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The blue oil fern is a species of lithophytic and epiphytic fern native to Southeast Asia—ranging from Thailand, Southern China, and Vietnam. Best known for its remarkable blue leaves, Microsorum thailandicum (steerei) comes from shaded, humid, jungles in limestone regions, where the blue leaves serve as an adaptation to capture more light in these low light understory conditions.
Potting Mix: my Microsorum ferns are potted in a standard “terrestrial orchid mix” that I use for paphipedilums or phragmipediums orchids—it’s bark and pumice at a ratio of about 1-to-1—if you don’t have access to pumice, you can substitute large-chunk perlite instead. The roots of the ferns are planted into that mix and then top-dressed with sphagnum moss around the top of the pot and snugged in close to the base of the plants. This top-dressing of sphagnum moss helps ensure localized humidity is maintained where new roots and leaves begin—consider that if you were growing only on bark, leca, or pumice, that top layer (and base of the plant) may get quite dry, preventing the success of fragile new growth points.
Humidity: Every care sheet say, “high to very high humidity is required”, and as with any plant from a tropical jungle, yes more humidity is probably ideal; so if you can offer that, please do. However, the humidity where I grow my ferns is low and often below 50%rH. I believe with consistent moisture at the roots and localized humidity (provided by the sphagnum moss), you can grow this fern without fussing much with humidity. As long as the roots do not ever go bone dry; however, if the roots dry hard in a dry climate, it could quickly kill the fern.
Microsorum thailandicum photo – the leaves in the middle probably got a bit too much light at one point.
Watering: The potting media should be kept continuously moist. I’ll often let my pot sit in a shallow tray of water for 3-5 days a week and now the roots of the fern poke out the bottom and grow into the water. I also routinely water once a week and flush about a full pot of water through the media. That chunky orchid potting mix I mentioned above ensures good airflow in the root zone (which prevents rot), so if you use a potting mix that’s more compact (like pure peatmoss), you may not be able to irrigate as often or liberally, and you definitely shouldn’t let the pot sit in water like I just suggested.
Water Quality: Opt for pure water (under 50ppms). Though this species is often found growing on limestone rock and I thought that my alkaline tap water might be alright, they didn’t like tap water and they really sulked. It’s possible that was related to chlorine or seasonal temperatures more than anything, but it aligns with other grower’s feedback on this topic. Pure water (like RO, distilled, or rainwater) is best. Typically for many ferns, the mineral content of my tap water (~250ppm TDS) is too much, so after about 6 months of inactivity I switched to bottled RO (reverse osmosis) water and after this change growth followed.
Pure water is often needed for plants with sensitive roots which may not be adapted to mineral-heavy soils. I also use this RO water for select carnivorous plant species like Nepenthes and butterworts. At this point, after testing both options, I can strongly recommend that pure water is preferred based on the difference I’ve seen in growth.
Nutrients & fertilizer: We’ve already covered that blue oil ferns are slow-growing, but that’s also an indication you’ll want to avoid excessive feeding. Orchid fertilizer for epiphytic plants is ideal. I also use the tiniest amount of organic fertilizer including bloodmeal and rock dust—but literally a tiny pinch 2x per year during active growth.
Being native to limestone regions, the blue oil ferns may have a higher reliance on calcium carbonate. If your fern seems to be struggling and you’re offering all of these other options, consider adding a small amount of oyster shell to the potting media; you can get this at a pet supply store. In a pinch, you could use a bit of crushed eggshell instead of oyster shell. Disclaimer: this is an untested/unverified recommendation, and while I do occasionally flush the potting media with my tap water (to provide some small amount of calcium), but I don’t add oyster shells. This is a common practice with terrestrial or lithophytic orchids from similar habitats that are reliant on higher levels of calcium.
Light: Low-to-moderate. I have my plant growing under LED grow lights on a timer of 12h day/12h night. They get about 12w per square foot of coverage which doesn’t include the additional light they get from the east-facing window—I consider this area “moderately bright” compared to most grower’s conditions. You could likely grow them with as little as 8w/sqft without much issue, but you’ll need to experiment to confirm. It’s reported that while the blue oil fern prefers lower light of the understory, it can tolerate brighter “sub-understory” light levels, which means they may grow higher up in the trees, and can manage more light than many other ferns or tropical plants. It’s also noted that, “some growers have had excellent success maintaining the [blue] color in very bright light.” To be clear though, this doesn’t mean DIRECT SUN and you should definitely avoid anything greater than 40% filtered sun.
General principals for standard culture of the Blue Oil Fern, Microsorum thailandicum
Querying other growers online, these are other common threads of information I’ve collected:
- They grow slowly
- They need consistent root moisture, high humidity and seem to do well with wetness (making them ideal candidates for semihydro, self-watering, or drip-irrigation setups).
- They don’t need a lot of fertilizer.
- They’re pretty resilient overall and easy to grow tolerating extremes of low or moderate light, wetness with short periods of dryness, and even variation in humidity.
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