To Bury or Not To Bury

By far the most important factor in deciding where to position a tuber is where the plant itself locates its own tuber.  An epiphyte like Sinningia douglasii will always have its tuber exposed.  A hillside species like S. hatschbachii will almost always have some portion of its tuber free of the soil.  On the other hand, the "meadow species" like S. curtiflora and S. tubiflora, which usually grow in flat areas in full sun, have tubers well down in the ground, to protect them from heat.

Another factor is esthetic.  A tuber such as that of S. ramboi demands to be exposed to view.  S. cardinalis, which does not normally have an attractive tuber can be buried or exposed depending on the quality of the tuber.

Another possible factor is the presence of perennial stems.  If buried, the base of such stems may generate roots.  This could be true of plants with persistent living stems, such as S. reitzii, and of plants which retain the stubs of previous years' stems, such as S. guttata.  I have not yet tried growing either species with its tuber completely buried, but it could be a worthwhile test.

Species which bloom directly from the tuber (e.g. S. defoliata) should not be buried.  There is no sense in making it difficult for the flowers.

Tuber Position Table

This table contains my deductions and experience about where to position a sinningia tuber.  Question marks mean I do not have enough recent experience with the species to make a recommendation.

Species

Tuber Position Notes

S. aggregata ?
S. aghensis This species, like S. sp. "Ibitioca", does not seem as cold-tolerant as most other sinningias. Therefore, if growing it outdoors, covering the tuber with at least a little soil might be a good idea.
S. allagophylla This is one of the "meadow species".  When grown from seed, it positions its tuber well down into the soil.  Therefore it seems logical to place its tuber below the surface.
S. amambayensis This species normally buries its tuber.
S. araneosa My experience with this species is contradictory.  If something works for you, stick with it.
S. barbata This species grows in forest litter.  Its "tuber" is a thickening at the base of the stem.  This structure should be underground.
S. brasiliensis ?
S. bulbosa It is hard to imagine how a species with a massive tuber like this one would normally have it completely underground.
S. bullata This species grows on hillsides and cliff faces.  Erosion usually exposes a substantial portion of the tuber.  The tuber is not particularly attractive, so exposing it is a matter of preference.
S. calcaria This species grows on hillsides.  Erosion usually exposes a substantial portion of the tuber.  I have not tried burying the tuber completely, but most of the tuber is underground.
S. canescens ?
S. carangolensis ?
S. cardinalis This is not a fussy species.  Over or under, it will still thrive.
S. cochlearis ?
S. concinna This miniature is probably best grown with its tuber just beneath the surface of the growing medium.
S. conspicua Peter Shalit has written that the stem-tuber joint of this species is weaker if the joint is above the soil line, which argues for burying it slightly.
S. cooperi This species is mostly epiphytic.  Since it is used to having its tuber exposed, growing it with the tuber exposed seems logical.
S. curtiflora This is one of the "meadow species".  When grown from seed, it positions its multiple tubers well down into the soil.  Therefore it seems logical to place them below the surface.
S. defoliata This species has flowerstalks emerging directly from the tuber.  One suspects that it grows in nature with the flowering part of the tuber exposed, and it seems logical to grow it that way in cultivation.  The tuber will sometimes form a secondary tuber or two, so leaving room for expansion is a good idea.
S. douglasii This species is mostly epiphytic.  Since it is used to having its tuber exposed, and since the tuber is quite attractive in a muscular sort of way, there is no reason not to grow it with the tuber well exposed.
S. elatior This is one of the "meadow species".  When grown from seed, it positions its tuber well down into the soil.  Therefore it seems logical to place its tuber below the surface.
S. eumorpha Peter Shalit has written that the stem-tuber joint of this species is weaker if the joint is above the soil line, which argues for burying it slightly.
S. gigantifolia The plant extrudes its tuber, in part as a thickened stem.
S. glazioviana This "river species" positions its tuber underground, and I think it should be kept that way.
S. globulosa My seedling positioned the tuber half above the soil line.  The tuber is dark and interesting, so I keep it in that position.
S. guttata I grow this species with the top of its tuber at the soil line.
S. harleyi ?
S. hatschbachii This species grows on hillsides.  Erosion usually exposes a substantial portion of the tuber.  The dark red-brown tuber is attractive, so it is worth growing exposed.
