The noble acanthus plant is a flowering perennial that thrives and is hardy in USDA zones 5b through 9b and can easily be winterized. It produces large, architecturally interesting foliage that is crenelated and sports sharp thorns. Acanthus blooms from spring through early fall and even into late fall in certain climes. It requires overwinter mulching below USDA zone 7 to help it survive the winter. In climates cooler than its natural range, the foliage will die back after the first hard frost and can be sheared off and mulched over, making way for fresh growth in the spring.
Cut back any dead, damaged or diseased acanthus foliage in the fall after the first frost. In climates colder than USDA zone 7, it is likely that the frost will kill the plant's top foliage. If this is the case, simply shear off the dying foliage at the crown of the plant just a few inches above the soil. Discard the cuttings in the compost pile or trash bin. Decaying foliage should be taken off of the plant in winter to prevent it being a breeding ground for disease during the winter months.
Water your acanthus well and deeply in the fall before the first hard frost comes. Water just around the base of the plant out to the drip line. This will moisten and fortify both the soil and plant roots against the coming winter drought conditions.
Mulch over the cut-back crown of the plant to cover it, or mulch around the base of the intact foliage in temperate climes where the plant successfully overwinters. Ensure that the mulch layer is at least 3 inches deep, and use an organic material such as shredded bark, cocoa bean hulls, leaf mold or compost for best insulation and soil enrichment.