Rarest cactus species in cultivation

[Driller64]

What are the rarest cactus species currently in cultivation? One of them, I'm guessing, would be Yavia cryptocarpa.

[Aiko]

And Whitesloanea crassa, i think.

[A. Dean Stock]

If you mean "rare" in terms of the number of people that have seen it or grow it, then Opuntia diploursina would qualify as one of the rarest. If you mean the "rarest" in nature then perhaps not.
Dean

[C And D]

I would say Aztekium valdezii

For now...,
I'm sure there are many here that can't wait for their own A. valdezii
and within ~10 years they may be some what common!!! or not...

but in a couple years there will be something rarer, and the next great "ultra rare new species" species will be on all our wish list's.

There is always something new, that what makes this hobby so great.

[SnowFella]

What about the Galapagos endemic species? They ought to be pretty rare in cultivation due to the strict laws they have.
Brachycereus nesioticus particularly I could imagine.

[phil_SK]

Do you mean the dozens and dozens that are hardly ever met in cultivation - Pseudoacanthocereus sicariguensis, Weberocereus frohningiorum, Pereskia marcanoi - things that nobody's heard of and that are not grown much due to their size or uninspiring nature, or rare plants (ie ones that there are very few of in habitat) but that have found their way into cultivation, like Chamaecereus silvestrii, which is thought to be extinct?

[DaveW]

Try and find a genuine Disocactus ackermannii or Schlumbergera truncata and not the hybrids that go around under these names, but a propagation of known habitat origin. Cacti that were discovered generations ago but now so hybridised in cultivation the original authentic collected stock has been lost. Therefore only a few botanical gardens have what could be regarded as genuine material.

Not sure Yavia is all that rare these days, I have had four from different sources over the years since it was discovered. You can get both plants and seed in Europe, but obviously you need to grow from seed if outside the EU. What is rare in one country is often quite common in another.

New discoveries are always initially rare, but whether they remain rare is dependent on how quickly they are propagated and who does the propagation. As Phil says some plants are very common in cultivation but far rarer in habitat, Echinocactus grusonii being an example.

[Saxicola]

I think you need to define what you mean by "rare", much like Dean says. Do you mean a plant that is very rare in the wild, or do you mean the fewest plants in cultivation? People have been discussing the former, but if you actually meant in cultivation I'd bet some really spiny Opuntia or Cylindropuntia would be up there. There are plenty of plants that aren't uncommon in the wild but almost no one actually cares about growing.

I used to work on a group of tropical plants (a genus of ginger) with around 100 species. They are very attractive plants yet only about 10 species could be said to have been grown by decent number of people, and well over half the genus were literally not in cultivation at all by anyone in the world aside from me. Cacti have a larger group of people interested in them but I'm positive that a large percentage of species are grown by no one or a few people at most.

[DWDogwood]

A whole gang of the columnar types are rare in cultivation.
Find a Stenocereus aragonii for instance.
But usually there's only a couple of folks in the market for them and they're just a name on a page in the big picture of things.
What about that "new" Utah Escobaria vivipara v grayensis?
Saw it in a German journal. Nobody's heard of it that I've contacted.

[A. Dean Stock]

Wow! That's a new one for me. I work on Utah cacti and I've never heard of an Escobaria vivipara var. grayenis. More importantly, I've seen Escobaria in every area of Utah and doubt that any population of E. vivipara not already named (like deserti) is worthy of taxonomic distinction. I doubt that it is a described variety.
Dean

[Robb]

I used to work on a group of tropical plants (a genus of ginger) with around 100 species. They are very attractive plants yet only about 10 species could be said to have been grown by decent number of people, and well over half the genus were literally not in cultivation at all by anyone in the world aside from me.

That's fascinating Saxicola, it sounds like a cool job.
Like DaveW said, common plants in some countries are rarer in others. For example, here in New Zealand plants such as Ariocarpus are quite hard to come by, and I imagine Aztekuim valdezii will not become readily available for many years.
I guess some Pygmaeocereus could be considered rare. The genus interests me, and I have found it difficult to come by seed sources for some of the little known forms of bylesianus.

