Filmy ferns are very enticing horticultural subjects, and are sure to become a weakness of many rare miniature plant addicts. Part of what makes them so alluring is their unique frond shapes and textures, as well as their semi-translucent leaf tissue which makes them look like living gems. Let’s learn more about these amazing plants, and how to grow them….

About Filmy Ferns

Filmy ferns occupy a number of genera, including but not limited to Hymenophyllum, Trichomanes, and Crepidomanes. The approximately 650 species are extremely varied in frond form, size, and growth habit. Some species are no larger than ¼” tall, and others have fronds much larger. The fronds may be entire, as a disc or fan, or intricately divided and feathery. Fronds come in a wide spectrum of colors, with most being shades of green, but some may have a glaucous appearance. Most have tissue which is only one cell thick, making it translucent to some degree, and earning them the “filmy” moniker.

Diminutive filmy ferns create fine carpets along horizontal or vertical surfaces, and large species create clumps and draperies on trees and rock faces.

Filmy ferns’ environment is usually very wet, often in cloud forests and fog prone locations, as well as rainforests with significant precipitation, which maintain the humidity levels and generally cooler temperatures necessary for their survival. They grow in areas where they are sheltered from dry air, such as pockets of moss, wet wood, and rock crevices. The plants may also be near splashing, dripping, or seeping water, which supplies constant moisture to their delicate fronds, and creates a protective barrier against air movement.  

How to Grow Filmy Ferns

Most filmy ferns will require different growing conditions than the majority of other tropical plants which you may want to grow. Therefore, you’ll need specialized growing tanks solely for filmy ferns and other plants and bryophytes which do well in similar conditions.

With this in mind, let’s take a look at some culture tips for these delightful but challenging plants.

Humidity and Moisture

Filmy ferns require exceptionally high humidity, as would be found in their natural habitat. They have evolved to be adapted to these conditions, which is why their fronds are extremely thin (“filmy”) and have little to no cuticle:

Hymenophyllaceae are a rare example of an evolutionary shift of adaptive strategy from typical vascular plant adaptation to the poikilohydry most typical of bryophytes.” [1]

In layman’s terms, the quote above means that, similar to mosses, filmy ferns have adapted to utilize their environment as way to maintain tissue hydration, as opposed to having structures or mechanisms for doing so within their foliage.

Since their leaves are so susceptible to drying out in the absence of moisture, you need to keep the humidity as high as possible. A Relative Humidity (RH) of at least 90% may be necessary to prevent desiccation, with higher RH being helpful.

Filmy ferns should be kept in extremely moist or wet conditions, which helps maintain high humidity and can also support tissue hydration by direct contact. Their rhizomes should never be allowed to dry out, and should be constantly in contact with, or covered by very moist or wet substrate. Many species benefit from frequent misting with pure water, which mimics their natural habitat.

Air flow

Air movement should be avoided as much as possible. Even under humid and wet conditions, sometimes filmy ferns will dry out when exposed to air flow, even if it’s not directly blowing on the fronds. It’s best not to have fans in a filmy fern growing container, although in some cases you can pull it off if done thoughtfully and carefully.

When growing filmy ferns in a grow tank with other plants, you can use fiberglass window screen to reduce air flow over the fronds, as well as to hold in humidity and soften the lighting (see accompanying images).

Container

As can be deduced by the humidity, moisture, and air flow sections of this article, filmy ferns should be kept in an enclosure which is entirely, or mostly sealed. Glass aquariums, jars, plastic tubs, and deli cups are good options.

Water

Never use tap water for filmy ferns. Only use distilled or RO water, which mimic the purity of rain and mist from their natural habitat.

Substrate

The best substrate will depend on the natural habitat of the species, though in most cases sphagnum moss will be an excellent choice. As mentioned previously, the rhizomes should be covered by, or at least in constant contact with very wet substrate.

Placement

Filmy ferns should not be planted in open spaces. They should be placed in sheltered locations, where wood, rocks, moss, and other surfaces will protect them from air movement and bright light. They will benefit from having their fronds close to, or partially enveloped by wet substrate and hardscape elements. Basically, the more sheltered you can make them, the happier they will be.

Do not plant filmy ferns high up in a tank, where they will be too close to light, heat, air movement, and other conditions that will harm them.

You may experiment with growing filmy ferns near water features, such as waterfalls and pond banks, where the rhizomes and fronds will receive constant moisture and humidity.

Filmy ferns can be grown on horizontal or vertical surfaces, though each species may have a preference. Keep in mind that vertical surfaces are more challenging to keep saturated.

Smaller species do well when partially enclosed in pockets and walls of sphagnum moss, which maintains high humidity in the direct vicinity of the fronds, and also may offer moisture to those which directly contact the wet substrate.

Submersed growth

Some species of filmy ferns grow in areas where water is constantly flowing over the plant, or where the entire plant is seasonally flooded – you can experiment with growing them submerged in water. It’s important to use pure water, and to ensure that it stays fresh and does not stagnate.

If the species is suitable for growth under water, within a few weeks you should notice healthy new growth of rhizomes, and newly forming fronds. Aquatic leaves will usually be morphologically different from emersed leaves, and will die if taken out of water long enough to dry out completely.

Growing filmy ferns in water is a fun experiment, and when successful it results in beautiful and interesting growth. I’ve found that the growth rate under water is significantly faster than in terrestrial conditions.

Lighting

Bright light should be avoided for filmy ferns.

