Terrestrial vs Epiphyte potting medium

[Havoccity]

Whats the difference? As far as I can tell, the only difference is how fine the medium is.

Also, why do terrestrial mixes need to be finer anyways? Do terrestrial roots not do well if the medium is too airy or something?

[Roberta]

Good question!

Think for a moment on how these grow... An epiphyte grows on the trunks or branches of trees, with their roots in the air. They get their nutrients from detritus that washes down from the forest canopy. (so not much) This addresses several factors that are addressed in the basic "how to's" of orchid culture:
Drainage - how well drained is the side of a tree? Very
Air - roots in the air get lots of it, continuous air movement. They get wet in the rain, and dry quickly.
Humidity - that keeps them from desiccating in all that fresh air.

Some epiphytes are more forgiving of drying out than others - the function of pseudobulbs as storage devices. Also, that white spongy coating on roots - velamin - is also a storage mechanism for water.

But this is why, if you can provide sufficient moisture, epiphytes tend to grow very well well mounted, it is just like "home". Growing in pots is a compromise, to deal with the fact that we typically do need to provide more moisture/humidity than we could if the roots were bare. But the medium still needs to provide as much air as possible in the root zone, because that's the environment that epiphytic plants have evolved in.

So the roots of a terrestrial orchid tend to need to stay a lot wetter, air is not such a big factor, likely they get more nutrients because soil holds more, and the roots are in constant contact with that soil.

Short story... epiphytes need "humid air" around their roots. Terrestrials ("normal plants) tend to need constant moisture.

There are many orchids that can grow terrestrially as well as epiphytically, if on the ground likely in leaf litter, And some mostly-terrestrial orchids can grow as epiphytes in extremely wet environments (like the Amazon)

[Subrosa]

Terrestrials vary more in their substrate requirements than epiphytes. In a good greenhouse you can mount almost any epiphyte to a board and with proper watering and fertilization have it thrive, but terrestrials need a suitable substrate, be they woodland, grassland, or bog dwelling orchids. As an example, the substrate you would want for a Cypripedium reginae would kill a Cypripedium acaule in short order.

[Havoccity]

Many thanks guys!

A follow up question if you don't mind. I've noticed that many terrestrial orchids are very heavy feeders when it comes to fertilizing. Would it be okay to use regular potting soil or compost instead then? I know they're used with Phaius, but what about other heavy feeders like Anguloa, Cymbidium, etc?

[Subrosa]

Originally Posted by Havoccity

Many thanks guys!

A follow up question if you don't mind. I've noticed that many terrestrial orchids are very heavy feeders when it comes to fertilizing. Would it be okay to use regular potting soil or compost instead then? I know they're used with Phaius, but what about other heavy feeders like Anguloa, Cymbidium, etc?

For some, definitely. For others you could amend potting soil with sand to get better drainage and be OK. Although they're not strictly speaking terrestrials, I grow Catasetum in a substrate consisting of pure composted cow manure on the bottom with a layer of mulch on top. By the time the roots reach the manure, the plants are ready for all that fertilizer and the plants don't seem to mind the lack of airiness in the substrate. Most of my orchids are bog/ wetland species, and although the true bog dwellers need the low nutrients conditions of a Sphagnum bog, species found in wet meadows, fens, and swamps can live in very rich soil.

[Roberta]

Cymbidium and Anguloa (and related Lycaste) aren't really terresterial. Cymbidiums will grow in sandy soil (well drained) but still need a lot air in the roots zone. I grow them in small bark that stays damp, but drains very well. (My soil is heavy clay) They grow fast, and big so they need more fertilizer than most orchids - I solve the "management" problem by giving the pots a top-dressing of time release fertilizer like Osmocote or Nutricote. I do know people (mostly of a prior generation) who mix composted manure into the medium, But the medium is "almost terrestrial", still airy.

Anguloa and Lycaste are definitely epiphytes. For me, the hybrids do OK in pots with medium/small bark, the species actually do better in baskets with sphagnum - stay damp, but lots of air. They don't grow significantly faster then the rest of the orchids, so don't have any special fertilizer needs.

In general, the big determining factor in fertilizer need is growth rate. A Cattleya may add one or maybe two new growths a year. Slowly. So it really doesn't need much fertilizer. The Catasetinae reinvent themselves every year (new huge leaves, new root system) in a few months... grow at a rate similar to a tomato plant, so need fertilizer to support that. Fertilizer minerals are used when new tissue is created, "maintenance" is by photosynthesis (the carbs)

[Ray]

I am going to disagree with the "heavy feeders" characterization, but I think that's nothing more than a historical misinterpretation of observations.

In order for any plant to add a pound of mass (i.e., carbon fixation = growth) it must absorb and process abut 200 pounds of water but only about 5 grams of NPK fertilizer.

Unlike epiphytes, terrestrial orchids tend to grow in relatively-quick, seasonal spurts, so the "instantaneous demand" for fertilizer might appear to be greater, even though it isn't, overall.

Even more astounding to me is that as plants lose about 95% of their water to transpiration, that "200 pounds" to be processed means the plant actually has to absorb around 4000# of water to gain a pound!

[Roberta]

Just to muddy the waters some more, there are terrestrials and then there are terrestrials. Subrosa pointed out that the meadow dwellers need conditions more like other plants - wildflowers, after all! Then there are the Mediterranean-zone terrestrials (European and Australian) that are native to poor soils and are extremely seasonal, withdrawing to dormant tubers in the hot, dry months of summer characteristic of that weather pattern, emerging briefly when they get a few months with some rain. So there's no "one size fits all" ... learning about and understanding habitats is, at least to me, one of the delights of orchid-growing. A lifelong adventure,