Parkway visitors have once again noticed a dense fern covering much of the pond at Sailor Bar this year. This floating fern is in the genus Azolla, often called mosquito ferns, and is believed to be native to California.
While non-toxic or otherwise dangerous, a dense population of Azolla can block light from the water and deplete water oxygen levels as they decompose. This year, the Azolla is covering roughly two-thirds of the pond.
“Last year we saw a large Azolla population in the same area,” said Liz Bellas, Director of Regional Parks. “This year’s population doesn’t appear to be quite as large, and we haven’t received any reports of dead or injured wildlife related to the Azolla.”
In California, the general consensus is that we have two species present – Azolla filiculoides, or “Common Mosquito Fern,” and Azolla microphylla, also known as “Mexican Mosquito Fern.” This latter species is sometimes called Azolla mexicana – it’s not clear whether A.microphylla and A.mexicana are separate species.
Both species of Azolla are thought to be native to California, although they have boom-and-bust population cycles that make them seem like they are invasive. Like last year, it seems that this year the populations in the Sacramento area are on the larger size.
The Department of Regional Parks
has worked with UC Berkley to determine why this fern is so abundant and what, if anything, we can do to curb its growth. Last year, the fern died off and sank once the weather warmed up and water began flowing through the pond, and we expect the same to happen this year.