The question of whether a fern becomes a weed in your garden depends entirely on the context and location. Ferns are an ancient and diverse group of plants that reproduce through spores rather than seeds or flowers. While many species are prized for their texture and shade tolerance in ornamental landscapes, others possess traits that allow them to spread aggressively and outcompete cultivated plants. A fern is not inherently a weed; its status is a subjective judgment based on where it grows and how its growth habits conflict with your gardening goals.
How Context Defines a Weed
The term “weed” is an ecological and human-defined classification, not a formal botanical one. A plant is considered a weed when it interferes with human activities, aesthetic preferences, or the health of desired plants. For instance, a Maidenhair fern planted intentionally is a valuable ornamental specimen, but the same fern sprouting uninvited in a manicured lawn instantly becomes undesirable.
This subjective definition is clear when considering species with notorious spreading habits. The hay-scented fern (Dennstaedtia punctilobula) is a sought-after native groundcover in naturalized areas, yet it is regarded as a weed in formal gardens due to its vigorous, colonizing nature. This fern can also release chemical compounds, a process called allelopathy, that discourages the growth of surrounding plants. Other ferns, like the Japanese climbing fern (Lygodium japonicum), are classified as noxious invasive species because they outcompete native flora on a landscape scale.
Aggressive Growth Patterns of Ferns
The primary reason certain ferns become unwanted weeds lies in their effective methods of vegetative spread. Ferns employ two main strategies for colonizing new territory, allowing them to persist and expand rapidly.
The most common mechanism for localized spread is the rhizome, an underground stem system from which new fronds emerge. Ferns with short, tight rhizomes, such as the Lady fern, are “clumping” and remain relatively contained. Conversely, aggressive ferns like the hay-scented fern have “running” rhizomes that extend horizontally beneath the soil surface, forming dense, competitive colonies far from the parent plant.
Another element is the production of spores, the reproductive structures ferns use instead of seeds. Spores are microscopic, dust-like particles produced on the undersides of the fronds and are easily dispersed long distances by wind, water, or human activity. This allows a single fern to unpredictably colonize new, distant areas of the garden, sometimes sprouting where no parent plant is visible.
Strategies for Controlling Unwanted Ferns
Managing a fern that has become a weed requires a targeted approach that addresses its aggressive subterranean and aerial growth.
Manual and Cultural Control
When manually removing unwanted ferns, pulling the visible fronds is insufficient, as the underground rhizome will quickly regenerate. To achieve lasting removal, you must dig out the entire rhizome system, which can be an extensive, interconnected network of roots and stems. Cutting the fronds repeatedly throughout the growing season can also slowly deplete the energy reserves stored in the rhizomes, though this requires consistent vigilance over multiple years.
Environmental Modification
Another effective strategy involves modifying the environment to make the area less hospitable to the plant. Most weed ferns thrive in specific conditions, typically shade, ample moisture, and acidic soil. By improving drainage, increasing light exposure, or raising the soil’s pH slightly through amendments, you can discourage the fern’s colonization efforts.
Chemical Control
Chemical control is considered a last resort and presents unique challenges because ferns are non-flowering plants. Systemic herbicides, such as those containing glyphosate or triclopyr, are necessary to be absorbed by the fronds and translocated down into the rhizome system to kill the entire plant. Ferns often require multiple applications, and non-selective chemicals must be applied carefully to avoid damaging desirable surrounding vegetation.