Gardeners use the word "kill" with hesitancy, trepidation--and sometimes a long and unhappy explanation. There are times, however, when a once-welcomed plant tries to overwhelm its neighbors, or a new planting scheme requires removal of the old one. Ajuga, or bugleweed, can be a very aggressive ground cover, and the only way to deter it is to kill it. Knowing what kills ajuga also helps a gardener know what keeps it alive and flourishing.
Dig out ajuga plants, roots and runners included. Ajuga spreads by means of runners--ground-level stems that root and form new plants. This can be time-consuming because you must trace the path of potential new plants. Be prepared to dig one season and inspect the next for plants you might have missed. Ajuga is tenacious.
Cover large areas of ajuga to be removed with impermeable black plastic sheeting (weed mulch or heavy garbage bags). Anchor it with rocks, and leave the area alone for at least four weeks. This removes both light and water, drastically inhibiting growth. While this might not kill all plants, it will make it much easier to dig out roots.
Consider using a broad-leaf herbicide or a soil-poisoner to eradicate ajuga. The Ohio Farm Bureau provides a simple recipe based on history (the Romans sowed salt in the soil of Carthage so that farmers could no longer grow crops): Mix 1 gallon vinegar, 1 cup salt and 9 drops of liquid soap and pour directly on plants. Both herbicides and soil-poisoners must be applied with caution or they will affect nearby plantings. Both will affect new plantings for a season or longer.
Let ajuga dry out for a prolonged period. Ajuga is vigorous but does best in regularly watered soil. Prolonged hot spells without water will damage ajuga and make it particularly hard to spread.
Take care in applying driveway and sidewalk salt close to ajuga. Just as soil can be made inhospitable to plants by deliberately adding salt, measures taken to combat snow and ice can also cause damage. Try to keep a salt-free border of 6 inches between ajuga bordering driveways or walks and salted areas, or consider using sand instead.
Take a holiday from weeding. Ajuga is aggressive, but so is grass, and gardeners grapple with keeping each out of the other's way. To encourage ajuga to thrive, you must pull out weeds and grass; sowing new grass seed means removing ajuga. In general, grass will win a prolonged struggle if left to battle with ajuga.