Not all plants save the exotic colors for fall. Purple ninebark (Physocarpus opulifolius "Monlo"), also known as "Diabolo" ninebark, is a dark-leaved cultivar of the green-leaved deciduous ninebark shrub. The leaves are dark purple until they drop in the autumn, though they may fade in high heat. Purple ninebark grows in U.S. Department of Agriculture plant hardiness zones 2 through 7 in full sun or partial shade, and grows up to 10 feet tall and wide. Plant the shrub where you can enjoy its clusters of white or pink blooms in the spring or summer and their pretty contrast to the dark leaves.
Water regularly to keep the soil moist during the first year after planting the ninebark. Give the shrub at least 1 inch of water per week and water when the soil surface begins to dry. After the first year, the shrub is more drought tolerant and you can allow the soil to dry slightly between waterings.
Fertilize your ninebark in the spring with a general-purpose fertilizer such as a 12-4-8. Mix 1 tablespoon of fertilizer in 1 gallon of water. Wet the leaves and soak the root zone with the fertilizer solution.
Spread 2 to 3 inches of organic mulch over the root zone of the ninebark each spring to help keep the soil moist and weed-free. If your soil is neutral or alkaline, use a pine bark mulch or another acidifying organic mulch.
Prune the shrub in the spring before new growth appears. Thin out the center of the shrub by cutting several canes back to the base of the plant with pruning shears. Select about 30 percent of the older canes spaced evenly throughout the shrub for removal. Remove any dead, damaged or diseased canes as well.
Rejuvenate the shrub if it gets spindly by cutting the entire shrub back to 1 inch above ground level in the winter. Rejuvenation pruning every five years keeps the shrub looking its best.
Watch for powdery mildew, which appears as white, velvety patches on the leaves. Water the shrub in the morning and thin out interior canes annually for better air circulation to help prevent the mildew. Treat with a premixed fungicide spray, coating the leaves and re-treating after one week if necessary.