Thanks to Molly K. for another question she sent to me a bit back that I am now getting around to answering...
Is it possible to grow ferns from spores right in a terrarium? Would it have to be an open terrarium?
It is absolutely possible to grow ferns from spores- but it is not always easy to achieve. You can collect fern spores by putting a fern frond in an envelope and waiting for the spores to dry and fall off. Sow the spores by sprinkling on top of sterilized damp soil mixed with some sphagnum moss, with a drainage layer below, in a container with a lid that can be removed or propped a bit for air circulation. I have found that misting the spores with distilled water can water the spores without washing them completely out of the soil. Fern spores require about 2-6 weeks to germinate and are extremely delicate in their "baby" phase.
With that said I have had pretty bad luck germinating ferns from spores but am always collecting spores with the hopes of trying yet again. I have successfully sprouted ferns from spores in incubator trays but they always seem to fail eventually (most often when I am trying to "air" out the incubator container by removing the lid and the baby ferns always dry out). The only successful spore-grown fern I had was a maidenhair fern (a maidenhair is pictured above but this is NOT the one I germinated from spores- it never looked that good!) inside a large wine jug terrarium. It was doing great for a while but was very slight and sickly and eventually died when I tried to relocate it elsewhere in the same terrarium.
If you are willing to dedicate a terrarium to fern-spore germination it does provide a suitable environment for the process, but it can be maddening staring at an empty terrarium for 6 weeks wondering if the spores will germinate or not. If it looks like nothing is happening I encourage you to add other plants but you never know- those ferns may pop up some day!