them completely. Any segment of daylily root left behind can spawn new growth. Killing unwanted daylilies takes patience and a time, but it can be accomplished.
Kill Unwanted Daylilies
Mow the area where the daylilies you want to kill are growing. Set the mower blade to its lowest setting to remove as much of the plant growth as possible. If the daylilies are growing in a ditch or hillside, you may need to use a string trimmer/weed whacker instead.
Cover the area with a plastic weed barrier. Be sure to cover the entire area and a couple feet beyond to keep the unwanted daylilies from receiving sunlight. Stake down the weed barrier to keep it in place. Alternately, an inch-thick layer of newspaper can be used instead of store-bought weed barrier. Wet the newspaper to keep it from blowing away.
Cover the entire area with a foot of mulch. This will further prevent sunlight from reaching the unwanted daylilies and effectively kill the plants.
Leave the mulch in place for one year to completely kill off the daylilies underneath. If any daylilies manage to break though a gap in your barrier, spray with Round-Up.