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Is that fern a native nephrolepis or invasive imposter?

By Staff | May 8, 2019

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(Editor’s Note: This is part of a series of articles by members of the Sanibel Vegetation Committee dealing with vegetative matters of concern to island residents. For the other articles in the series, visit “http://www.mysanibel.com/Departments/Natural-Resources”>www.mysanibel.com/Departments/Natural-Resources.)

At the time of the city’s incorporation, the Sanibel community decided that our island shall remain a sanctuary island, living in harmony with the island’s wildlife and natural habitats. The community works to promote native plants, not only because our native animals are dependent on them, but because native plants require less maintenance. Native plants are adapted to the local climate and therefore, do not need to be irrigated after they are established, and do not need fertilizer or pesticides to thrive. All characteristics that promote a healthy ecosystem.

Of course, many plants that are not “native” do well here. We refer to them as “exotic” or “non-native.” We define a plant as “native” when a plant is documented as growing in Florida before the explorers arrived in 1500s. Exotics have been introduced since the 16th century and many of them are colorful additions to our native yards. We need to be aware of the exotics that have been proven to change the composition of plant communities by displacing native species, altering the ecological balance and hybridizing with natives. These plants are called “invasive exotics.”

Florida has documented 80 invasive, exotic species that are disrupting native plant communities and 86 species that have the potential to be disruptive (www.fleppc.org).

Often invasive, exotics can be hard to tell from their close native relatives. Sanibel’s sword or “Boston” ferns are an example of that dilemma. Boston ferns have been admired and valued for decades. They can be seen in homes and public buildings almost everywhere. Native sword (or Boston) ferns, cultivars and exotic sword ferns were marketed and sold as landscape and house plants.

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The native Boston fern (Nephrolepis exaltata) and the invasive, exotic Boston fern or tuberous sword fern (Nephrolepis cordifolia) are two sword ferns that have been planted on Sanibel that can be particularly difficult to separate. The frond on the left in the side-by-side photo is the native Boston fern. It grows up to 4 feet in length and is much more flexible appearing to “weep.” The invasive tuberous sword fern, on the right, is shorter growing only 3 feet long and is stiffly erect.

The upper leaflets or pinnae from the invasive tuberous sword fern are 2 inches long and .25 inches wide. They are mostly straight but can be curved with a blunt tip. They are very closely spaced on the frond. The lower pinnae of the native Boston fern are 3 inches long and .50 inch wide. They are slightly curved becoming sickle shaped, narrowing to a point at the tip. The pinnae of the native Boston fern have more space between pinnae (.25 to .5 inches).

The invasive, exotic tuberous sword fern (N. cordifolia) continues to be sold in the landscape and nursery trade and is rapidly spreading into native plant communities. It is now found in hammocks, marsh edges, flatwoods and conservation areas. The Florida Nurserymen and Growers Association encourages a phase-out of the tuberous sword fern from the landscape market. On Sanibel, we can choose not to landscape with these ferns and to find native alternatives for our yards. Native alternatives include: peperomia (Peperomia obtusifolia), white plumbago (Plumbago zeylanica), coontie (Zamia integrifolia) and wild coffee (Psychotria nervosa). By choosing to plant native, you are choosing to maintain the natural balance of our valued ecosystems and support the wildlife that make Sanibel their home.

Where can I learn more about native plants on Sanibel? The Vegetation Committee hosts free plant walks from November to April at City Hall to view and discuss the use of native plants. Everyone and their questions are welcome.

For more information, visit www.mysanibel.com/Departments/Natural-Resources or contact the Natural Resources Department at 239-472-3700. Photos of the invasive exotic plants “Worst of the Worst” and the city’s “The Alien Invasion” brochure can also be found on the website.

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