Water clover
Marsilea does not look like a typical fern with highly dissected fronds, but it belongs to a group of aquatic or marshy ferns that includes Azolla, Ceratopteris and Salvinia. There are about 65 species in the genus, most of which are found in tropical Africa. All species have long, creeping rhizomes, which branch and produce upright fronds bearing four leaflets, giving the appearance of a four-leaved clover.
Ferns generally reproduce by division, creeping rhizomes or spores and Marsilea are typical in this respect. Along with many other aquatic ferns, they have what are known as sporocarps; specialised leaf branches that enclose the sori, which produce spores, and are typically found at the base of the frond stalk.
The sporocarps are hard, bean-like structures that can remain viable for over one century if kept dry. In cultivation, to aid germination, scraping the sporocarps, until you can see the white inside, is a good idea. Once the sporocarp is put in water, in bright light, germination will happen within minutes. It can take as little as three months from spore to a mature, sporocarp-bearing plant.
The plants can easily be cultivated in tropical pond margins, as an aquatic in aquaria or in pots if they are immersed in water, allowing the leaves to float. Water clovers do not tend to be as popular amongst fern growers, as more familiar terrestrial and epiphytic ferns, perhaps because they do not look fern like.
At the Oxford Botanic Garden, Marsilea quadrifolia is grown in loam, at the base of other aquatic plants, such as papyrus and swamp lily, on the margins of the Lily House pond, where moisture is constant. The plants spread by rhizomes, forming a dense mat. This species is said to be hardy down to -15 degrees Celsius, but we have never put it to the test.
Marsilea aegyptiaca is on the IUCN Red List of threatened Species, but is rated as Least Concern. Marsilea quadrifolia however is rated as Vulnerable. Threats are more limited in arid and semi-arid regions; the main threats are from the destruction of wetlands by draining for agriculture and pollution from fertilizers. Marsilea aegyptiaca is found in parts of North Africa, Madagascar and the Arabian Peninsula. In the Mediterranean, it is found in Egypt, where it is abundant and Tunisia, where it is rare. In these areas, it grows in canals, pools, flooded fields and rice fields.
Further reading
Jones DL 1987. Encyclopaedia of ferns. Timber Press.
Hoshizaki BJ and Moran RC 2001. Fern grower's manual. Timber Press.
Huxley A 1999. The new RHS dictionary of gardening. Groves Dictionaries Inc.
Lucinda Lachelin