Ferns reproduce through methods distinct from the seed-based system of flowering plants, and they definitely spread on their own. They are highly adept at colonizing new areas using two different propagation strategies. This natural tendency to spread can be vigorous, allowing them to quickly establish large colonies in favorable environments. Understanding these dual methods of reproduction is key.
Spore Dispersal: The Reproductive Cycle
The most unique method of fern propagation is sexual reproduction through microscopic spores, which allows for long-distance colonization. The familiar, leafy fern is the diploid sporophyte generation. It produces specialized structures called sori, usually visible as dots or lines on the underside of mature fronds. Sori are clusters of tiny spore cases, known as sporangia, where spores are created through meiosis.
When the spores are ripe, a mechanism within the sporangia snaps open, forcefully ejecting the lightweight spores into the air. Wind currents carry these spores, sometimes many miles, to new locations. If a spore lands in a moist environment, it germinates into a tiny, heart-shaped structure called a prothallus, which represents the haploid gametophyte generation.
This prothallus is a separate, short-lived plant that produces the male and female reproductive cells. For fertilization to occur, a thin film of water is needed to allow the motile sperm to swim to the egg. After fertilization, the resulting zygote grows into the new, leafy sporophyte fern, completing the two-stage life cycle known as the alternation of generations. This spore-based system is effective for colonizing distant habitats.
Underground Expansion: Spreading via Rhizomes
Ferns also use an asexual method for local expansion by utilizing specialized underground stems called rhizomes. These creeping structures function like runners, storing food and sending up new fronds and roots along their length. The behavior of these rhizomes determines whether a fern is considered a “running” or a “clumping” type.
Running ferns possess long, thin, and rapidly extending rhizomes that aggressively spread outward, often traveling several inches to a foot or more each year. The Ostrich Fern (Matteuccia struthiopteris) is a prime example, quickly forming large, dense colonies that can dominate a garden space. This aggressive spreading allows the fern to rapidly fill in an area, creating a lush, uniform groundcover.
In contrast, clumping ferns have short, tight rhizomes that expand very slowly, causing the plant to grow outward from a central point. The Japanese Holly Fern (Cyrtomium falcatum) is a clumping type that gradually increases its diameter over many years, maintaining a defined, vase-shaped form. Knowing a fern’s rhizome habit is important, as a running variety will quickly occupy a large footprint, while a clumping variety will remain neatly contained.
Managing Fern Growth in the Landscape
Managing fern growth requires a two-pronged approach focusing on both aerial and subterranean reproductive structures. To limit long-distance spread via spores, remove the fertile fronds before the sori mature and release their contents. This process, often called deadheading, should be done before the dots on the frond undersides turn dark brown or black.
Controlling local expansion from running rhizomes is a matter of physical containment. Aggressive runners, like the Ostrich Fern, can be managed by installing a root barrier, such as sturdy plastic or metal edging, buried vertically to a depth of 10 to 12 inches. Routine root pruning with a sharp spade at the edges of the planting area is another effective containment method.
For clumping ferns, management is simpler, involving division every few years to maintain their size and vigor. This is done by digging up the entire clump and cutting the root mass into smaller sections. Ensure each new piece has healthy fronds and roots. Planting running ferns in containers or dedicating them to a large, contained area is a simple way to enjoy their texture without worrying about an aggressive takeover.