Blue Oil Fern Care & Culture – Microsorum thailandicum (steerei)

In Houseplants & Tropicals
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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 strap leaves, Microsorum thailandicum (steerei) comes from shaded, humid, jungles in limestone regions, where its blue leaves serve as an adaptation to capture more light in these low light understory conditions.

In terms of care, I find them relatively easy. However, they’re very slow growing and require consistent care over a long period of time—meaning you can’t forget to water them for a week or two. If you’ve ever grown Paphiopedilum slipper orchids, you’ll understand the patience required to keep slow-growers like these. Growth changes are measured in months and years rather than days or weeks and sometimes you might not find them as rewarding because they mostly exist like a plastic plant, rather than flourish and size-up like a philodendron. This plant is probably more ideal for people who easily adhere to a routine, are not forgetful, and can provide that weekly consistency for many years. Erratic conditions (especially with regards to watering cadence) will likely limit success but I’ll get into the specifics of care and culture below. By the way, if you really like blue plants and that’s what has attracted you to the blue ferns, I also encourage you to check out Begonia pavonina.

Before / After 10 months in my care
Photo of my Blue Oil Fern (Microsorum steerei)

 

Care & Culture of the Blue Oil Fern – Microsorum thailandicum

I’ve had my two blue oil ferns for about 10 months and though they’ve grown slowly, they’re still alive and doing well. I should note…nearly all previous ferns I’ve tried growing have died within a few months, with the exception to some weedy ferns that popped up in a terrarium, so I consider this a win. My two blue ferns produced an abundance of new leaves in spring-to-summer when the temps are warmer (23-28C), but for most of the winter (when temps averaged 18-22C) they didn’t grow much at all. This inactivity could have been related to poor water quality (I was briefly using tap water), but I’ll know better after a few years growing them to observe seasonal changes.

Potting Mix: I have my ferns 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 ferns are planted into that mix and then top-dressed with sphagnum moss around the base. 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, it’s 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 too much with humidity, as long as the roots do not ever go bone dry. Time will tell if my plants “thrive” (as in get very large)…but at this point after nearly a year, they’re actively growing and not dead…so *my opinion* is that humidity requirements for the blue oil fern are mostly negligible (again, provided you don’t allow the roots to completely dry out).

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. 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.

Water Quality: Opt for pure water. Though this species is often found growing on limestone rock and I thought that my alkaline tap water might be alright, but they didn’t seem to flourish when given tap water. 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 is better. Typically for many ferns, the mineral content of my tap water (~250ppm TDS) is too much, so after about 6 months of inactivity I started using bottled RO (reverse osmosis) water and after this change growth followed.

Pure water like RO or distilled, is often needed for plants with sensitive roots—it’s the equivalent of using rain water (which you could use instead). I also use this RO water for select carnivorous plant species like Nepenthes and butterworts. At this point, after testing both options, I think I can reliably 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 alkaline tap water (to provide that calcium), I don’t add oyster shells; however, 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.” [source] 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:

  1. They grow slowly
  2. 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).
  3. They don’t need a lot of fertilizer.
  4. 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.

 

Key information on growing the Blue Oil Fern from the Exotic Rainforest

One grower, Jay Vaninni, notes that he has dozens of mature blue oil ferns growing along side his collection of high-light Neoregelia bromeliads and others in his personal collection growing in deep shade—both groups “look just fine.” Vaninni shares, “[the blue oil ferns] do appear to look considerably better when watered with reverse osmosis or rainwater and fertilized with very dilute fish emulsion and/or kelp extract.”

“we have several of the blue ferns growing [attached to] limestone […] you may want to place your plants in a moderately lit [vase or terrarium] with several inches of gravel in the bottom [and add water to about 50% the height of the gravel] so the gravel is always wet. This will insure high humidity inside the container [and then grow the ferns either on a limestone rock with sphagnum moss, or in substrate planted directly on top of the gravel.] Mist the ferns daily as well as keep the root system packed with damp orchid moss until you are certain the roots are firmly attached. We often feed the fern with a very dilute orchid fertilizer.” [source]

 

Blue Oil Fern Species Name: is it Microsorum thailandicum or steerei?

It never fails that any time I go to write a post on a plant’s care, the taxonomy monster jumps in and makes a simple topic exponentially more complicated (and boring), and why should ferns be any different?! You may find “Blue Oil Ferns” listed as either Microsorum steerei [source 1source 2 (GBIF)] or Microsorum thailandicum [source (GBIF)]. It seems that people believe that M. steerei and “M. steerii” are synonyms of M. thailandicum; however, both species are currently accepted registered species names—whether or not the blue oil fern is only M. thailandicum, I don’t know, but my ferns (acquired in 2020) were tagged as, “Microsorum steerei”. If M. thailandicum is indeed a synonym of M. steerei, then technically M. steerei would be the accurate name, as it was first registered in 1933 before M. thailandicum in 2001. It is possible the true “Microsorum steerei” is just a different but similar species and that’s why some believe the former name is a synonym for Microsorum thailandicum. Whatever the answer, I don’t have it because I haven’t been able to find clarity (from credible sources) on whether both species are considered blue oil ferns. Assuming there are two species, given their general habitat and range, the care outline above should still be applicable. If you have more accurate information on the taxonomy of the Blue Oil Fern, send me a message on Instagram and let’s discuss so I can update this record for others.

 

Links & Additional Reading About Growing Microsorum thailandicum & steerei

 

Photos of my Blue Oil Ferns

Taken June 2021

When I got my Blue Oil Ferns – End of September 2020

 

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