You know how there are some people who say, “_______ is the BEST way to grow orchids”, when referring to mounting orchids, using bark, sphagnum or semihydro? And then someone else will say, “no, that doesn’t work, I kill everything in ______” and then someone else will pipe in and say, “everything works differently in your individual climate”
It’s true! Your climate (how dry/most, warm/cool, breezy/stagnant, etc) will affect the rate of evaporation and how fast a media and your orchid’s roots dry out. But maybe there are some nuggets of science that can help us all understand why one method or potting media works perfectly for one person’s collection, while that same method could be catastrophically-bad for another person’s orchid collection…
Testing Water-Holding of Different Media
I ran a pretty simple experiment to test the water-holding capacity of different orchid media and then tracked the subsequent dry-out rate in my climate. The results were pretty interesting and it turned out that media which held the least water dried the fastest while media that held more water, would stay moist longer.
Critical Questions: Which orchid media holds the least vs. most water and what effect does that total water-holding capacity have on the dry-out rate? Extrapolation: which potting media dries the fastest vs. slowest (or too fast in a dry climate vs. too slow in a humid climate)?
How much water can each media type hold when fully saturated
* Roughly 1/2 cup of Media by volume was used and soaked for 30 mins
- LECA: 27.4g of water
- Pine Bark: 37.02g of water
- Blended (Fir bark, pumice, sphag): 50.77g of water
- Sphagnum: 102.4g of water
From these results, we can see that LECA holds the least amount of water, while sphagnum holds the most. Sphagnum holds nearly 4 times as much water compared to LECA, and nearly 3x as much water as pine bark. A blended mix leverages both the water-retentive quality of sphagnum and bark to hold about 1/2 the amount of sphagnum alone.
If we think about this data, it might be clear why some people love sphagnum, while others love LECA! If your climate has a low humidity (under 40%), then LECA (especially the top layer) could go bone dry within in a day or two. Conversely, if your humidity is high, then saturated or old / compacted sphagnum moss might be too wet even after a full week—which is bad because wetness and compaction are favorable for some fungal pathogens like fusarium and may lead to root rot.
The value of these media types may change according to the needs of the orchid too. Some species need fast-drying conditions and well-oxygenated roots (like vandas and “classic phals”), while others (like phragmipediums and coelogyne) may require continuously moist root and suffer if the roots become too dry.
This is why I use a blended mix. I can customize the dry-out rate based on the needs of the orchid in combination with variables like my climate. If I have an orchid that needs very moist roots, I may use pure sphagnum (or a lot of sphagnum in the blend); if I have an orchid that needs moist but airy roots, I’ll use a bit of sphagnum but also add bark and perlite; if I have an orchid that has roots which must dry quickly, then I won’t use any sphagnum at all and instead I’ll use large chunk bark (or even wine corks) so that air can flow freely though the media (increasing the evaporation rate). You can check out all of the potting mix “recipes” I use for my orchids here.
This type of simple experiment is probably a bit of a no-brainer. It might seem like common sense, but for a lot of us sometimes bridging that common sense with our understanding of new topics helps us go, “ah haaa!” It also helps illustrate the value of testing and analyzing things so that we can become better growers and I hope this simple comparison gives others (new growers especially) some perspective on potting media options and their relationship to your climate and plants. It’s also illuminates why some things work for one grower, while they might not work for you…it likely just comes down to your conditions and variables such as media water retention, evaporation, and humidity.
If you want to see the rest of the results, check out this google sheet. I’ve tabled the timed readings and included some graphs that illustrate the dry out period over time.
Anyways, remember to always test your theories, don’t accept what other growers say as fact and enjoy the process of learning about plants. This is how you’ll become a better grower 🙂