S. helioana This species has flowerstalks emerging directly from the tuber.  One suspects that it grows in nature with the flowering part of the tuber exposed, and it seems logical to grow it that way in cultivation.  Why make it any harder for it to flower?
S. hirsuta See S. kautskyi.
S. iarae This species has one of the most attractive tubers.  The conical top suggests that it is exposed in nature.  Grow it that way.
S. incarnata This is one of the "meadow species".  When grown from seed, it positions its tuber well down into the soil.  Therefore it seems logical to place its tuber below the surface.
S. insularis I grow it with about 1/4 the tuber above the soil line.
S. kautskyi See S. hirsuta.
S. lateritia ?
S. leopoldii Tuber partially exposed.
S. leucotricha This species forms substantial tubers with age, which argues for letting more of the tuber show above the soil as the plant gets older.
S. lindleyi This unique species has a thick stem which seems to function like a tuber.  Sometimes it's perennial, sometimes it isn't.  At any rate, burying it is out of the question.
S. lineata This species forms very large tubers with age, but they are flat gnarly disks.  We saw some in Brazil in 1999, buried in riverbank mud, with just the top surface showing.  I don't think it hurts to raise the tuber, but there isn't much advantage in it.
S. macrophylla This species is closely related to S. speciosa.  Perhaps its tubers should be treated in the same way.
S. macropoda This species forms substantial tubers with age, which argues for letting more of the tuber show above the soil as the plant gets older.
S. macrostachya This species forms substantial tubers with age, which argues for letting more of the tuber show above the soil as the plant gets older.  In this case, burying the tuber to bring soil into contact with the stem bases might be a mistake.
S. magnifica This species blooms in late fall and winter.  Burying the tuber might help to ensure bloom.
S. mauroana This species blooms in late fall and winter.  Burying the tuber might help to ensure bloom.
S. micans I had two tubers, grew them partially exposed, and one year neither one came back.
S. muscicola This miniature is probably best grown with its tuber just beneath the surface of the soil.
S. nivalis ?
S. nordestina This species buries its small tubers.  Nothing is gained by exposing them.
S. piresiana Because its tuber is attractive, this species should be grown with the tuber partially exposed.
S. polyantha I grow this with its tuber partially exposed.  In nature, it grows in sandy soil near the ocean.  I believe that when we saw the plant, the tuber was completely covered, but that was ten years ago.  My memory of what happened ten minutes ago is not reliable.
S. pusilla This miniature is probably best grown with its tuber just beneath the surface of the soil.
S. ramboi In my opinion, the most beautiful tuber.  Don't bury any more of it than necessary!
S. reitzii This can be grown exposed or not.  Its stems are perennial, so it is possible that they will develop roots if their bases are buried. 
S. richii This species grows in forest litter, so its tubers should be buried, but not deeply.  Room should be allowed for expansion, as it will make satellite tubers if given space to do so.
S. rupicola ?
S. sceptrum This is one of the "meadow species".  When grown from seed, it positions its tuber well down into the soil.  Therefore it seems logical to place its tuber below the surface.
S. schiffneri No problem here: it doesn't have a tuber.
S. sellovii It is hard to imagine how a species which eventually develops a massive tuber like this one would normally have it completely underground.
S. speciosa According to Ingrid Lindskog, the traditional Swedish way of growing S. speciosa is with a shallow layer of soil over the tuber, because it makes roots all over its surface.  Given my history with this species, you should listen to her and not me.
S. striata ?
S. sulcata My guess is that its tuber position preference would be similar to that of S. sellovii, but having never successfully grown this species, I can't be sure.
S. tuberosa This species has flowerstalks emerging directly from the tuber.  One suspects that it grows in nature with the flowering part of the tuber exposed, and it seems logical to grow it that way in cultivation.  Why make it any harder for it to flower?  My seedling has positioned its tuber so that it projects well above the soil line, so I have no intention of putting it deeper into the soil.
S. tubiflora This is one of the "meadow species".  When grown from seed, it positions its tuber well down into the soil.  Therefore it seems logical to place its tuber below the surface.
S. valsuganensis I grow it with the tuber mostly buried.
S. villosa ?
S. warmingii This species grows relatively tall, but may not be one of the "meadow species".  I grow it with the tuber top at or just below the surface.
S. sp. "Black Hill" Like S. reitzii, this can be grown exposed or not.
S. sp. "Ibitioca" This species, like S. aghensis, does not seem as cold-tolerant as most other sinningias.  Therefore, if growing it outdoors, covering the tuber with at least a little soil might be a good idea.