Old garden plants like mint and daylilies have a well-earned reputation for hardiness. They can also be downright invasive. Most gardeners who’ve fallen for the charms of these plants without creating a boundary for them end up having to destroy whole groups of plants and start over, sadder but wiser and determined to isolate these plants or use only non-invasive varieties. When Mentha species and Hemerocallis grow out-of-bounds and become weeds, totally eradicating the lot may be your only option.
Pour boiling water on plants on successive days. Mint and daylilies each tolerate a wide range of growing conditions, but boiling water will scald plant tissue and cook roots.
Spray mint with vinegar or other acetic acid using a household spray bottle. Mint needs alkaline soil to grow and the acid will kill the leaves and leech into the soil.
Over water daylilies—they can’t stand wet soil.
Dig at least a foot around all plants and cultivate deeply, working in and lifting tubers and rhizomes as you go. Collect all roots and take them to the municipal compost; they’ll continue to grow all over your little compost heap and you will simply have moved them to another part of the yard.
Work some well-rotted compost into the soil to provide fresh nitrogen and cover the area with a black plastic tarp. Pin the edges down with tent stakes or landscaping timbers.
Leave the tarp on for 2 to 3 weeks. The heat will force growth but the lack of light will kill new plants.
Remove the tarp and cultivate, picking out any roots or plants that have died. No matter how lifeless they may appear, never turn mint or daylily remains back into soil as compost.
Dig up as many roots as possible. Mint and daylilies both grow by sending out underground roots to begin new plants; wait for these new plants to start sprouting before you proceed with the next step.
Spray the plants and the entire area with an herbicide containing glycophosate, a broad-spectrum herbicide marketed for use on grassy (daylily) or broadleaf (mint) weeds.
Repeat applications as directed on the herbicide label, usually no more frequently than 10 to 14 days apart. Remove and destroy dead plants.