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Grass Carp Weed Guide: Aquatic Plants They Eat vs. Avoid

Grass carp are a popular tool for controlling some pond weeds. These vegetarian fish can eat large quantities of aquatic plants, making them valuable for aquatic weed management in ponds and lakes. In fact, young grass carp have been known to eat 50% to 300% of their body weight in plants per day under certain conditions. Using grass carp can reduce the need for chemical treatments and help with DIY pond maintenance. However, grass carp are picky eaters – they won’t eat every type of weed in your pond. This article explains which aquatic plants grass carp prefer to eat, which they might eat if necessary, and which they typically avoid. We’ll also cover what to do when grass carp won’t eat certain weeds or algae, including alternative treatments like aquatic herbicides and algaecides.

Using Grass Carp for Pond Weed Control

Grass carp (Ctenopharyngodon idella), also known as white amur, are plant-eating fish native to Asia. They have been introduced across the United States since the 1960s as a biological control for nuisance aquatic vegetation. Most stocked grass carp are sterile triploid fish, so they cannot reproduce and overpopulate. (Many states require this as a condition of stocking) Pond owners often stock grass carp because a few fish can consume a lot of pond weeds daily, helping keep growth under control naturally. Grass carp offer a long-term weed control method (they live 5–10+ years) that can be cheaper and last longer than repeated chemical treatments. However, success depends on the types of plants in the pond – grass carp will only eat certain species and will ignore others. Knowing their diet preferences is key to effective pond weed management.

Aquatic Habitat Damage from Grass Carp

There are downsides to using grass carp. When they consume all or most of the vegetation in a pond, it can harm the overall ecosystem by removing habitat, structure, and food sources for other fish, insects, and wildlife. Without plants, the pond becomes vulnerable to excessive algae growth, leading to poor water quality and lower oxygen levels. This imbalance often results in fewer fish and a less healthy aquatic environment.

Plants Grass Carp Prefer to Eat

Grass carp generally prefer soft, tender submerged plants with non-fibrous stems and leaves. These are the “salad” that grass carp find tasty and will devour first. If your pond has these weeds, grass carp can often provide significant control. Common aquatic plants that grass carp readily consume include:

Hydrilla – A problematic invasive weed (hydrilla is one of the grass carp’s preferred foods in many states).
Pondweeds (Potamogeton spp.) – Such as American pondweed and bushy pondweed; these are favorite foods for grass carp.
Elodea (Waterweed) – A soft submerged plant that grass carp eagerly eat.
Coontail – A common underwater plant with feathery leaves; grass carp will readily consume coontail.
Southern Naiad (Najas) – A delicate aquatic plant that carp enjoy grazing on.
Eurasian Watermilfoil – A feathery submerged plant that grass carp will eat (carp often munch on milfoil, although it’s not always their top choice).
Bladderwort – A soft, free-floating plant (with underwater leaves and bladders) that grass carp readily eat.
Chara (Muskgrass) and Nitella – These are algae-like submerged plants (macroalgae) that grass carp often graze on when available.

Note: While grass carp do eat Eurasian watermilfoil, some experts rank milfoil lower on their preference list. Texas Parks & Wildlife, for example, notes that milfoil is “typically low on their dietary preference list”. Grass carp will consume milfoil if it’s present, but they might choose other soft plants first. In general, any submerged weed that is soft and non-woody is fair game for grass carp – they feed on these favorites from the top of the plant down, often clearing out a pond’s submersed vegetation over time.

Plants Grass Carp Will Eat If Necessary (Less-Preferred Vegetation)

There are some plants that grass carp might eat when their favorite foods are scarce, but these are less preferred and often not reliably controlled by carp. In other words, grass carp won’t seek these out if tastier options are available. In some cases, they will nibble these plants only if they are very hungry or if stocked at high densities, and even then results can be mixed. Examples of such vegetation include:

