Recently, some Kauai home gardeners brought a mysterious, green land plant that looks like limu to our College of Tropical Agriculture and Human Resources office for identification.

For residents in wet areas, you may notice this growing on your sidewalks and driveways, making for hazardous and slippery conditions. Exactly what is this “land limu”?

Although this organism resembles green limu or seaweed, it isn’t a seaweed but actually belongs to the Nostoc genus of cyanobacteria. Cyanobacteria, also known as blue-green algae, are single-celled bacteria that can photosynthesize like plants. At 3.5 billion years old, cyanobacteria are thought to be one of the oldest organisms on earth.

Historically, it influenced the development of the planet’s current oxygen-rich environment, allowing life to thrive. Nostoc species (likely Nostoc commune in Hawaii) are known by other names, such as star jelly, witch’s butter and meadow ears.

Nostoc thrives in tropical and wet conditions. You may notice it popping up after a big rain or in areas receiving too much irrigation. This alga flourishes in phosphorus-rich environments, which can be remedied with proper fertilizer management. Nostoc clusters can invade lawns in areas with poor grass growth due to soil compaction.

Hawaiians call Nostoc commune “limu ka kanaka,” which translates to “man-striking moss.” This is due to people striking the ground hard from slipping on this alga.

As Nostoc creates a danger for homes, gardens and greenhouses, removal may be desired but can also be very difficult. Nostoc is very hardy; it can produce a coating that protects it against drought and extremely high or low temperatures. Thus, simply drying Nostoc out may not be enough to eradicate it.

Studies recommend heating in very dry conditions at 176 degrees for 36 hours or at 110 degrees over four days. This can be accomplished by either oven drying or solarization, a technique that involves covering the area with a clear, thin plastic sheet to heat the underneath surface. After killing the alga, the plant material is suitable for composting.

If heating under dry conditions is not an option, the next alternative is to shovel the clusters to remove them. Moss and algae sprays, potassium salts of fatty acids or copper-based products may be utilized to kill clusters before removal. Always read and follow label directions for these products. Do not rake or till before killing because clusters will break into many pieces and spread across the area.

Lastly, improve drainage to reduce standing water that allows Nostoc to establish.

If you do not mind Nostoc growing in your area, then there are uses for this alga. Being a blue-green alga, Nostoc is a nitrogen fixer, which is an organism that converts atmospheric nitrogen gas into a form that plants and lifeforms can use for food. Nostoc can provide nitrogen to plants through a symbiotic relationship, like in wetland rice, or themselves can be a source of nitrogen, other nutrients, as well as plant growth hormones. Studies have shown slight increases in crop growth with addition of fertilizer made from Nostoc.

Joshua Silva is a junior extension agent for the University of Hawaii College of Tropical Agriculture and Human Resources. He is the coordinator for the Kauai Master Gardener program and focuses on different areas of sustainable agriculture. Email him at jhsilva@hawaii.edu, or call at (808) 274-3478.