I assume it never gets wet. The process of moistening the center of a really tight ball of moss is to soak it. For a long time. Spraying and misting will never fully wet a really tight packed moss ball.

I admit I am confused about how it works with air exchange

This professional catasetum grower has moist sphagnum, and packs it like he does in the video : Click Here

That's pretty much my method.

I use besgrow: Spagmoss Sphagnum Moss - At the Root of Healthier Plants | Besgrow

---------- Post added at 06:26 AM ---------- Previous post was at 06:21 AM ----------

Me too, but with dry spag.

---------- Post added at 06:30 AM ---------- Previous post was at 06:26 AM ----------

Here's a bit of pure speculation for you. I think tightly packed spag can't expand with water in the same way as loosely packed spag.

A thought experiment would be to take a single dish sponge and put it into a cup or aluminum can (stronger than a cup) and get it wet. Then put three sponges in the same sized vessel and wet them. I bet the inability for the three sponges to expand would mean they absorb less and dry more quickly. Yet there is still a ton of air space in the "mix."

Maybe it's kind of like - loose sphagnum probably allows for pools of water to be formed among the media. That is, when enough water gets on the bits of the loose sphagnum segments, the water and loose sphagnum combination can form localised layers or pools/clumps of water through out the pot region. And when you have some (or a lot) of these water clumps, then it becomes a bunch of pools.

Pathways, directions and speed of aerated water movement in the media and pot will be affected by the pool regions. Perhaps the more pools there are, or the bigger these pools are, the more it affects the movement (or no movement) of oxygenated (aerated) water, which the roots (or parts of roots) need to survive.

The non-uniformity of this loosely-packed water soaking media could mean significant variability (variations) in aerated water moving activity - some regions might have slow water movement - or hardly any aerated water movement.

But - when the sphagnum is firmly packed and the medium approaches a tighter network or has more uniform water spreading and propagation (movement) properties throughout - working collectively as a wick, then aerated moisture/water can keep propagating (at some desirable rate) without getting hung up too much in any particular region - provided the amount of water we add each time (when it is time for watering) doesn't overload the system - in which case - drowning of roots could still occur.

When packed firmly, the water that is being evaporated (away from the surface of the pot and through holes etc) probably pulls/draws the water from the inner regions (of the packed sphagnum) much more uniformly than loose sphagnum - maintaining some 'suitable' rate of water movement throughout the pot. And - as we know - moving water (even slowly) that contains oxygen, will sustain the roots. But water moving at too slow a pace, or hardly any movement at all (if any medium gets water-logged or saturated) can result in roots somewhere in the pot running out of oxygen, and dying.

The key factor is - adequate oxygenated water movement. The roots generally occupy a fraction of the region of the pot/medium. So we can assume that there will be lots of oxygen carrying water in the medium. The main thing is that the oxygen carrying water in the various regions of the pot keeps moving adequately around any roots regions, and also keeps moving adequately in general - to avoid stagnation. Firmly packed sphagnum can provide the necessary conditions for this needed water movement.

If there is pooling of water (eg. water-logged loose packed sphagnum), then the movement of oxygenated water in the medium might have more variability in speed and directions of movement. Sections of roots in certain regions might use up all the oxygen where oxygenated water movement is very slow (or none at all) - and the roots could take a bad turn after a while.

It could also turn out that loosely packed sphagnum is fine too. It likely depends on how much water we add (and how we apply it - such as controlled spray doses or pouring it into the pot etc), and how we control the system - so that water logging in any region(s) of the pot doesn't occur.

It may be harder to get things right (or do what we need) with loosely packed sphagnum - such as working towards keeping the bulk of the sphagnum moist (without water pooling, without saturation of any region) when watering the media.

Also, good air-movement in the growing environment is likely to help the movement of oxygenated (aerated) water along (better than still-air environment), as well as providing other healthy benefits for the orchids.

We might also need to consider what happens if the medium has some pathways of aerated water movement slow down considerably, or no movement at all --- maybe the possibility of some kind of unwanted bacterial activity or some kind of unwanted fungal activity in certain regions or pockets of media.

One other consideration - pot contents isn't going to be just sphagnum. After a while, there's going to be lots of roots. So how the water will move among roots plus sphagnum is something to consider as well.

In short, when inside the pot, we don't want sphagnum to have water looking watery - like this: Sphagnum - Wikipedia

just my 2c, I am still experimenting with sphag. I purchased a few nice standard cattleya divisions from notable west coast growers Steven Christofferson and OV orchids and saw they both use sphagnum (and sometimes with plastic pots). I asked both of them since I had previously been using a standard coconut husk or bark mix.

Slightly different but overall similar take home message: I was told to wet the sphag and to really squeeze the water out, and then pack it tight. (Also never to overwater, when in doubt wait a day or two). I like the fact that, instead of watering at fixed intervals or guessing when to water, now I let the plant tell me. Yes conditions will dictate how fast the pots dry but their growth patterns really impact how quickly they dry out. (I only use clay pots).

I read a Chadwick article about the benefits of sphagnum on cattleyas although they caution about using it on pots bigger than 4".

Knocked a few plants out of their 5" pots lately, root growth seems quite good overall. The top was crusty and algae laden but the sphagnum on the inside looked remarkably fresh. I think it is a world of difference using squeezed sphagnum packed tight versus the loose sphagnum which really soaks up water imo.

Jmoney -
First, Welcome!

Rather than getting hung up on the details of "which medium" and wet vs dry, packed vs loose, etc... take a step back and examine "what" you're trying to achieve. The "how" will vary depending upon your conditions. For Catts, no matter the medium or container the critical factor is being able to dry out between waterings, and lots of air in the root zone. So it depends on your ambient humidity, temperature, whether you are using plastic pots or open baskets, how frequently you water, etc. There is not one right way... Personally, because I am an enthusiastic water-er and mostly grow outside, I need to make sure that I have an environment where the plants can dry out. My best results come from using large bark, wood (best) or plastic (OK) baskets, and hanging the Catts, especially the larger ones (which exposes them to the breeze) Sphag wouldn't work well for me at all for that group. (It works great for orchids that need to stay damp like Oncidinae and PLeurothallids and Maxillarias) But if you live in a drier climate and/or don't water often, it might work for you.

I gather, from reading bits and pieces of this long thread, that you are mostly talking about catts. I use Roberta's method for most every other type of orchid. I have very good success mixing a touch of any bark, cut up corks etc. with my spagh. I wait to water until the top is crusty, then I drizzle water. If you pour it on, you'll be wiping the floor. For my oncids and phals, I will water from below about half the time. I have failures but, never from root rot. Also my 2c.

I think that's a good approach. I've definitely seen very successful cattleya seedling growth in orchid nurseries ---- with firm-packed spaghnum. As long as the spaghnum doesn't get super saturated, I really do think what people say is true ---- and makes sense ---- as in the firm packed spaghnum behaves like a sponge, and the water in there can get distributed around ------ and not form gluggy regions that could drown or smother roots.

Thanks Roberta! I have been using sphagnum for years but in the beginning it was very much a love/hate. (back then it was mostly phrags). Plants really perked up and put on a growth spurt, but after a year or so invariably they would decline. They were potted "loose" and when I finally got around to repotting it was a slimy mess with dead roots. I chalked it up to rapid breakdown of the moss and needing to repot earlier.

under my conditions the tight packing method produces totally different results. Not saying one is better than the other, and clearly depends on one's growing conditions and watering habits, but just saying the two are night and day in terms of properties...