Duckweed & Watermeal – These are tiny floating plants that can carpet a pond’s surface. Grass carp may consume duckweed or watermeal, but are unlikely to eat enough to control it. Often, carp ignore these tiny plants if other food exists. When they do eat them, it usually requires a high stocking rate of carp in a pond with little other vegetation. Even then, control is inconsistent – you might see some reduction, but carp often cannot completely eliminate duckweed or watermeal.
Filamentous Algae (Pond Scum) – Filamentous algae are stringy, mat-forming algae on the water surface or bottom. Generally, grass carp do not prefer to eat algae. They might sip on algae clumps only when starving, and usually they won’t keep up with an algae bloom. In most cases, grass carp will not control filamentous algae in a pond.
Water Hyacinth – A floating plant with thick, spongy leaves and purple flowers. Grass carp rarely eat water hyacinth unless nothing else is available. According to Texas biologists, triploid grass carp are “not a viable control option for water hyacinth since they rarely eat the plant unless all other vegetation is removed”. So if hyacinth is present alongside more palatable weeds, carp will avoid it.
Eurasian Watermilfoil (revisited) – As noted, milfoil is not a favorite. Carp might nibble on milfoil only after they’ve eaten other plants. Because of this, experts rarely rely on grass carp alone for milfoil control. Carp would only make a dent in milfoil if no tastier weeds remain, and even then, complete control is unlikely.
Muskgrass (Chara) – Chara is a macro-algae that resembles a submerged plant. There’s mixed information about grass carp and chara. Some sources list chara/muskgrass as a food grass carp will consume, but Texas experts note it is “not preferred” by carp. In practice, grass carp might eat chara if other soft plants are gone, but they often leave large chara beds untouched.
Other Tough or Bitter Plants: In general, grass carp tend to avoid or only lightly graze any plant that is tough in texture, has a strong taste/odor, or isn’t soft enough. For example, adult pondweeds with very broad leaves or certain tough species of submerged plants might only be eaten as a last resort. Carp choose tender greens over anything coarse or fibrous.

Plants Grass Carp Avoid (Weeds They Won’t Eat)

Despite their appetite, there are many aquatic plants that grass carp simply won’t eat. Grass carp dislike vegetation with hard, woody, or fibrous stems and leaves. They also tend to avoid most emergent and floating-leaf plants (the ones that stick out of the water or have tough floating pads). If your pond is dominated by these species, grass carp will leave them alone, and you’ll need other control methods. Key plants that grass carp typically avoid or fail to control include:

Cattails – Tall reed-like emergent plants with fibrous stalks. Grass carp do not eat cattails and are not effective at controlling them. The woody stems are unappealing to the fish.
Bulrush and Sedges – These are other shoreline rushes/reeds with tough, fibrous structure. Grass carp will ignore bulrush (and similar sedge or rush plants).
Water Primrose – A creeping emergent plant with woody stems and yellow flowers. Carp will not consume water primrose; it’s on the “do not eat” list for grass carp.
Water Lilies and Spatterdock – Floating-leaf plants (water lilies have round pads; spatterdock has large heart-shaped leaves). Grass carp avoid water lilies – the leaves and stems are too tough/stringy for them. Both white waterlilies and yellow spatterdock are typically untouched by grass carp.
Watershield – Another floating-leaf plant with small oval leaves and a gelatinous coating on stems. Grass carp won’t eat watershield. The texture and taste deter them.
Arrowhead & Burreed – Emergent broadleaf plants found in shallow water (arrowhead has arrow-shaped leaves; burreed has burr-like seed heads). Grass carp do not graze these at all. The firm, fibrous leaves make them unpalatable.
Large-Leaf Pondweed – Certain pondweed species with large, tough leaves (e.g. Illinois pondweed or large-leaf pondweed) are usually ignored by grass carp, even though carp eat the softer pondweeds. The fish seem to know the difference and avoid the thick, broad leaves.
Woody Shoreline Plants – In general, any woody or thick-stemmed plant (like shrubs or tree saplings growing in the water, or woody vines) will not be eaten by grass carp. For example, purple loosestrife or alligator weed are invasive emergent plants that grass carp won’t control.
Filamentous Algae – Worth mentioning again: algae mats and scums are avoided by grass carp. Many pond owners hope grass carp will clean it up – but they won’t. Carp might take an occasional bite but will not significantly reduce filamentous algae or planktonic algae blooms.

