Commonly known as dusty miller, Centaurea cineraria displays velvety white leaves that provide an interesting contrast to deep green foliage plants, or brightly colored annuals or perennials. Dusty miller is perennial in U.S. Department of Agriculture plant hardiness zones 7 through 10; but in cooler climates, the plant is often grown as an annual. Propagate dusty miller by dividing the roots when the plant is not actively growing in autumn or early spring.
Prepare a planting spot in full sunlight or partial shade by spading the soil thoroughly to a depth of at least 8 to 10 inches. Remove stones and dirt clods, then dig 2 to 3 inches of organic matter into the top 6 inches of soil, along with an all-purpose, 12-12-12 fertilizer. Apply the fertilizer at a rate of about 1 1/2 tablespoons per plant.
Water the dusty miller plant deeply one to two days before propagating the plant. Well-hydrated plants tolerate division better than plants with dry roots.
Insert a sharp shovel deeply into the soil about 6 to 8 inches from the plant. Rock the shovel back and forth to loosen the roots. Repeat on all four sides of the plant.
Lift the loosened plant carefully from the ground, and shake excess soil from the roots. If the soil is badly compacted, rinse the roots with a garden hose or dip them in a bucket of water. Remove only enough soil to allow you to see the natural divisions between the roots.
Pull the plant apart carefully at the natural divisions. Be sure each division has four or five shoots and a healthy root system. Trim damaged sections with a clean, sharp knife, and dispose of old, woody or nonproductive sections.
Dig a hole in the prepared area. Make the hole double the diameter of the plant's root ball but no deeper than the height of the root ball. Place the plant in the hole, making the top of the root ball even with the surrounding soil. Allow 6 to 12 inches between each plant.
Water the plant deeply enough to completely saturate the plant's root ball. After planting, water as needed to keep the soil lightly moist until the dusty miller displays new growth. Thereafter, water to a depth of 6 inches when the top 1 inch of soil feels dry to the touch. Don't water if the soil is moist. Although dusty miller is a disease-resistant plant, it is susceptible to rust in humid conditions.
Mulch the dusty miller lightly with 1 to 2 inches of pine needles, dry grass clippings or other natural material to keep the roots evenly moist and cool. Mulch also helps keep weeds under control.