Weeds are the bane of gardeners everywhere. But sometimes, the gardener herself is the unwitting source of the problem.

That’s the case for a small number of aquatic invasive plants like hydrilla and water hyacinth. Popular in backyard water gardens in the United States, these weeds quickly spread to natural waterways where they choke out other marine life, degrade water quality and clog recreation areas.

While most plants have a low probability of becoming invasive, the ones that do can cost a pretty penny—the United States alone spends $110 million in control and management efforts every year.

One hope lies in identifying potential invaders before they’re imported, says Nature Conservancy ecologist Doria Gordon. She co-authored a recent study evaluating a risk assessment tool for invasive aquatic plants. The researchers found that the tool—modified from a system originally developed by New Zealand’s Biosecurity Program—could predict with 91% accuracy whether an aquatic plant would become invasive or not.

I talked to Doria about her research into early detection of invasive freshwater plants.


Q: What exactly is a “water garden”? Are they a growing trend in the U.S.?

Water gardens are generally backyard ponds that are created by homeowners as a landscaping feature. They sometimes feature specific types of aquatic plants, can have ornamental fish and, more rarely, are used to produce fish like tilapia as food sources. They are generally shallow and use pumps and filters to circulate the water and maintain water clarity. Waterfalls are often incorporated into the design of water gardens, both for aesthetics and to ensure sufficient oxygen in the water. Many homeowners hope that native amphibians, birds and other wildlife will use their water gardens.

While this trend in the U.S. is derived from thousands of years of water gardening in the Middle East, Asia, and Europe, its popularity has been increasing: between 1998 and 2003, the number of homes in the U.S. with water gardens quadrupled, from 4 million to 16 million and supported $1.56 billion in retail sales in 2003 alone.

Q: Invasive aquatic animals like Asian carp and zebra mussels are causing serious economic and ecological damage in places like the Great Lakes. What’s the impact of invasive aquatic plants?

Like the animals, there are a few aquatic plants—such as hydrilla and water hyacinth—that can have expensive and harmful impacts in aquatic systems. These species can multiply and spread rapidly, degrade water quality, choke out native species and clog boat motors, waterways, water intake or outflows at dams, water treatment plants, and other facilities. Control and management of these species is estimated to cost the U.S. over $110 million every year, but this number may be a substantial underestimate since Florida alone has spent $20 million in some years on hydrilla control (Koschnick 2007).

And these are just the problems from the plants themselves that have been introduced. Recent research has demonstrated that most of those plants come with hitchhikers—many of them invertebrates—that can have unintended negative consequences as well. In fact, Keller and Lodge (2007) found that 90% of the plants available on-line arrived with associated live invertebrate animals. So homeowners can accidentally move species like zebra mussels and Asiatic clams—and new species—across the landscape with their water garden and aquarium plants.

Q: How quickly do these aquatic invasives spread from the backyard to public waterways?

The frequency and rate of movement from water gardens into more natural systems varies with many factors. Distance to freshwater is a factor, as is the environment and weather. Animals can carry some species, but flooding will carry many. And some of the movement is purposeful—people sometimes directly dispose of unwanted aquarium contents and water garden “weeds” into natural water systems.

So it’s not necessarily a predictable event, but the evidence is clear that it happens with high enough frequency that many of our natural freshwater systems have been invaded by multiple species.

Q: If you’re a water gardener, what plants should you avoid? What are some common plants used in water gardens that are particularly troublesome?

Few species are invasive everywhere they might be grown, but some of the bad actors in many states include water hyacinth, flowering rush, purple fanwort, Brazilian elodea, parrot feather, yellow flag iris, yellow floating heart… the list is long.

On the positive side, there’s also a long list of attractive native and non-native plants—which will also vary by location—that can be used instead. Native examples include species like cardinal flower, marsh marigold, arrowhead and pickerel weed.

Q: Your research looks at how New Zealand has handled invasive plants and animals. What lessons can we gain from the Kiwis?

The Australians and New Zealanders have pioneered the work to develop tools that help to identify invasive plants and animals before they are introduced. These countries have correctly identified their vulnerability—islands provide some of the best examples of invaders causing extinction and substantial economic harm—and they have the ability to protect their borders more completely than continental countries. Invasive species are not stopped by political boundaries, so effective boundary protection would require that surrounding countries adopt consistent regulations and approaches.

While islands may be most vulnerable to invaders because their flora and fauna are generally either fairly unique (it’s no surprise that Hawaii has the most stringent regulations in the U.S. about species movement across its borders) or a subset of that on the mainland, all habitats appear vulnerable to invasion. Invasive species were estimated to cost the U.S. at least $120 billion annually in the early 2000s (Pimentel et al. 2005).

So it pays for us to learn from advances made in countries like Australia and New Zealand. And it’s a great sign that the North American Plant Protection Organization (Mexico, U.S. and Canada) has agreed upon protection approaches for invasive plants and plant pests. I understand that Canada and the U.S. are now using similar risk assessment approaches for plants, which is an expansion of the Australian approach. 

Q:  It’s hard to blame the backyard water gardener if she doesn’t know the plants she’s buying are invasive. Shouldn’t stores be prohibited from selling invasives? Or at least be required to label invasives as such?

It’s true, and casting blame is not a useful exercise. There’s a lot of confusing information available to gardeners. More websites promote species (native and non-native) than provide information about which are likely to have unwanted impacts.

So we need better information available to homeowners. There are a few programs around the country that are taking on this need. Just to give a couple of examples, the GreenThumb program started by The Nature Conservancy in the Florida Keys certifies plant nurseries that agree not to carry invasive species and adopt a number of other best practices for conservation. The PlantRight program in California has a great website that informs homeowners and businesses alike about which species to avoid in various regions of the state. This program was developed by the horticultural industry with the intent of educating consumers about species to avoid.

And a number of species are regulated, either federally or at state levels. Those species are generally not available for water garden use, though most can probably be obtained over the internet. However, regulations generally include species already present and causing harm unless they have clear potential to cause substantial damage to agriculture. By that time we’re trying to catch up and reduce impacts rather than taking the much less expensive and more effective approach of preventing the introduction of high risk species from the start. The work I’ve been involved with to identify effective tools for predicting species likely to cause harm would best be used for screening purposes before the species is permitted to be imported.

[Image: water garden featuring water hyacinth on left. Image source: Flickr user geraldfigal via a Creative Commons license]

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  1. There is no photo of hydrilla in this post, so I googled it.

    I have hydrilla in my pond! All of my water garden plants came out of local waterways, so I assumed they were all natives and never gave it another thought. I knew about the invasive water hyacinth, but not hydrilla!

    What should I do with it?

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