Ferns are non-flowering plants that reproduce and expand differently than seed-bearing plants. The method and speed of expansion vary dramatically among species, meaning there is no single answer to how they spread. Understanding the biological mechanisms behind fern propagation is necessary for managing their growth in a garden setting. Some species remain neatly contained, while others rapidly colonize large spaces, making knowledge of their growth habit essential for successful cultivation.
The Dual Nature of Fern Propagation
Ferns use a two-part life cycle to ensure both local expansion and wide-ranging genetic dispersal. The visible, frond-bearing fern is the sporophyte generation, which produces reproductive units called spores. These spores are typically found in clusters called sori on the underside of mature fronds. When released, millions of spores are carried by the wind, acting as the primary method for colonizing new, distant habitats.
If a spore lands in a moist, protected location, it germinates into a tiny, heart-shaped structure known as a gametophyte. This inconspicuous stage is responsible for sexual reproduction, producing male and female sex cells. Because the sperm cells must swim through a film of water to reach and fertilize the egg, this limits where a new sporophyte can successfully establish.
For established ferns, local spread is achieved through vegetative expansion using specialized underground stems called rhizomes. The rhizome extends horizontally beneath or just on the soil surface, connecting to the parent plant. New fronds and roots emerge from nodes along the rhizome, allowing the fern to clone itself and form dense colonies. This asexual propagation ensures efficient expansion into the immediate surrounding area.
Identifying Fern Growth Habits: Clumping vs. Running
The physical structure of a fern’s rhizome dictates its visible growth habit, which falls into two main categories: clumping and running. Clumping ferns, sometimes called crown ferns, have short, thick rhizomes that grow vertically or suberectly. This architecture causes new fronds to emerge tightly packed together, creating a dense, fountain-like cluster that expands very slowly outward. Clumping ferns are generally considered non-aggressive and remain neatly contained in their planting location.
Examples include the Japanese Tassel Fern (Polystichum polyblepharum) and the Sword Fern (Polystichum munitum). The short, stocky nature of their rhizomes means their spread is predictable, typically expanding only a few inches annually. They form an elegant, well-defined mass, making them ideal for borders, containers, or structured garden beds.
In contrast, running ferns are characterized by long, thin, creeping rhizomes that spread aggressively beneath the soil surface. These rhizomes travel significant distances, sending up new, scattered fronds along their length. This results in a much looser, more open colony of ferns that can quickly colonize a large area. Prime examples of this running behavior are the Ostrich Fern (Matteuccia struthiopteris) and the Bracken Fern (Pteridium aquilinum). The rhizomes of these running types are built for rapid lateral expansion, often pushing out several feet in a single growing season under optimal conditions. This makes them highly effective as groundcovers but can present a challenge in smaller, contained garden spaces.
Techniques for Controlling and Encouraging Fern Spread
Managing the spread of ferns depends entirely on correctly identifying their growth habit. For running ferns, control is often necessary to prevent them from overtaking a garden bed or lawn.
Controlling Running Ferns
One of the most effective physical control methods is installing a non-porous root barrier vertically into the soil around the planting area. This barrier should extend at least a foot deep to block the horizontal path of the creeping rhizomes, preventing them from extending beyond the designated zone. Alternatively, running ferns can be successfully contained by planting them in large, bottomless containers sunk into the ground. Regular maintenance is also required, which involves root pruning by slicing vertically through the soil with a sharp spade once or twice a year, just outside the desired boundary. For aggressive species like Hay-Scented Fern, methods like repeated mowing or solarization, which involves covering the area with clear plastic to superheat the soil, may be necessary to deplete the rhizome energy reserves and manage the population.
Encouraging Clumping Ferns
Clumping ferns, by their nature, require encouragement to increase their presence and are easily propagated through division. This involves digging up a mature clump and carefully separating the tight crown into two or more smaller sections, each containing a portion of the rhizome and a few healthy fronds. Division is best performed in the spring just as new growth is beginning, or in the fall after the growing season has concluded.
For both types, providing optimal growing conditions will encourage healthy spread. Most ferns prefer well-drained soil rich in organic matter and consistent moisture, particularly those that naturally grow in woodland environments. Ensuring adequate shade also prevents the fronds from scorching, allowing the plant to allocate its energy toward robust rhizome growth and the production of new fronds.