A recent scientific discovery, the oldest fossil of a land plant, is a fossil of sphagnum! Sphagnum is a veritable living fossil. Full article is here.
Sphagnum is prized for its rot resistant properties, which is why it is used as a medium for cloning and rooting other plants. A stalk, stem, or leaf of another plant can be cut off and the cut end can be inserted into wet sphagnum dead or alive and the decay resistant nature of sphagnum keeps the cutting from rotting before it can heal and grow roots. Pro-tip: placing cuttings in live sphagnum is far more effective because live sphagnum actively emits acids and sugars that both protect and feed the cutting you are trying to clone. In ancient times, humans even used sphagnum bandages to keep their wounds from rotting. More details here.
Rot Resistance Varies
Not all sphagnum species deliver the same level of decay resistance that you want when using sphagnum as a growing medium for other plants. The saying “easy come, easy go” applies to sphagnum species. The fast growing species are the least decay resistant.
Sphagnum species are best classified on a spectrum corresponding to how far above the surface of the water/ground they grow. The fastest growing species are the lowest species in terms of distance from the surface of the water/ground, in fact, they grow right beneath the surface of the water, they love high water nutrient levels, low acidity, and weak light. These fast growers are the fastest to rot. The higher above the waterline or ground that a sphagnum species grows in the wild, it has access to lower amounts of water and nutrients, and so the topmost species above the ground simply do not have the resources to grow fast. These species are high above the water and ground and so they are rarely shaded by other plants or rocks, they get blasted by full sunlight and so they evolved a strategy of growing tissues that last a long time by not decaying because if they did decay, the nutrients for rebuilding the tissues would simply not be available. These high-up species are slow moving tanks, highly acidic, dessication resistant, and their tissues “tan” dark colors to prevent sunburn: deep dark purple or nearly blackish brown are common colors for these types of sphagnum. In the wild, the top two species in this classification are fuscum and austinii, the slow brown tanks of the sphagnum world.
More details on the decay resistance of several different sphagnum species can be found here.
Below is a picture of two solid mounds of sphagnum austinii growing almost knee height above the ground in full sunlight. These mounds are actually a miniature forest of sphagnum austinii stalks growing tightly packed together. Each stalk is several inches to 24 inches long! The individual sphagnum stalks hold each other up be leaning on each other. The color brown does not mean dead in this species, it is the plant tanning to protect its chloroplasts from being sunburned. Here is a study of a real bog, comparing the height above the ground/waterline and acidity of several sphagnum species including fuscum and austinii. Note the extreme acidity of these two species gets to a 2.6 pH. That explains the decay resistance.
I have to grab lunch, but will post more interesting science articles on tips for growing sphagnum faster. The articles will cover optimal light intensity, photoperiod, temperature, and water pH. There is even an article that shows how mixing in a very small amount of sugar into the sphagnum’s water can boost its growth rate by 40 times its normal growth rate.