Understanding Flumioxazin

Background

Friends of Great Hill Pond works to manage the invasive aquatic plants in Great Hill Pond, including fanwort (Cabomba caroliniana). Some of that work uses a herbicide called flumioxazin, applied by a licensed contractor, Pond and Lake Connection, under a permit issued by the State of Connecticut. Flumioxazin is sold for aquatic use under several brand names, including Clipper.

This page summarizes what flumioxazin is, how it is used in this setting, and what published research and regulatory reviews report about its behavior in water, its effects on aquatic life, and its human health profile. It is written as a plain-language overview of the evidence. Each claim links to a primary source, and the full list of sources appears at the end.

What Flumioxazin Is and How It Works

Flumioxazin is a fast-acting contact herbicide used to manage floating, emergent, and submerged aquatic weeds and algae [1][9]. "Contact" means it works on the parts of the plant it actually touches, rather than being carried throughout the plant. Its movement within a plant is limited [8].

It works by blocking an enzyme called protoporphyrinogen oxidase, usually shortened to PPO [1][4]. This enzyme is part of the pathway plants use to build chlorophyll. When PPO is blocked, the plant accumulates compounds that, in the presence of light, produce reactive forms of oxygen that break down plant cell membranes [8][9]. Because light is required, flumioxazin is described as a light-dependent contact herbicide [4]. Weed control research groups classify it as a "Group 14" herbicide, the group name for PPO inhibitors [1].

Flumioxazin is most effective on young, actively growing weeds, and it works best in water with a pH below about 8.5 [1]. The reason for the pH limit is chemical: at higher pH, flumioxazin breaks down in water very quickly, sometimes before it can act on the plants, which is discussed further below [1].

The current product label lists fanwort (Cabomba caroliniana) among the submerged plants controlled by subsurface application, along with others such as coontail, hydrilla, curly-leaf pondweed, and Eurasian and variable-leaf watermilfoil [1].

How It Is Applied

For aquatic treatment, flumioxazin is applied by a licensed applicator, under a state permit, to the water over the target area. Pond and Lake Connection applies the product from an airboat, at labeled rates that are adjusted for water depth, plant species, density, biomass, and other conditions present at the time of treatment.

The label places several limits on how often and how much of a waterbody can be treated. The same section of water may not be treated more than six times in a year, and normally not more than once in any 28-day period, though in areas of dense growth the remaining weeds may be treated within 10 to 14 days [1]. In dense weed populations, no more than half of the waterbody is to be treated in a single operation [1].

The label also directs applicators to manage a specific risk. When a large mass of weeds dies and decomposes at once, the decomposition can lower the dissolved oxygen in the water and harm fish. To limit this, the label directs treating one third to one half of the water area in a single operation, waiting at least 10 to 14 days between treatments, and beginning along the shore and working outward so fish can move into untreated water [1].

Because flumioxazin breaks down faster as pH rises, the label directs that the product be mixed with water at a pH of 5 to 7, using a buffer if needed [1]. It also notes that the water around dense mats of submerged plants can rise above pH 8.5 by the middle of the day through photosynthesis, so applications made early in the morning and early in the season tend to be more effective [1].

What Happens to It in the Water

Flumioxazin does not persist in water. It is broken down by three natural processes: reaction with water (hydrolysis), sunlight (photolysis), and microbes [9].

Hydrolysis is the dominant process, and its speed depends strongly on pH. Published half-lives, meaning the time for half of the material to break down, are roughly four days at pH 5, about one day at neutral pH 7, and on the order of 15 to 20 minutes at pH 9 [4][9]. In many lakes and ponds, the half-life is less than a day [9]. Sunlight adds a second pathway, with a photolysis half-life in water of about one day [4].

When flumioxazin breaks down, it forms several smaller compounds, the main ones abbreviated APF, THPA, and 482-HA [4][6]. Studies submitted to regulators found that these breakdown products are generally less toxic to aquatic life than flumioxazin itself, so breakdown is largely a detoxifying process and the parent compound is treated as the main residue of concern [7]. One exception noted in review is the APF breakdown product, whose toxicity to aquatic invertebrates approaches that of the parent [6].

Some of the flumioxazin applied moves from the water into bottom sediment. Rather than building up there, the compound and its breakdown products in sediment are subject to microbial breakdown, and studies describe the sediment acting as a place where the material is degraded rather than stored [6]. Flumioxazin also has a low tendency to accumulate in the tissues of aquatic animals [4][10].

Swimming and Recreational Exposure

The current product label sets no waiting period for swimming or fishing. It states directly that there is no post-application holding restriction against using treated water for drinking or recreational purposes such as swimming and fishing [1].

Consistent with this, EPA's registration review did not identify dietary risks of concern, including from drinking water, and did not identify residential exposure risks of concern, the category that covers recreational contact such as swimming in treated water [2][3].

The label does set short waiting periods for some other uses of treated water. Treated water may not be used to irrigate food crops until at least five days after application, and it sets separate, generally shorter, waiting periods before treated water is used to irrigate turf and landscape ornamentals [1].

