Pure sphagnum moss is a popular choice for potting media—and it’s a good option…but it’s not flawless. Compared to peatmoss, sphagnum is sustainable because it’s the dried and processed new growth of moss (which only takes a few-year cycle to create) rather than the peatmoss underlayer of a bog (which takes centuries to accumulate). Sphagnum moss is especially ideal for lithophytic or epiphytic plants like orchids and aroids which need moisture at the root zone, but also lots of airflow (because their roots normally grow attached to trees rather than in the ground). Sphagnum is so water retentive that it holds around 18x it’s dry weight in water—that means a cup of sphagnum moss can hold about 3/4 cup of water—amazing right!?
The thing about sphagnum is that when you use fresh new sphagnum it initially has structure and porosity, but over time that sphagnum degrades and essentially becomes peat—it fragments, compacts and the properties change as it ages. Also, the more consistently wet you keep sphagnum moss, the faster this degradation will happen. In lay terms…the moss rots and becomes composted as yeast, other fungi and bacteria break down the cells.
This process has implications for growers and I often see people glowing over how amazing their plants are performing after 1 or 2 months rooting in sphagnum…but there are some things to be wary of…
Why pure sphagnum moss can become a bad choice for plants
After an extended period of time (8…12…16 months), you have a few big challenges to deal with. Those are:
- The root zone becomes acidic. This is bad for some species of plant which come from alkaline habitats—like Alocasia Dragon Scale, Paphipedium Orchids, or Philodendrons. Excessive acidity can kill their roots. You can add oystershell to buffer this drop in pH and prevent acidification…but if you’re not aware of this problem, then you’re probably not adding oystershells, right? AND you shouldn’t add oystershells to all potting media, a good majority of plants are not from alkaline habitats and they do need some acidity (but not too much!)
- The media decays and the root zone becomes anaerobic – this happens as the moss starts to fragment, compact and the air pockets plug up. This process eventually limits airflow, especially when saturated after a heavy watering, then bacteria and fungi use up the available oxygen making the root zone anaerobic! When conditions are anaerobic like this your potting media becomes perfect environment for fermentation!Think for example…how sauerkraut is made…under anaerobic conditions! You take fresh cabbage, you put it in salt water to kill off dangerous pathogens, cover the cabbage underwater in an air-restricted container, and yeast (under those anaerobic conditions) turn the sugars in the cabbage into lactic and acetic acid! And when you pickle cabbage like this, the acidity drops to 3.5pH or lower. Acidic conditions like this will kill roots and THIS is often what growers incorrectly refer to as “root rot from overwatering.” It’s the exact same process that happens when you use peatmoss or peat-based soils without adding enough drainage materials (like pumice, perlite, rock, or sand) to give the potting mix structure, porosity and airflow. It looks like root rot from “overwatering”, but the root-zone just became anaerobic and THAT is what killed the roots…not the water!
- To avoid both 1 and 2, you need to repot more often than regular potting media and you must also remove most of the old media—which…good luck doing without damaging all of those beautiful roots on particularly rooty plants like Alocasias or Philodendrons.
Sphagnum moss still has it’s value for growers—I use it often! And I’m definitely not saying “you shouldn’t use sphagnum moss.” If you LOVE using pure sphagnum just the way it is, then go ahead and you do you! But…if things change and your beautiful roots suddenly turn to mush, you’ll understand the mechanisms that changed and/or you’ll know what to watch out for.
Is sphagnum moss is a bad choice for growing plants?
No…it just means the success you see early on may not be representative of the full-term cycle of this specific media. It also means you need to be aware of what to watch for, avoid, or what to do if you’ve had a plant in sphagnum for over a year.
Are there workarounds to make sphagnum last longer?
Yes. You can add structure to your sphagnum, which can help slow compaction—or you can use it as a component of a potting mix so that you’re gaining some water retention, but still allowing for good oxygen movement after many months—though that’s pretty much two ways of saying the same thing. With a more airy media, just beware that you’ll likely have to water a bit more often because with more air movement and less moss, the media will hold less water and it will evaporate a bit faster.
I mix at least one part large-chunk perlite or pumice to one part sphagnum moss, OR I’ll use sphagnum moss as a top-dressing above bark-based media to help localize moisture retention at the top of the pot (where new roots start) without putting it down throughout the root zone!
Photo of Paphiopedium Orchid with Sphagnum Top Dressing
(which keeps the base of the plant moist, where new roots emerge)