I have seen Russelia equisetiformis (firecracker plant, coral plant) for sale in Iowa once, maybe twice. The ex-job has had them. I don't know if they sold, or if they died really fast, but either way, they weren't around for very long. I have no idea what they'd be like to grow indoors, but they do get a lot bigger than I expected from the (mostly close-up) pictures in the books. I'd think providing adequate light would be a problem.
I've never seen Salpiglossis cvv. (painted tongue) plants for sale, nor can I recall seeing the seeds anywhere. The seeds part could just be me not paying attention, but I know I would remember seeing the flowers.
The commenters at davesgarden.com remark a couple times on the plants' tendency to wilt suddenly for no apparent reason, so it could be that it's difficult for growers to maintain a healthy-looking stock. Where they are sold, they are apparently typically marketed as annuals, though I believe they are actually perennials if protected from cold (USDA zone 8-11).
I encountered one reference to Salpiglossis that suggested its unavailability is mostly a matter of fashion, not a matter of difficulty. Fern also claims that Salpiglossis has come "roaring back" into fashion recently. I can't say I've seen any evidence of that here in Iowa.
While researching this post, I've had a tough time keeping Schizanthus cvv. (butterfly flower, poor man's orchid) distinct in my mind from Salpiglossis; the two names both begin and end with "s," are nearly the same length, and are alphabetically close to one another, so I guess my brain would like them to be the same plant. Having now seen several photos of different varieties of each, though, I believe I can finally tell them apart. If nothing else, there's a clue in the name: schizo- means "split," so that'd be the one with the split petals.
I've never seen Schizanthus around here either, though there are a number of websites referring to growing them from seed, or growing them indoors, so apparently this really is something that people do. I couldn't swear I've never seen seeds for sale.
They're typically sold as annuals, but in mild climates may be short-lived perennials, so maybe it's the "short-lived" part that's the problem? They also need a lot of sun, are slow to start from seed, and at least one site said they prefer cool, humid conditions. Any of those could be a problem; all of them together seems like it would definitively rule them out as indoor plants. And yet, there they are in the books (Crockett and Kramer) anyway.
I've only seen Senecio confusus once or twice; the first time (photographed above) was at the ex-job, and I can't recall for sure whether there was a second time or not. Its identity may be somewhat in flux: I found some sites claiming that it had been changed to Pseudogynoxys chenopodioides. Plant List says that both Pseudogynoxys chenopodioides and S. confusus are accepted names for (I assume) different species, so I don't actually even know what plant we're talking about.
It's another vining plant, that has the potential to get big (vines to 10-12 feet / 3.0-3.7 m long), which is possibly the only reason one needs not to grow it in one's house. That said, it's apparently also really easy to propagate, so it could probably be restarted regularly, whenever it gets too large. It also needs a lot of light.
Solanum pseudocapsicum (Jerusalem cherry; winter cherry; various botanical synonyms including S. capsicastrum) has never interested me that much, even when I saw it in books, but while I was reading through them in preparation for these posts, I noticed that it seemed to come up quite a bit (Kramer, Crockett, Powell). I've only ever seen it at the ex-job. If I remember correctly, that only happened a couple times, and they dropped a lot of leaves shortly after arrival on both occasions.
The consensus at davesgarden.com is that they need cool bright conditions in the winter. They're possibly also not helped by their fairly plain appearance when not bearing fruit (though that doesn't stop people from growing Citrus), their appeal to bugs (not personally experienced, but that's the claim here) or the poisonous foliage and fruit.
Only the Kramer book mentioned Sonerila cvv. (sonerila), and there's remarkably little about them on-line either, so it's entirely possible that they're not missing from retail so much as not yet arrived in retail. I had to go through four pages of Google results to find a single mention of them as a cultivated plant, and that (WARNING: autoplay video/audio) wasn't really what you'd call informative. (Houseplantsguru.com, on page 5 of the Google results, covers most of the same information in a much less chaotic context, and page 6 gives us the French version, which I can't read but possibly some of you can.)
There's near-universal consensus that they're attractive plants (many of the Google results are links to sonerila drawings, paintings, or photography), so I have to assume that there's a problem with them being too slow, or too touchy, for mass production. They do apparently demand a lot of warmth and high humidity, if nothing else.
