doughnut effect
 n.— «Asters are easily propagated by division in the early spring. Simply dig the clump up once the shoots appear in early May, and gently pull the crown apart. When they require dividing, you will notice new growth around the outside of the clump only. The center will look dead; this is often called the doughnut effect. Dig, divide and toss the old center out, then replant the healthy plants from around the outer edge of the clump in a full sun location, with a moist yet well-drained organically rich soil.» —“Asters trump mums for their hardiness, color palettes” by Nancy O’Donnell Chicago Tribune Oct. 5, 2008. (source: Double-Tongued Dictionary)

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