Plant Encyclopedia

An odd woodland fern, Japanese grape fern, Botrychium japonicum


Japan is home to an astounding array of fern species, including the grape ferns or moonworts, genus Botrychium. Two commonly seen species in Japan are B. japonicum and B. ternatum, both winter green, summer dormant plants. This article deals with the former, a larger plant in all aspects.

B. japonicum is a winter green species rarely more than 30 cm tall. It has two distinctly different looking fronds. The one that looks like a typical fern frond is called the trophophore and is sterile, while the other one bearing the spores is the sporophore. New growth of both frond types occurs in late summer and early fall. The trophophore is 3 times pinnate. Just after emerging it branches off from the sporophore and separates again into three equal sections, each growing up to a length of 10 or more centimeters. The entire blade is roughly triangular in shape; 20 cm or less at its base and tapering down to its end. The pinnae are toothed sharply and are highly veined. The stipe has a light coating of small scales about halfway up and then becomes naked.

 

Botrychium japonicum frond

The sterile frond of Botrychium japonicum is divided equally into three sections and is three times pinnate.

The sporophore rises to twice the height of the trophophore and ends in a cluster of opposite branched stems bearing perfectly round spore cases. The lowermost stems are divided again into smaller opposite branches and are long, up to 3-4 cm in length each. These stems progressively get shorter further up and half way up the entire cluster they cease to be branched. The small spore cases group tightly near each other on these stems, giving the appearance of a cluster of tiny grapes (actually, they look like fish eggs to me!). They start out bright green, but eventually turn yellowish, then orange and finally brown with age.

Botrychium japonicum can be found on Honshu, Shikoku, and Kyushu in Japan as well as Korea, Taiwan, and China. It is most commonly seen in moist deep woods in moderate to heavy shade. Occasionally it is found in conifer plantations, but most often in mixed broadleaf forests. On Kyushu it is usually seen at mid level elevations (~300-600 meters). Plants occur singly or or in small, loose groupings.

This is a strange group of ferns very unlike most except their near cousins, the adder’s tongue ferns, genus Ophioglossum. The best time to see them is in mid fall just after the fronds have emerged. This fern is peculiar in that it seems to prefer mature hardwood forest, or at the very least old plantation forest. You never see it in young plantations. It is more curious than lovely, but I’m always pleased to find one, especially since they are seen only here and there. I’m always a bit startled when I come up on a group since they “magically” appear in places that were empty all spring and summer, being a summer dormant species. Moreover, plants can remain dormant underground for years before appearing again, so any given plant may not be seen for several seasons.

 

Botrychium japonicum adult

Botrychium japonicum is at its best in the months of October and November, just after emerging.


Botrychium japonicum sporophore

The sporophore is the fertile frond, producing the spore, and the reason why these ferns are commonly called "grape ferns" or "flowering ferns".

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The genus name, Botrychium, is from the Greek word botrys meaning “grape”, a clear reference to the cluster of tiny spore cases at the end of the fruiting frond. The Japanese name is oohanawarabi, meaning “large flowering bracken” from the words oo = big, hana = flower, and warabi = bracken. Bracken is a term used in the west primarily for the genus Pteridium, but it is more broadly used in Japan with the word warabi for any fern that resembles that genus. Perhaps the plant seems to be a flowering bracken because of the large sporophore which is quite showy when the spore sacs turn yellow . The reason it is called “big” is that it is larger than the other common species in Japan, B ternatum, fuyunohanawarabi (“winter flowering bracken”).

This fern may present a problem in cultivation, though I’ve never tried growing any Botrychium species. Some say they can be transplanted by digging wild plants up with a good section of earth and placing it in a similar woodland. Part of the problem may lie in the fact that these ferns are strongly associated with a fungus symbiont, and may require at least some of their nourishment from them. Some species of this genus have highly reduced trophophores, potentially increasing that dependence. These are therefore probably best appreciated in their native homes, though the more insistent fern lovers may want to try them. It is also possible to try germinating spore provided the correct fungal symbiont can be grown alongside – undoubtedly another difficult process.

A cool species to see in the wilds of Japan, but not a very likely candidate for the garden. This odd genus of woodland fern, though widespread and diverse over much of the northern hemisphere, largely remains an enigma for gardeners.

 

2 Responses to “An odd woodland fern, Japanese grape fern, Botrychium japonicum”

  1. Gerhard Stickroth says:

    Hi Botany Boy,
    excellent articles. I am very much interested if you can tell me anything about winter hardiness of Botrychium japonicum.
    Regards,
    Gerhard Stickroth

    • tommy says:

      Gerhard,

      My guess would be they are hardy to around USDA cold hardiness zone 7, but not lower. These are evergreen ferns, so in the colder areas where they are found in northern Honshu I’d guess that the foliage is killed back from time to time. I don’t think they can handle frozen soil, so protection will be needed in colder areas.

      Tom

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