When most people look out at the San Joaquin River, the last thing they expect to see is a blanket of bright-green plants stretching from bank to bank with geese and ducks using them to literally walk on water. But as our friend Richard Sloan recently noted, two invasive species—Parrot Feather and Spongeplant—are now creating real challenges for paddlers, fishermen, and wildlife along the Fresno reach of the river.

Both plants are beautiful, hardy, and widely sold in the aquarium and water garden trade. And that’s probably how they ended up here.

A Problem That Starts With Good Intentions

Many invasive species don’t arrive through malice—they arrive through good intentions. A homeowner buys a plant for their backyard pond. An aquarist trims excess growth and wants to “return it to nature.” In each case, these actions may seem harmless, but the result is not.

Once released into a slow-moving river or backwater habitat, non-native plants have everything they need to explode in growth:

• Warm water
• Long summer days
• Few natural predators
• High nutrient loads from upstream sources

Parrot Feather (Myriophyllum aquaticum) and Spongeplant (Limnobium laevigatum) thrive under these conditions, forming dense mats that clog channels, slow water, trap sediment, and crowd out native vegetation. What looks like a lush green carpet is actually a sign of an unhealthy river system struggling to breathe.

Accidental Releases Are More Common Than People Think

There are three main pathways that likely introduced these plants into the San Joaquin River:

1. Aquarium and Pond Dumping

This is the most common culprit. When a tank becomes overgrown or a backyard pond is being cleaned, people sometimes dump plants—and occasionally fish—directly into the river.

A single handful of Parrot Feather can regenerate into acres of plant growth within a few seasons.

2. Stormwater Drainage

Aquarium trimmings tossed into Fresno area storm drains and canals often end up in the San Joaquin River.

If a fragment survives the journey, it establishes.

3. Escape From Backyard Water Features

Parrot Feather and Spongeplant are popular landscaping plants because they’re pretty, cheap, and tough. But they spread aggressively. Birds, raccoons, and other animals can carry pieces from yard ponds to natural waterways.

It only takes one tiny fragment—literally a piece the size of your pinky nail—to start a brand-new colony.

Once Established, They’re Hard to Remove

Unlike native plants, which grow with the seasons, invasive aquarium plants grow almost year-round in our climate. They:

• Outcompete native fish nursery habitat
• Slow water flow and increase flood risk
• Impede kayakers and canoes
• Hide hazards below the water
• Reduce oxygen and increase water-temperature stress for fish.

As Richard noted, these plants now reach all the way across some stretches of the river, making paddling feel more like pushing through wet carpet than moving across open water.

What We Can Do—Starting With Education

At the Fresno Aquarium, one of our core missions is to help the public understand how everyday choices affect local waterways. Preventing new introductions is far cheaper—and far more effective—than trying to remove established infestations.

A Few Simple Ways to Help Protect the River:

• Never dump aquarium plants or fish into any waterway or storm drain.
• Seal unwanted plants in a plastic bag and place them in the trash.
• Choose native or non-spreading alternatives for backyard ponds.
• Support local control efforts aimed at removing invasive aquatic weeds.

The San Joaquin River watershed is one of the Central Valley’s most valuable natural resources. But it needs our help. The “green carpets” we see today are reminders that small actions at home can ripple across an entire watershed.

Working together, we can keep these invaders from spreading further—and protect the San Joaquin River for fish, wildlife, and future generations.

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