Understanding the Relationship Between Orchid Media & Water A simple experiment

In Orchids
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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 Capacity 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. Which makes sense…if you have a half-glass of water and a full-glass of water…which would would take longer to fully evaporate? Probably the latter, right?

Critical Questions:

  • Which orchid media holds the least vs. most water?
  • What effect does that total water-holding capacity have on the dry-out rate?
  • Which potting media dries the fastest vs. slowest (or too fast in a dry climate vs. too slow in a humid climate)?

 

Experiment / Process

Roughly 1/2 cup of each media was soaked in water for 30 mins; then they were removed and weighed (subtracting the original dry weight to calculate the total water held). Repeated checks were taken hourly for the first day and then three times a day for the following 3 days. 

 

Results – Click to view

Answer #1: How much water can each media type hold when fully saturated?

  • 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

What is the dry-out rate of different media?

From these results, we can see LECA holds the least amount of water, while sphagnum holds the most. Sphagnum holds nearly 4x as much water compared to LECA, and nearly 3x as much water as pine bark. The blended mix leveraged both the water-retentive quality of sphagnum and bark, but also had some pumice which gave the media structure for airflow leading to a faster dryout rate; however, it still initially held about 1/2 the amount of sphagnum alone and nearly double that of LECA.

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 me, sometimes bridging that common sense with my understanding of new topics helps me 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.

With that, remember to always test your theories, don’t accept what other growers say as fact—you might be surprised what you testing things for yourself. Above all…enjoy the process of learning about plants, it’s how you’ll become a better grower 🙂