Ferns are a diverse group of plants often admired for their lush foliage and adaptability in various garden settings. Gardeners frequently wonder if these ancient plants pose an invasive threat. While many ferns are well-behaved garden inhabitants, certain species can exhibit aggressive tendencies or become truly invasive, disrupting local ecosystems. This article aims to clarify the distinction between aggressive and invasive ferns and provide guidance for managing them.
Understanding What Makes a Plant Invasive
An invasive plant species is not simply one that spreads rapidly. It is non-native to an ecosystem, exhibits aggressive growth, and causes environmental or economic harm. These plants spread quickly, outcompeting native flora for essential resources like light, nutrients, water, and space. This competition reduces native plant diversity and negatively impacts local wildlife.
It is important to distinguish this from an “aggressive” plant, which spreads quickly within a garden but is often native or does not cause significant ecological damage. A small percentage of introduced plants escape cultivation and become pests in natural areas, often lacking natural predators or controls.
How Ferns Spread
Ferns employ unique reproductive strategies. Unlike flowering plants, they reproduce through spores, not seeds. These tiny, dust-like spores are produced in clusters called sori, found on the undersides of mature fronds. Spores are easily dispersed by wind, water, animals, or human activity, allowing them to travel considerable distances. If a spore lands in a moist, shaded environment, it can germinate and develop into a new fern plant.
Many ferns also spread vegetatively through underground stems called rhizomes. These rhizomes grow horizontally, sending up new fronds and roots, allowing a single fern to form a large colony over time.
Identifying Truly Invasive Fern Species
While many ferns are welcome additions, several species are truly invasive, posing significant ecological threats. The Japanese climbing fern (Lygodium japonicum) is a prominent example, invasive across the southeastern United States, including Florida, Georgia, Alabama, and the Carolinas. This vine-like fern can grow up to 90 feet long, forming dense mats that smother native vegetation and acts as a “fire ladder,” carrying fires into tree canopies.
The Old World climbing fern (Lygodium microphyllum) is particularly aggressive in moist habitats of South and Central Florida. It rapidly invades natural areas, blanketing large expanses by climbing over trees and shrubs, and also creates a severe fire hazard.
The Hay-scented fern (Dennstaedtia punctilobula), though native to the Eastern United States, can act as a “native invasive” in the Northeast. It spreads aggressively via rhizomes and spores, forming dense monocultures that outcompete hardwood tree seedlings. Its success is partly attributed to its tolerance of varied conditions and low palatability to deer.
Managing Problematic Ferns
Managing aggressive or invasive ferns requires prevention and control. Preventative measures include selecting non-invasive fern species suitable for the local climate and avoiding known invasive varieties. Properly siting ferns, considering their mature size and spread, can also help contain growth.
For existing problematic ferns, manual removal can be effective for smaller infestations. This involves diligently digging up the entire plant, including its root system or rhizomes, as any left-behind fragments can regrow. Repeated cutting or mowing of fronds can deplete the plant’s energy reserves, especially if done before spore release.
Cultural practices like thick mulch or black plastic can smother ferns by blocking sunlight. For severe infestations, chemical control using systemic herbicides like glyphosate or triclopyr may be considered. These herbicides are absorbed by the plant and circulated throughout its system, targeting the roots for more effective eradication.
However, herbicide application must be done with extreme caution to minimize harm to non-target plants and the environment, always following product label instructions. Consistent monitoring and follow-up are essential, as ferns are persistent and often require repeated efforts.