Dryopteris marginalis will spread very slowly. The clump may get a little bigger, but it won't produce offsets.

Thanks for the reply! So, I guess if the D. marginalis does well and I want more, I'll have to buy more instead of making my own.

Since I originally posted, I have also acquired a Northern maidenhair, Adiantum pedatum. Kind of a shot in the dark, since apparently maidenhairs have a slightly difficult reputation, but we'll see how it does. What I'm able to find is that it might spread and be able to be divided, but probably slowly.

I've also discovered that my planting bed may be a lot shallower than I thought - I don't know what's underneath, but it seems like a concrete slab, about 6-8 inches down. Can't determine if it's under all the garden area. If so, I'll just have to depend on the fact that ferns are shallow rooted -- !

...And now I've somehow got an Autumn Fern (Dryopteris erythrosora) and a Painted Fern (Athyrium niponicum.) Both jumped into my bag at the Rittenhouse farmer's market. The "one each of whatever I can find" non-plan goes against everything I've been taught, design-wise, but the space is really small and I'm rationalizing it as a test to see which plants do well.

There is a Delaware Valley Fern Society, you can look them up on line. Mostly interested in native ferns but they have some avid Fern gardeners, and they run interesting field trips.

Thanks for the recommendation! I guess I'm sounding like a fern enthusiast in the making. Probably I am.

Unfortunately, my physical ailments make field trips very difficult, but I do like to get out to our superb public gardens when I can. I saw a lot of very nice ferns at the Jenkins Arboretum just this past week, which were worth navigating the difficult steep slope. Though I was tempted to try some fern-napping, I got my northern maidenhair fern there through legitimate means at the plant sale.

So far I've got 2 Autumn Ferns, 2 Japanese Painted ferns, 2 Ostrich, 1 Northern Maidenhair, 1 Eastern Wood Fern/Marginal Fern, and I think I'd better knock off the fern acquistions as summer is setting in.

I don't expect any of these to reach their full height potential, since the garden area is underlaid by a concrete slab of some sort, so there's about 4-8 inches of soil in most places. To compensate, I've been mulching very thickly with shredded wood mulch. The soil, what there is of it, is kind of clayey and spongy, but it seems to retain moisture pretty well and has a decent worm population, so I'm hopeful that if I continue to lay down layers of mulch it will build up its organic matter and continue to retain moisture well as we get into the hotter months. Also potentially concerning is the amount of wind that comes through the space - you wouldn't think it, since this little patio is between two solid rows of three-story row houses to north and south, but apparently the east-west winds enjoy the wind tunnel and come through on a regular basis. The fern fronds flap in the wind pretty often, so I guess I'll see which ones do OK despite this.

The ferns I like best so far are the autumn ferns; they look nice almost right away, the fronds are a nice triangular shape and the new fronds are such a pretty color. The ostriches are getting established and continue to unfurl new fronds. I expect them to do well and to spread without too much encouragement. The Marginal fern put on four fronds (and lost a fifth to my clumsy pruners - whoops) and then stopped for the moment - it looks OK, though the fronds are on the thin side. The Maidenhair has put out two small, pale, spindly fronds and hasn't managed any more yet. The painted ferns are also slow to establish.