Are Sword Ferns invasive in Zone 5?
Hi Marsha, which is the plant you're referring to as Sword Fern? There are two that go by this name; Nephrolepis, and Polystichum munitum. The first is the Boston Fern, and is not hardy for the colder areas and is usually grown as a lovely houseplant. I'm thinking that this is not the plant you're referring to.
Polystichum munitum is the native fern of high shady evergreen forests. It requires lots of organic matter, moisture and full shade.
I would seriously doubt if it would ever be referred to as 'invasive' - most people love it and want to grow more of it, as it's one of the few plants that actually thrive under big evergreen trees like the Giant Redwood (Sequoia giganteum).
Any plant that has really specific requirements like these lovely ferns will never have the chance to become invasive in other situations.
These plants are hardy in any zone where they would get adequate moisture, and they will tend to go dormant and lose their leaves if it's too cold for them.
Freezing temperatures for an extended period in the winter can burn the leaves, but they will replace the foliage with fresh new ones, emerging from the center in the typical rolled fiddle head format; these are not edible however.
Hope that helps ease your mind about these great plants!
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