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 more sustainable because it’s the dried new growth of live moss (which only takes a few-year cycle to create); peatmoss on the other hand is the underlayer of a bog (which takes centuries to accumulate). Sphagnum moss is especially ideal for lithophytic or epiphytic plants (like orchids and some aroids) which need moisture at the root zone, but also lots of airflow. The roots of those types of plants normally grow attached to trees rather than in the ground and moss helps create a microclimate that is humid and not too dry. Sphagnum holds a LOT of water. In fact it can hold about 18x it’s dry weight in water—meaning a cup of sphagnum moss holds about 3/4 cup of water—which is amazing!
The thing about sphagnum is that when you use fresh new moss it initially has structure and porosity. Over time though, the sphagnum degrades and essentially becomes peatmoss—it fragments, compacts and properties change as it ages. Also, the more consistently wet you keep sphagnum moss, the faster this degradation happens. In lay terms…the moss rots and becomes composted as yeast, other fungi and bacteria break down the cells and this changing quality tends to be more drastic than any other potting soil option.
The implications for growers are simple: they can be blindsided if unaware if this change. I often see people gushing over how amazing their plants grow in sphagnum after only 1 or 2 months…but then sometimes people start having issues of root rot. So let’s cover the challenges of moss…
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. Sphagnum moss is acidic. It’s generally around a pH of 4.2, but that can go lower over time. This can be bad for some species of plant which come from alkaline habitats—like Alocasia Dragon Scale, Paphiopedilum Orchids, or Philodendrons. These plants are often not adapted to tolerate acidic conditions because their conditions are naturally never acidic. Why does this matters: pH affects the availability of nutrients and plants adapted to basic or alkaline conditions have specific adaptations to overcome the challenges of growing in alkaline soil, along with some dependencies on those conditions to thrive. Putting those same plants into really acidic conditions often causes significant issues like nutrient toxicity for select micro nutrients and/or nutrient deficiency of macro nutrients. You can add oyster shell to your media, which works as a buffer and prevents acidification, but if you’re not aware of this problem, then you’re probably not adding oyster shells, right? However, to be explicitly clear, you shouldn’t add oyster shells to all potting media, a good majority of plants are not from alkaline habitats and are conversely not adapted to alkaline conditions. Many tropical plants do need some amount of acidity.
- The media decays and the root zone becomes anaerobic. This happens as the moss starts to age, fragment, compact closing air pockets. This process eventually limits airflow, especially when the media is saturated after a heavy watering. In wet conditions with limited air gaps, bacteria and fungi use up the available oxygen, making the root zone anaerobic. When conditions are anaerobic your potting media essentially becomes an 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 anaerobic conditions) turn the sugars in the cabbage into lactic and acetic acid! When you pickle cabbage like this, the acidity drops below 3.5pH. 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). You really want to add structure to your potting mix which allows improves airflow.
- When using sphagnum moss, to avoid both 1 and 2, you need to repot more often than regular potting media. You also have to remove most of the old media—which is challenging if you don’t want to damage roots, especially on really rooty plants like Alocasias, Begonias, or Philodendrons.
Sphagnum moss still has it’s value for growers—I use it often! I’m definitely not saying “you shouldn’t use sphagnum moss.” If you LOVE using pure sphagnum just the way it is, then continue using it for sure. But…if things change and your beautiful roots suddenly turn to mush, this may help you understand the variables that changed and what to watch for in the future.
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 cycle of the 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—Nepenthes growers often cut their sphagnum moss 1:1 with large perlite. You can use sphagnum as a component of a potting mix; then you’re gaining some water retention, but still allowing for good oxygen movement after many months. Beware with a more airy media you’ll have to water more often. More air movement means faster evaporation and less moss means less water retention.
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)
