Acanthus plants are striking in the garden with their tall stalks, dark green spiky leaves and beautiful purple and white flowers. They make a wonderful back border or walkway plant. You may know the Acanthus as Bear's breeches, which is their common name. They are easy to grow in gardening zones 6 to 10 and they will spread by themselves. These beautiful flowers will go dormant in the heat of the summer and come back in the fall, so you should plan for this and plant a summer flower to take it's place.
Dig a hole a few inches larger and six inches deeper than the container in which you purchased your Acanthus. Acanthus can be grown from seeds or root cuttings, but they have a hard time making it through the first winter. It is better to start with a plant and propagate with it. Acanthus should be planted in the spring, after the last threat of frost has past. The location should be full sun to partial afternoon shade.
Place a few inches of fine gravel in the bottom of the hole. Then mix the dug out soil with one part compost to two parts soil. Place enough of the mixed soil back into the hole so that the plant is positioned at the same level it was in it's original container. Acanthus isn't too particular about the soil it's in as long as it drains well. Placing the gravel at the bottom of the hole will ensure good drainage.
Take the plant out of the container and gently knock off the dirt. Place the plant in the hole and spread out the roots a bit. Fill halfway with your soil mixture, and water the soil and roots. Continue to fill to the top of the hole with soil and press down firmly.
Water the plant well immediately after planting. Water every other day while the plant is flowering, unless you have rain. In the summer, the spike will die back and you can cut back watering to once a week until fall, when the flower starts to return. Once the plant is established, water only when the top of the soil feels dry.
Fertilize with a general purpose flower fertilizer in the spring and again in the fall. These plants do not need much fertilizer and if planted in a flower garden will do well with the fertilizer you use for the rest of the garden.
Place a few inches of mulch over the plant when it dies back in late fall or early winter. This will help keep the roots warm and the plant will come back in the spring. Watch for new plants to shoot up close to the original. Cut them out immediately if you do not want the plant to spread.
Start new plants by taking cuttings from the roots in late fall or early winter. Lay them on a bed of compost and keep moist. They will sprout in a few weeks. Pot them up indoors and transplant them out in the spring.