Underpotting orchids

[ArronOB]

I have read many times that some orchids (especially oncidium types) like to be underpotted, in the sense of being restricted to a small pot.

My question is: Is it simply the case that their preferred conditions of water and air movement are more likely to be found in a tight pot, or is there some other mechanism at play. Hormones or something?

Cheers
Arron

[Ray]

Just supposition on my part here, but based upon observation, it appears that plants prefer to be well mechanically anchored, and it does vary. Vandas don't seem to mind hanging out in free space, while others seem to prefer "cast in concrete" stability, or do not thrive, or almost certainly won't bloom.

Phalaenopsis would be somewhere in-between: when grown in pots large enough that the aerial roots can reach into the medium and act as "guy wires" to stabilize the plant, they stop producing more aerial roots.

I put an Onc. Sharry Baby clone from a 2" pot into a 12" S/H pot as an experiment. It grew for several years with no blooming, even though the newest growths were definitely mature enough, but once the pot (18" tall) was full of roots - BOOM!

[estación seca]

Nobody restricts their roots in habitat, and they don't grow in pots in habitat. I suspect the tiny pot thing has to do with the fact we grow most of them under very unfavorable conditions compared to habitat.

The chief things lacking in most people's grow areas are adequate air circulation and humidity. Heat is too expensive in winter and air conditioning in summer for really good air exchange, and this leads to stagnant, wet air - so roots don't get the air they should.

[ArronOB]

Originally Posted by estación seca

The chief things lacking in most people's grow areas are adequate air circulation and humidity. Heat is too expensive in winter and air conditioning in summer for really good air exchange, and this leads to stagnant, wet air - so roots don't get the air they should.

Yes, but shouldn’t the stale air problem be worse in a tight pot ?

[aliceinwl]

Speculation on my part, but it seems that most of the orchids that like tight pots are epiphytes or lithophytes. In nature these plants are going to be growing in minimal substrate and roots are going to be adhering to or weaving into nooks in rocks or bark.

When overpotted, the issue seems to be with interior portions of the substrate retaining too much moisture. I think the tight pot, is one way to force growers not to use too much substrate. It also ensures roots have adequate air exposure: in a big pot the roots would be buried in the interior.

In terms of pots restricting air flow, many orchids do need extra ventilation: slitted sides, clay, wood baskets, net pots, etc. Sometimes one can get around this with a very coarse substrate in a traditional pot, but usually size plays a big role: smaller pots tend to have proportionally greater surface area that facilitates the rapid evaporation required. When these orchids get to the point where they’re all root in the pot with very little substrate, this is probably the closest approximation they can get to conditions in the wild (while potted).

I agree that tight pots are our attempt to make a bad situation livable. I would wager that if these tightly potted orchids were grown under their preferred natural conditions they would out perform their potted brethren.

[estación seca]

Maybe we're using the word tight in different ways... to most gardeners "tight" refers to a pot barely able to fit the roots. It does not refer to cramming potting medium into the pot as tightly as possible.

Decades ago orchid growers used osmunda fern fiber as medium. This was, indeed, packed into the pot as firmly as possible with a stick. But it isn't available any longer, and people no longer pot like this except when growing in long-fiber sphagnum moss.

I suspect most of these plants "preferring" tight pots would do well in packed sphagnum if watered properly. I may pot one of the tiny Tolumnia hybrid meristems I bought at our October meeting in packed sphagnum to see what happens. A lot of Oncidium hybrids are sold nowadays in sphagnum moss, but in the ones I have seen it is not packed very tightly.