“Light and desiccation responses of filmy ferns can be seen as an integrated package. At low light and windspeed in humid forests, net radiation and saturation deficit are low, and diffusion resistance high. Water loss is slow and can be supported by modest conduction from the sub-stratum.” [1]

The technical jargon quoted above can be translated loosely into layman’s terms to mean that lower light environments are more supportive of keeping filmy ferns hydrated.

Medium to low light, and indirect light, are ideal. Very shady spots are acceptable, but some species will grow substantially slower, or even stop growing if light is too dim.

Temperature

Most species prefer to be on the cool side, as they come from shaded locations, near water, as well as misty cloud forests at high elevations. Some will be OK in intermediate to warm conditions, but avoid hot temperatures. In summery weather and locales, keep them in the coldest part of your home. Air conditioning may be necessary to protect them from excess heat.

Fertilizer

Using chemical fertilizers on filmy ferns is ill-advised. If you want to try fertilizing, use urea free formulas, or even better, natural fertilizers such as worm castings. Again, fertilizing filmy ferns should be considered an experiment, with the understanding that some species may not fare well.

Companion Moss and Plants

Protect small and micro species of filmy ferns from rampant mosses and plants, which will choke them out. Larger filmy ferns should be OK with moss growth, but keep it in check so it doesn’t get too out of hand.

Filmy Fern Growth Rate

Many filmy ferns are extremely slow growing, so patience will be necessary. As long as the rhizomes stay healthy, and proper conditions are met, they should plod along at their own pace.

When newly planted, they will take a long time to acclimate. After that, if no growth is seen for several months, but rhizomes are healthy, try increasing the light slightly, and possibly the humidity and moisture as well. It is advisable to alter only one condition at a time, to avoid shocking the plant, as well as so you can track how each change affects the plant.

Possible Problems with Filmy Ferns, and Their Causes

Fronds are turning a shade of black or brown, or have dark edges or patches.

Some possible causes:
  • The humidity and moisture are too low
  • There is too much air movement
  • The temperature is too high
  • The light is too bright
  • Fertilizers or other chemicals have contacted the leaf
  • Inappropriate water was used
  • The fronds are old and are naturally dying as the plant puts out new growth.

Rhizome tips are no longer green, or rhizomes are dying

Some possible causes:
  • The humidity and moisture are too low
  • The rhizomes were allowed to dry out
  • There is contamination from chemicals, pathogens, mold
  • The substrate is too far degraded
  • Inappropriate water was used

The plant is alive and healthy, but isn’t growing

Some possible causes:
  • The plant needs more time to acclimate
  • There isn’t enough light
  • The plant wants more water

Conclusion

As you can see, filmy ferns require extremely specialized conditions, and present a number of challenges in culture. Great care and patience will be required, but if you are successful, you’ll be rewarded with beautiful and unique plants that are a joy to behold.

References

  1. Michael C. F. Proctor, “Light and desiccation responses of some Hymenophyllaceae (filmy ferns) from Trinidad, Venezuela and New Zealand: poikilohydry in a light-limited but low evaporation ecological niche,” Annals of Botany, Volume 109, Issue 5, 1 April 2012, Pages 1019–1026, https://doi.org/10.1093/aob/mcs012
  2. “Hymenophyllaceae,” Wikipedia, https://en.wikipedia.org/wiki/Hymenophyllaceae

Image Credits

  1. Hymenophyllum nephrophyllum Kidney Fern Ship Creek Beach 2.JPG by MurielBendel licensed under the Creative Commons Attribution-Share Alike 4.0 International license and was modified with general enhancements for color and contrast. Description: Kidney fern (Hymenophyllum nephrophyllum) near Ship Creek Beach, West Coast, South Island, New Zealand.
  2. WP-Hymenophyllum-Exkursion nach Berdorf (Luxemburgexkursion) 011.jpg contributed by Oliver s. licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license and modified with general enhancements for color and contrast.
  3. Crepidomanes minutum utiwagoke01.jpg by Keisotyo licensed under the Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license and was modified with general enhancements of cropping, color, and contrast.
  4. Filmy Fern (3162870189).jpg by Jason Hollinger licensed under the Creative Commons Attribution 2.0 Generic license and was modified with slight adjustments to contrast, color, and exposure. Description: Hymenoglossum cruentum (Cav.) C. Presl. 20090101 Parque Nacional Chiloe, Chile
  5. Hymenophyllum obtusum (palailaulii) (5868436134).jpg by John Game licensed under the Creative Commons Attribution 2.0 Generic license and modified by adjusting contrast, color, and cropping. Description: Hymenophyllum obtusum (palailaulii). Broader fronds and distinctively branched hairs help distinguish this small but attractive filmy fern from the related Hymenophyllum (Sphaerocionium) lanceolatum. The linear bright green fronds are the fern Adenophorus tenella. Epiphytic on a shady log, Poamoho Trail, Oahu.
  6. Mindo-Cloud-Forest-05.jpg used from Public Domain courtesy of Ayacop and modified with slight adjustments to contrast and color. Description: Cloud forest near Mindo, Ecuador
  7. Cloud forest panama 2.jpg contributed by DirkvdM licensed under the Creative Commons Attribution 1.0 Generic license and modified with adjustments to contrast and color. Photograph by Dirk van der Made (en:User:DirkvdM – for more photos see en:user:DirkvdM/Photographs). A cloud forest near Santa Fe, Panamá, April 2004.
  8. Monteverde Reserve Costa Rica 17.jpg by Cephas licensed under the Creative Commons Attribution-Share Alike 4.0 International license and modified with slight adjustments to color and contrast. Description: Monteverde Cloud Forest Reserve, Monteverde, Costa Rica