Background
Live Sphagnum is a popular companion moss in the carnivorous plant hobby and is often used as a top dressing in potted Nepenthes. It can also be used in terrariums and paludariums as a carpeting moss or for texturing. Its appeal lies in its soft, pillowy appearance, lime green coloration, and ease of growth. However, apart from aesthetics, live Sphagnum also has beneficial properties, which will be described later on, but first, let's get down to its care and cultivation.
Light
Most Sphagnum sp. thrive in bright, indirect light but can also tolerate lower light levels. An indication of sufficient light is the coloration of the moss itself - lime green in bright, indirect light and medium/dark green in lower light levels. When given the adequate exposure, an established colony of Sphagnum grows quickly, relative to most other species of moss.
When exposed to sufficient light, Sphagnum forms a lush, dense, lime-green colony. (Spot the leech)
Humidity & Airflow
In short, try to maintain humidity around 65-70%RH with gentle air movement.
Sphagnum, like most moss species, grows naturally in areas of substantially high, stable humidity. I grow my large colony outdoors in tropical Singapore, where the nursery humidity averages 65%RH in the day, and 85-99%RH at night. During hot and dry spells with strong air movement, humidity in the afternoon can drop to as low as 45-50%RH. Exposure to short periods of low humidity is usually not detrimental but will likely result in the drying or die-back of growth tips, as shown in the photo below. Extended periods of wildly fluctuating or low humidity coupled with strong air movement will result in the eventual death of the colony by desiccation.
Many indoor growers cultivate Sphagnum in fully sealed terrariums - this is a good method to maintain humidity indoors but excessively high humidity (>80%RH) stifles the growth of Sphagnum and promotes the growth of mold and fungus because of the absence of air movement. Rather than focusing on excessively high humidity (>80%RH), keep the colony in a terrarium or similar enclosure that allows for gentle air movement and exchange - this can be done by propping the lid or leaving a small gap open - and an average, stable humidity of about 70%RH.
Moderately dried out patch of Sphagnum top dressing caused by a sudden hot and dry spell.
Watering
Sphagnum is somewhat surprisingly not as thirsty as you might imagine. I grow my colony directly on my perforated nursery benchtop, which allows for complete drainage when watered. You might choose to grow your colony in a shallow but wide plant pot or tray, and drainage holes are a must.
Keep Sphagnum moist at all times, but never sopping wet or sitting in stagnant water for extended periods, especially when exposure is on the lower end. When watering is needed, drench the moss colony thoroughly and allow excess water to flow through. If grown in a terrarium or similar, a good misting will suffice but again and especially so in this situation, avoid a sopping wet colony sitting in stagnant water. Overwatering coupled with low exposure can easily kill off your colony by drowning.
The initial stages of dehydration often present as white tips on new growth (arrow), which are especially tender and prone to drying out.
Advanced dehydration causes the browning out and death of tissues. Immediate rehydration can sometimes promote regeneration of new growth after a couple of weeks.
Temperature
Most Sphagnum species cultivated locally grow well in our tropical lowland climate. For reference, during the hottest periods of the year, the Sphagnum colony I grow does just fine in 39C afternoons for extended periods. Night temperatures average 22-24C.
Red species like S. capillifolium however, do require cool nights (at least 20C or lower) to maintain its coloration.
Fertilisation
A light and regular fertilisation regime significantly boosts the growth of Sphagnum. I would personally recommend a soluble general purpose fertiliser like a 5-5-5 or 10-10-10 sprayed onto the colony once a week. If using a higher NPK ratio, dilute down to at least 10-10-10 to prevent over-fertilisation, which will result in algae blooms. Also, choose an organic, non-salt-based soluble fertiliser as this will prevent the build up of mineral salts especially when the colony is grown enclosed in a tank or container.
Avoid slow-release pellets as these cause a localised concentration of excess nutrients that can and will result in algae blotches.
Because my colony is grown outdoors and is therefore subject to changes in light exposure, I fertilise weekly in periods of fair weather and stop completely during the monsoons. If your colony is grown indoors under artificial lights with a fixed photoperiod year-round, weekly fertilisation throughout the year is fine.
Benefits of Sphagnum
When used as a top dressing in potted plants, live Sphagnum maintains acidity of the medium by continually pumping hydrogen ions (H+) into it. This is especially beneficial for most carnivorous plants that thrive in acidic soils.
Because of its inherent acidity, Sphagnum is also antimicrobial, which coupled with its water absorption capacity and physical structure, makes it an excellent rooting medium for stem and leaf cuttings. A live colony will work better than its dried counterpart as a propagation bed because the continuous new growth promotes a micro-environment balanced in moisture, humidity and air circulation. Dried Sphagnum gets soppy and mucky over time, and the compaction of the fibres makes it prone to causing root rot when used as a propagation bed.
Common Issues
Algae - probably the most common annoyance which can quickly overrun a colony if left to be. Algae is easily prevented by balancing light, moisture (both of the moss itself, and humidity) and nutrients. An excess of any one of these three factors can cause an algae bloom.
To start, grow the moss colony drier - though a bog/swamp-dwelling species in the wild, Sphagnum easily gets infested with algae if grown sopping wet and excessively bright, in cultivation. Drop the moisture and light when initially starting off a colony, while maintaining 60-70%RH humidity. Observe new growth for etiolation (dark green, spindly-looking sad growth) and increase the exposure if needed. The same goes for fertilisation - start off low and slow, then increase the frequency. Cut back if algae starts to grow.
Mold/fungus - usually seen when grown in excessively high humidity (>80%RH), with inadequate air flow. Can also be caused by dead moss, plant debris, or other dead organic matter - all of which are broken down by fungi or mold and cause a temporary localised colonisation until the organic matter is completely broken down. To reduce the occurrence of mold or fungus, maintain gentle air movement over the moss colony and keep free of organic debris.
Mold that has colonised an area of dead Sphagnum moss.