The Autumn Fern (Dryopteris erythrosora) is a popular perennial valued for its distinctive coppery-red new fronds that mature to a glossy dark green. Gardeners often seek this fern for its color interest and manageable size. Autumn Ferns are reliably clump-forming, meaning they spread very slowly and are not considered aggressive. Their growth pattern results in a dense mound that increases in diameter gradually over many years.
Understanding the Autumn Fern’s Growth Habit
The Autumn Fern’s slow spread is controlled by its root structure, which consists of short, thick rhizomes. These specialized underground stems are the clumping type, unlike the running type found in aggressive ferns. The rhizomes expand the existing crown outward at a measured pace, rather than sending out runners that create new plants far away.
This clumping habit means a single plant will slowly increase in size, forming a dense, vase-shaped mound. It typically reaches a mature spread of 18 to 36 inches over five to ten years. The growth is concentrated at the center, resulting in a compact footprint that rarely requires containment. This slow growth rate benefits gardeners who prefer a tidy, predictable arrangement.
Ferns also reproduce using microscopic spores, which are contained in structures called sori on the underside of the fronds. While spores are carried by wind, they require specific, moist, and sterile conditions to successfully germinate. Unwanted spread from spores is uncommon in a typical garden environment, making rhizome growth the primary factor in the plant’s expansion.
Intentional Propagation Through Division
Because the Autumn Fern does not aggressively spread, gardeners rely on division to create new plants and manage mature clumps. Division is required when the central crown becomes overcrowded, which can lead to a hollow center or smaller frond production, typically every three to five years. This process is most successful during the dormant season, either in early spring or in the late fall after the first hard frost.
To propagate, the entire fern clump must be carefully dug up using a shovel or garden fork. Excess soil is removed to expose the network of rhizomes and roots. A sharp, sterilized knife or spade is then used to cut the root ball into smaller sections.
Each separated section must contain a portion of the thick rhizome, a healthy mass of fibrous roots, and at least one growing point or crown. These divisions are immediately replanted at the same depth as the original plant and thoroughly watered. This manual separation is the most reliable way to increase the number of Autumn Ferns in a landscape.