Giant Reed (Arundo donax)

Giant reed (Arundo donax), commonly referred to by its genus, is a tall bamboo-like invasive plant that has spread throughout the rivers and creeks of the American southwest. It was originally brought over from the Mediterranean to be used to stabilize streambanks, as a windbreak, or for roofing material. Since its introduction, it has spread rapidly - sometimes growing to comprise more than 50% of a stream’s total vegetative cover. Arundo has widespread and devastating impacts to the areas it is introduced to.

When arundo (or similar invasive riparian weeds like tamarisk) takes root, it physically changes to nature of the stream. By sending down deep roots, the arundo can withstand light flooding and therefore, the stream gets funneled along a straight path which scours out the streambed and eliminates pools and eddies that would otherwise give shelter to aquatic animals and their eggs. When arundo establishes itself in a thick stand, it crowds out any larger trees (like sycamores or cottonwoods) that would be able to provide shade to the stream, and rather allows sunlight to shine directly into the stream and warm it beyond temperatures that our aquatic animals can tolerate. Furthermore, arundo uses significantly more water than native vegetation in the same area. It is difficult to measure the difference in water use between the two due to the variability in a study area’s climate and plant communities, but most studies show that arundo uses three to eight times more water than the native vegetation it displaces.

In terms of natural disasters, arundo infestations directly contribute to creating worse wildfires and floods. Arundo dries out in the summers and its tall thick stands easily catch fire and can add to the intensity of a wildfire, and also allow the fire to spread quickly up and down a dry streambed. Arundo also grows thick enough to trap sediments in streams - leading to a lower water capacity - and it slows the flow of water during high-runoff events - leading to water piling up and spilling out of the stream channel.

Furthermore, arundo deprives our local wildlife of the habitat they need. Birds are unable to nest in the reeds, deer can’t graze on it due to the harsh alkaloids that it transports to its leaves, and larger mammals can’t even move through it.

Luckily, arundo infestations are containable and eradication is possible. Arundo can’t germinate from seeds in our region, meaning there is no seedbank for arundo to regrow from and seeds can’t blow in or out of the infested area. Arundo in our region spreads from fragments and cuttings of parent plants - so if an arundo stalk is cut or snapped off, the broken piece can become an entirely new individual plant. Similarly, when arundo stalks are scoured out by floods or burned off by fire, they can regenerate from their deep roots. With this knowledge, we can work from the headwaters of a river to the mouth of it and be sure that the plant does not regenerate upstream of where we’re working.

We’ve been working along other agencies for years to eradicate arundo from the Santa Clara River. It’s a lot of work, but it has been shown time and time again to be worth the effort. In the long run, removing arundo saves money by reducing fire and flood risk, increasing the availability of water, improving habitat quality, improving streamflow and sediment transport. In 10 years, removing arundo can have a conservative economic benefit to cost ratio of 2:1.