A butterfly bush can either be a beautiful bush of flowers that attract butterflies to your garden or a massively reproducing weed that inhibits the growth of your garden. Killing the roots of a butterfly bush by way of herbicide is challenging. Also, cutting the stems off at the base only encourages it to grow. There are, fortunately, several different ways of getting rid of this fertile plant.
The "Tug and Lug" Method
Dig around the roots of the butterfly bush and pull it from the ground. Place it in an area where it cannot easily re-root itself in the ground until it can be disposed of in the trash. Do not dispose of the butterfly bush in a compost heap--doing this would destroy the efforts of making the compost heap and give the butterfly bush an adrenaline kick to pump up its germination rate.
The Copper Spike Method
Cut the bush as low to the ground as possible. Nail the copper spike into the center of the root. A chemical reaction will take place that not only kills the root but helps disintegrate the root.
The Sun Deprivation Method
Cut the bush close to the ground and cover it with a black plastic, then boards with heavy rocks on top. This should deprive the plant of sunlight and kill the roots.
The Boiling Water Method
Boil water on your stove and pour it around the base of the butterfly bush. The plant is now cooking underground; be warned this will indiscriminately kill the plants around it.
The Bleach Method
Fill a spray bottle with bleach. Cut the bush as close to the ground as possible. Spray the stump with the bleach--saturate the stump thoroughly, concentrating the bleach on the stump and not the dirt around it, as it may affect nearby plants. The bleach chemicals will only last for a few days. Once the bush is dead, remove it and it is safe to plant in that spot again. If you get bleach on a plant you wish to keep in your garden, simply wash the bleach off. Rubbing alcohol works as well; be warned, it is as indiscriminate as bleach when it comes to killing plants.
The Salt Method (A Last Resort)
Salt the earth around the roots. Trace the circumference of the root in salt. The salt will seep down into the dirt and deprive the plant of water. Another technique is to boil the plant with salt water--whatever gets the job done. Be warned, though, that this method prevents any plant growth in this area for many years.