Effects on Fish and Other Wildlife

Regulatory reviews classify flumioxazin as practically non-toxic to fish on an acute basis, for both freshwater and marine species, and EPA's ecological review found no acute levels of concern exceeded for fish [4][10]. Because flumioxazin breaks down quickly and does not build up in tissue, the risk of accumulation in fish is low [4][10]. EPA's modeling did flag some chronic risk values for fish above its level of concern, but those estimates assumed the chemical was continuously replenished to hold a steady concentration, whereas real applications produce only a brief, declining pulse of exposure, which makes the chronic estimate conservative [4][7].

Aquatic invertebrates are a more sensitive group. State and federal fact sheets describe flumioxazin as moderately to highly toxic to aquatic invertebrates, with possible effects even below the label maximum of 400 parts per billion, and the product label states directly that the product can be toxic to aquatic invertebrates if not used according to directions [1][10].

The most sensitive organisms of all are non-target aquatic plants and algae, which is expected given that flumioxazin is a herbicide [6]. This is the basis for the label's drift and runoff precautions, which are meant to keep the product from reaching plants outside the treatment area [1].

Flumioxazin is classified as practically non-toxic to birds, mammals, and honeybees on an acute basis [4][10]. As with the fish estimates, a practical consideration beyond the chemical's direct toxicity is the drop in dissolved oxygen that can follow when treated plants decompose, which the staged-treatment directions on the label are designed to limit [1].

Human Health

Regulatory reviews characterize flumioxazin's human health profile as follows:

• The greatest human exposure is to the people handling and applying the concentrated product, not to the public. The label requires applicators and handlers to wear a long-sleeved shirt, long pants, chemical-resistant gloves, and shoes with socks, and notes that the product is harmful if inhaled or absorbed through the skin and causes moderate eye irritation [1]. For this reason, application is carried out by licensed professionals using specified protective equipment rather than by the general public.
• In laboratory animal studies, the toxic effects associated with flumioxazin include anemia and effects on the liver and the cardiovascular system, with rats appearing to be the most sensitive species [3][5].
• The most closely examined finding is developmental toxicity. In rat studies, exposure of pregnant animals produced fetal heart defects, most notably ventricular septal defects (an opening in the wall between the heart's lower chambers), by both oral and skin exposure routes [3][5]. These developmental effects were seen in rats but not in rabbits [3][5]. The proposed biological explanation is that blocking the PPO enzyme interferes with the formation of heme, a component of blood, leading to anemia and reduced oxygen in fetal tissue [11].
• After reviewing this database, EPA concluded that its regulatory limits are protective of infants and children for oral and skin exposure, and it set the additional children's safety factor to 1X for those routes while keeping the extra 10X factor for inhalation because an inhalation study was not available [5].
• EPA did not identify dietary risks of concern. Its estimates of everyday dietary exposure were a small fraction of the doses EPA considers safe: about 8 percent of the acute level for women of childbearing age, and about 7 percent of the chronic level for the general population and 16 percent for young children, all well below its level of concern [2].
• EPA classifies flumioxazin as not likely to be carcinogenic to humans, and as practically non-toxic to mammals on an acute basis [2].

Common Questions

Is flumioxazin banned in Europe?

No. Flumioxazin's classification and status in the European Union have changed over time and have been the subject of debate. It was classified in 2014 as toxic for reproduction in the more serious category (1B), was reclassified in 2019 to the less serious category (2), and the European Union then renewed its approval in January 2022 [11][12]. That renewal was contested, including objections raised in the European Parliament, largely because of the reproductive-toxicity classification history [12]. The European framework and the United States framework weigh these findings differently, which is why the regulatory outcomes differ.

Does flumioxazin build up in the pond over time?

The evidence indicates it does not accumulate as the active compound. Flumioxazin breaks down in water within roughly a day at typical pond pH, faster in more alkaline water and slower in acidic water, through reaction with water, sunlight, and microbes [4][9]. Some material moves into sediment, but there it is subject to microbial breakdown rather than long-term storage, and the compound has a low tendency to accumulate in aquatic organisms [4][6].

Why is timing and water pH mentioned so often?

Because both strongly affect how flumioxazin behaves. It breaks down faster as pH rises, so in more alkaline water it can lose effectiveness quickly, and the water around dense plant mats can become more alkaline through the day as plants photosynthesize [1]. This is why the label favors application to young plants, early in the day, and early in the season [1].

Summary

Applied as described here, by a licensed contractor, under a state permit, below the surface at low concentration, and in stages, flumioxazin is a contact herbicide that breaks down in water within about a day at typical pond pH and does not accumulate as the active compound. It is practically non-toxic to fish, birds, and mammals, is moderately to highly toxic to aquatic invertebrates and very toxic to non-target aquatic plants, and carries a well-documented developmental-toxicity finding in rats that regulators account for when setting exposure limits. The current label sets no waiting period for swimming or fishing in treated water, and directs staged treatment to protect against oxygen loss from decomposing plants.

Published by Friends of Great Hill Pond. greathillpond.org