Phantom Water Menace

Chinese water spinach (Ipomoea aquatica) is an edible plant; it is a popular vegetable in Asia. Chinese water spinach is also a noxious weed (Federal) because in the right conditions, it can take over lakes and ponds. The US Department of Agriculture allows states to decide whether to allow importation for human consumption via permit or not. The Iowa Department of Agriculture recently requested input from other states. I passed on Oregon’s experience and monitored the other responses. In general, northern states allow Chinese water spinach; southern states don’t.

In 2003, we were approached by USDA and asked whether we’d accept Chinese water spinach in Oregon. Portland State University stepped up and did a risk assessment: http://www.oregon.gov/OISC/docs/pdf/ipaq_ra.pdf

Here is an excerpt from the conclusion: “. . . due to the requirement for hot, humid conditions for growth and the failure of I. aquatica to establish in more temperate areas of Asia, where it has been an important food for several centuries, we conclude that there is a low risk that I. aquatica could establish, invade, and create a nuisance condition in Oregon rivers, streams, lakes, and drainage and irrigation canals.”

The authors even bought some plants at an Asian market and tried to rear it without success. This plant grows amazingly fast in the tropics and in greenhouses, but we don’t have to worry about it becoming an invasive weed in Oregon.

This is a good example of how weed problems are regional. Just because a plant is a weed elsewhere, doesn’t mean it would be a weed here. Before adding weeds or other invasive species to lists of prohibited species, we need to get in the habit of doing a risk assessment. In most cases, this is essentially a literature review answering these questions:

- What is the likelihood of introduction, establishment, and spread?
- What would be the ecologic and economic consequences if it did establish?

The Oregon Invasive Species Council is making good headway on doing risk assessments for all species on our list of 100 Worst Invaders. It won’t surprise me at all if some species currently on the list turn out to be phantom menaces. Others, of course, will pose more risk than we currently imagine. The value of doing these risk assessments is that it allows us to focus on the highest priority invasive species and not waste time and resources on low risk species like Chinese water spinach.

Dan Hilburn