Why do grass carp avoid these? The general rule is that if a plant is tough, thick, or woody, grass carp will leave it alone. Their preference is for tender submerged foliage. Additionally, many of the “avoided” plants are things like cattails or lilies that provide habitat or have defenses (thick cuticles, bitter compounds, etc.). Grass carp can physically eat only softer vegetation; they aren’t equipped to graze on wood-like stems or leathery leaves. This means stocking grass carp will not solve weed problems involving these types of vegetation. If your pond has mostly cattails, lilies, or algae, other control methods will be needed – grass carp will simply swim past those and search for softer plants.

What to Do When Grass Carp Won’t Eat Certain Weeds or Algae

If you have nuisance plants that grass carp ignore, you’ll need to rely on alternative pond weed management methods. Here are some tips to handle weeds that grass carp won’t eat:

Manual Removal: For shoreline plants like cattails or water primrose, physical removal can help. Cutting, pulling, or raking out these weeds can reduce their coverage. This is labor-intensive but effective for small areas. For floating plants like water hyacinth, you can use a skimmer or net to scoop them out regularly. While manual removal is environmentally friendly, it may be impractical for large infestations.

Aquatic Herbicides: Using an aquatic herbicide is often the most efficient way to control plants grass carp avoid. Always choose a herbicide labeled for the specific weed and follow the product instructions for safe use. A few examples of treatments:

Emergent Weeds (Cattails, Lilies, etc.): Use a systemic herbicide that kills roots. A glyphosate-based aquatic herbicide is very effective on cattails, bulrush, water lilies, watershield, and similar plants. For instance, a product like Glyphosate 5.4 Aquatic Herbicide (made for ponds) can be sprayed on the foliage; it will be drawn in and kill the entire plant (cattails, lily pads, and others) down to the roots. This prevents regrowth. (Tip: add a non-ionic surfactant when using glyphosate to help it stick to those waxy leaves.) Another option for floating-leaf plants is Navigate, a 2,4-D granular herbicide which is designed to target water lilies, spatterdock, and watershield. Always apply herbicides when the plants are actively growing for best results.

Floating Weeds (Duckweed & Watermeal): For tiny floating weeds that grass carp won’t control, consider an herbicide like fluridone or flumioxazin. Fluridone (brand example: Sonar™) is a slow-acting herbicide that can wipe out duckweed and watermeal over several weeks. Flumioxazin (brand example: Propeller™) is a faster treatment that causes a complete kill of duckweed or watermeal usually within 1–2 weeks. These products are very effective on duckweed/watermeal when used properly. For example, Propeller Aquatic Herbicide is a flumioxazin granule you mix with water and spray; it’s made to “get rid of tough weeds like watermeal and duckweed” (with visible results in under 14 days). When using such treatments, be mindful of water flow – they work best in ponds with little outflow (so the chemical stays in the water long enough to affect the plants).

Algae and Chara: For filamentous algae or chara (muskgrass) that grass carp won’t eat, the go-to solution is an algaecide. A common choice is a copper-based algaecide like Cutrine® Plus. Cutrine Plus is a liquid algaecide that controls string algae (filamentous algae) and chara effectively. It can be sprayed over the algae mats to kill them quickly. Another algaecide option is sodium carbonate peroxyhydrate (brand example: GreenClean) which releases peroxide and destroys algae on contact. When treating algae, it’s wise to treat in sections if the bloom is heavy – killing too much at once can deplete oxygen. Always follow dosage instructions; products like Cutrine Plus are labeled safe for fish when used correctly to kill algae and chara.

Environmental & Preventative Measures: In addition to direct treatments, consider steps to prevent weed problems. Pond dye can reduce sunlight and slow growth of submerged plants and algae (though dye won’t stop floating weeds like duckweed). Aeration can help circulate water and improve oxygen levels, which can reduce the severity of algae blooms and help beneficial microbes. Also, avoid excess nutrient runoff into the pond – too many nutrients fuel weed and algae growth. Good pond management practices will complement your grass carp and/or herbicide efforts, making your aquatic weed management more effective overall.

Safety and Best Practices: When using herbicides or algaecides, always read the label for proper application rates and any water use restrictions. It’s important to apply these products carefully in a DIY pond maintenance context. Treating a large amount of weeds at once can lead to oxygen depletion as the plants decay, which can stress or kill fish. To avoid this, treat heavy infestations in sections (e.g., a third or half of the pond at a time) and ensure good aeration during treatment. By following the instructions and taking precautions, chemical treatments can safely eliminate the weeds that grass carp won’t eat.