Answer: Hardy ferns are usually propagated by division when they are offered for sale. In nature, they are also propagated by spores rather than by seeds. It is rather difficult to provide the conditions the spores need to thrive and produce plants, so most people purchase small plants instead.
In addition to ferns, there are a number of plants used as greenery in cutflower arrangements and many of them are tropical plants. You might take a look at a houseplant supplier and see if they have anything that would look right in your arrangements.
I'm sorry I can't be more helpful.
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