(I'm fairly certain that I've seen Stanhopea spp./cvv. (no common name?) on the blog of at least one person I follow, but I can't remember who, or where. If I could remember that, I'd link to you, so if you think you might be the person I'm remembering, say something in the comments.)
I've never seen a Stanhopea for sale around here, even from specialty sellers or at the orchid show. Why not? No clue: they're apparently cultivated all the time. The short life of the flowers (~3 days, per Wikipedia), need for cooler temperatures (according to this care sheet, anyway) and odd growth habit (the flower spikes typically grow down instead of up, requiring plants to be hung for best display, and necessitating the use of special pots with holes in the sides) might all be factors.
Trachelospermum jasminoides (confederate jasmine, star jasmine) doesn't seem to be sold around here, though some of the true jasmines (Jasminum spp.) are. Not sure why this would be: a quick look around the internet didn't turn up anything especially damning. They can get to be big, heavy plants, but they restart easily from cuttings, so that shouldn't be a problem, and unlike a lot of the plants on these lists, Trachelospermum doesn't appear to require a cool winter (though cool winters are apparently not a problem, either). And I can't even remember the last time I heard so many people gush so hard about a single plant. (Not that the davesgarden.com people are reliable gushers: they think Aspidistra elatior is a nice plant too.) So there's no huge, obvious reason why Trachelospermum wouldn't sell in Iowa. I might even be interested in a little one, if only to see why it's such a big deal.
I really, really tried with Tolmiea menziesii (piggyback plant, youth on age), but . . . no. Just no. At least some of my problem is that it's too warm in here -- they do prefer cool, humid air -- but I was never able to get watering right either.
It's not like they're even pretty; I'd gotten one because I'd been under the impression that they were fairly easy plants. They grew well in the greenhouse at work, so I figured they'd grow well for me too.
I do still see Tolmiea occasionally -- the ex-job has had large hanging baskets once or twice, and I'm pretty sure I've seen 4" plants at one other place in Iowa City as well -- but it's pretty rare. Fine by me.
Veltheimia cvv. (forest lily, cape lily, bush lily) do still get sold, if you know the right places. Unfortunately, none of those places seem to be in Eastern Iowa. V. bracteata is apparently a very nice houseplant, for people who can provide enough light, and who don't panic at a short summer dormancy. (Which rules me out, twice.)
The Bulb Maven attributes their disappearance to the whims of fashion,1 rather than difficulty providing the right growing conditions. Could be, I suppose, though I have no memories of them ever being widely available.
As I noted, I'm mildly curious about Trachelospermum jasminoides, unless someone can provide a compelling reason for me not to be. Schizanthus or Stanhopea would be my wouldn't-it-be-cool-if plants, from this group, but I don't anticipate ever actually trying them.
- Rechsteineria cardinalis (cardinal flower): have never seen.
- Reinwardtia indica (syn. R. trigyna, yellow flax): have never seen.
- Rivinia humilis (pigeonberry, rouge plant): have never seen.
- Ruellia makoyana (monkey plant, Mexican wild petunia, trailing velvet plant): have never seen in person, though I see photos all over the place.
- Schizocentron elegans (creeping princess flower): have never seen.
- species Schlumbergeras: have never seen in person, though I'm pretty sure I've run into Schlumbergera gaertneri (now possibly Hatiora gaertneri) for sale a few times on the internet.
- Sprekelia formosissima (Aztec lily, Jacobean lily): have never seen, but I ran into a few sellers on-line while writing this.
- Streptosolen jamesonii (marmalade bush, orange browallia): have never seen.
- Tabernaemontana divaricata (crepe jasmine): have never seen for sale. No idea if I've ever seen them in person.
- Tetranema roseum (Mexican violet, Mexican foxglove): have never seen.
- Tetrapanax papyriferus (rice-paper plant): have never seen for sale, though I've seen it on blogs, Danger Garden in particular, so I know they're out there.
- Tibouchina semidecandra (princess flower, glory bush): have never seen.
- Tulbaghia fragrans (sweet garlic, pink agapanthus): have never seen.
- Vallota (apparently Cyrtanthus now; no common name?): have never seen.
- Zephyranthes candida (rain lily, zephyr lily, fairy lily): have never seen.