[Steve Johnson]

I can only go by rarity under cultivation here in the US. Pygmaeocereus comes to mind, although I'd also mention Gymnocactus ysabelae and Mammillaria deherdtiana. Aztekium ritteri is an obvious choice for rarity under cultivation -- at the growth rate of 1 mm per year, I'm not sure how many people have the patience to grow A. ritteri from seed.

[DaveW]

What was probably originally meant by the thread was desirable rarities. Many plants are rare in cultivation, as others have said, since they are not considered all that desirable, except to possibly a minority of growers. In the UK rampant Opuntia's are not grown very much as they take up too much greenhouse space, therefore some could probably be considered rare in cultivation here. However in climates where they can be grown outdoors they are probably more common.

The problem regarding worldwide distribution of rarities in future will be CITES if you are one of the collectors that must have ready made plants. At least for now seed is usually internationally available, therefore unless we can find nurserymen willing to raise them for us we will have to all get used to doing our own seed raising as the ready made plant position will not improve in future. Like it or not seedling grafting and then eventually de-grafting looks like the only way of obtaining reasonable sized plants of some of the slower growing rarer species in a reasonable time. If you are lucky enough to find a nurseryman to do it for you they are going to charge you for that effort since there will then be buyer competition forcing up the price, particularly for plants on their own roots, meaning their slower commercial propagation.

[Saxicola]

That's fascinating Saxicola, it sounds like a cool job.

It definitely was. It is the same type of job that A. Dean Stock has. I just took the "fly around the world to exotic locales" route to studying plants while he, as far as I know, took the more sensible "study what is in your own back yard" approach which lets him spend more time studying his plants in the field. A lot of the species I studied I saw only once or twice (some not at all) in the wild. Since no one had ever studied (most) of the genus in detail, ever, outside of herbarium sheets, I was able to increase our knowledge of the plants a great deal. But nothing like actually living in the region where they come from. When you do that you can actually study numerous populations in detail.

It is a long story (which I won't bore you with) as to why I don't do it anymore, but part of it was the difficulty in finding a job doing this in a place I wanted to live.

[phil_SK]

Appendix 4 of the New Cactus Lexicon comprises conservation assessments for the cactus taxa they recognise. These are drawn from a number of sources and, I think it's fair to say, are a bit rough and ready in places and plenty of species are listed as being data deficient. That said, it's still the most comprehensive listing I know of. Their conservative classification will, of course, disguise vulnerabilities of distinct populations that some might think merit species status. They use the IUCN Red List categories.

As far as I can see, none are listed as extinct.
Four are listed as EW:
Cereus estevesii (as CR-D/EW? where the D indicated it's CR because the population size is <50)
Echinopsis chamaecereus
Mammillaria columbiana ssp yucatanensis
Rhipsalis pentaptera (as CR-D/EW?)

For interest, some of the others that have been mentioned:
Pygmaeocereus bieblii VU
P. bylesianus LC
Turbinicarpus saueri ssp nelissae [Gymnocactus ysabelae] VU
Mamm. deherdtiana (ssp d. and ssp dodsonii) LC
Aztekium ritteri VU
Disocactus ackermannii DD
Yavia cryptocarpa CR
Echinocactus grusonii CR (not sure if the new populations were known when this was published)
Brachycereus nesioticus VU

There are plenty of other CRs, including (my selection):
Cleistocactus winteri ssp winteri (but not ssp colademono which is VU)
Cylindropuntia hystrix
Echinocereus ferreirianus ssp lindsayi and E. schmollii
Eriosyce aspillagae, laui, megliolii
Escobaria cubensis
Mamm herrerae, sanchez-mejoradae, schwarzii
Matucana madisoniorum
several melocacti
Opuntia chaffeyi
Thelocactus conothelos ssp argenteus
16 of the 36 Turbinicarpus species or subspecies they recognise, although there are, unusually for the NCL, lots of ssp recognised.

There are bound to be many more amongst the DD listings.