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‘Cocktail Whiskey’ begonia (Begonia semperflorens), ‘Sun Devil Extreme’ vinca (Catharanthus roseus), ‘Million Gold’ melampodium (Melampodium paludosum), and ‘Super Elfin’ impatiens (Impatiens walleriana) plants were irrigated with water treated with quinclorac, topramezone, imazamox, and penoxsulam to identify herbicide concentrations that cause phytotoxic effects. Plants were irrigated four times over a 10-day period with the equivalent of 0.5 inch of treated water during each irrigation and were then irrigated with tap water until they were harvested 28 days after the first herbicide treatment. Visual quality and dry weight data revealed that melampodium was the most sensitive of the bedding plants to quinclorac, imazamox, and penoxsulam, whereas vinca was the most sensitive species to topramezone. Noticeable reductions in visual quality and dry weight of melampodium were evident after exposure to 240, 580, and 10 ppb of quinclorac, imazamox, and penoxsulam, respectively, while dry weight of vinca was reduced after exposure to 110 ppb of topramezone. Current irrigation restrictions on imazamox, penoxsulam, and topramezone are adequate to minimize damage to these bedding plants if herbicide-treated waters are used for four irrigation events. However, irrigation restrictions should be established for quinclorac to prevent damage to sensitive bedding plants such as melampodium.

Native aquatic plants are important to maintaining a balanced ecosystem, but they often are displaced by exotic invasive plant species. The research on the control and growth of the invasive aquatic species hydrilla (Hydrilla verticillata) using sand substrates and controlled-release fertilizers (CRF) provides a potential production technique for other aquatic plants. We questioned if we could use hydrilla production techniques to grow southern naiad (Najas guadalupensis), a Florida-native aquatic plant that is often mistaken for hydrilla. We grew southern naiad cuttings in containers filled with 100:0, 75:25, 50:50, 25:75, or 0:100 coarse builder’s sand and sphagnum moss (by volume). Before planting, containers were fertilized with 0, 1, 2, or 4 g·kg⁻¹ CRF (15N–4P–10K). Containers were submerged in large storage tubs filled with rainwater and grown for 8 weeks. Southern naiad shoot dry weight was greater in the 100% sand substrate than that in the 0% sand substrate. Substrate electrical conductivity (EC) levels were greater in the 0% sand with no difference among the other substrates. Shoot and root dry weight of plants fertilized with 1–2 g·kg⁻¹ CRF were greater than 0 or 4 g·kg⁻¹ CRF. Substrate EC also increased as fertilizer rate increased, with the highest EC observed at 4 g·kg⁻¹ CRF. Based on our results, we would suggest growing southern naiad in substrates with 100% sand and fertilized with 1–2 g·kg⁻¹ CRF.

Herbicides that are labeled for aquatic use are often the foundation of aquatic vegetation management programs in the United States because many of these products, which are registered by the U.S. Environmental Protection Agency, are effective, selective, and relatively inexpensive. Resource managers are interested in reducing the use of synthetic herbicides and are considering alternative methods for aquatic weed control. We evaluated the effects of acetic acid and d-limonene on growth of the invasive small floating species feathered mosquitofern (Azolla pinnata) and common salvinia (Salvinia minima), as well as on the native emergent wetland plants cattail (Typha latifolia) and gulf coast spikerush (Eleocharis cellulosa). Acetic acid and d-limonene (alone and in combination) were applied once as foliar treatments to healthy plants, which were grown for 8 weeks after treatment to allow for development of phytotoxicity symptoms. All experiments also included diquat dibromide at three concentrations as “industry-standard” treatments for comparison. A 0.22% concentration of diquat dibromide eliminated all vegetation of all species. Most single-product treatments provided good control of invasive feathered mosquitofern with acceptable levels of damage to native gulf coast spikerush, but only 15% and 20% d-limonene treatments were effective on invasive common salvinia and selective for native cattail. Some combinations of acetic acid and d-limonene provided acceptable control of both floating weeds and selectivity for gulf coast spikerush, but all mixes caused unacceptable levels of damage to cattail. Treating these small floating weeds with acetic acid and d-limonene instead of diquat dibromide would increase material costs by 15- to 27-fold. Although these natural products may be useful in some areas where synthetic herbicides are discouraged, they are unlikely to be affordable options for most resource managers.

Topramezone and bispyribac-sodium were registered for aquatic weed control in the last decade. A primary target for these products is fluridone-resistant hydrilla (Hydrilla verticillata), which is one of the most invasive submersed weeds in the southeastern United States. Both products have water use restrictions that prohibit irrigation of turfgrasses with treated waters until the herbicides have degraded to very low concentrations. The objective of these studies was to identify the concentrations of topramezone and bispyribac-sodium that are phytotoxic to turfgrasses that are commonly planted in Florida. Three species of turfgrass were irrigated twice weekly with 0.5 inch of treated water for 4 weeks (eight irrigations total). Cumulative EC₁₀ values (the herbicide concentration that caused a 10% reduction in biomass compared with untreated control plants) after eight irrigations with water containing topramezone were 3.5, 4.3, and 17 ppb for ‘Palmetto’ st. augustinegrass (Stenotaphrum secundatum), ‘Pensacola’ bahiagrass (Paspalum notatum), and ‘Tifway 419’ hybrid bermudagrass (Cynodon dactylon × C. transvaalensis), respectively. Bispyribac-sodium was less toxic to all turfgrasses evaluated, with EC₁₀ values of 56, 16, and >800 ppb for ‘Palmetto’ st. augustinegrass, ‘Pensacola’ bahiagrass, and ‘Tifway 419’ hybrid bermudagrass, respectively. These results support label instructions and highlight the need to comply with irrigation restrictions because the typical use concentrations for submersed weed control with topramezone and bispyribac-sodium are in the 20–40-ppb range.

Topramezone is a 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide that was labeled for aquatic use in Florida in 2013 with a maximum submersed application concentration of 50 µg·L⁻¹. Preliminary greenhouse studies reported that the concentration of herbicide that reduces growth by 10% compared with untreated controls (EC₁₀) of topramezone in irrigation water applied to st. augustinegrass (Stenotaphrum secundatum) grown in 100% sand was 3.5 ppb. The objective of these experiments was to determine whether substrate carbon content influenced the response of ‘Palmetto’ st. augustinegrass to irrigation with topramezone-treated water. The herbicide was applied at concentrations ranging from 0 to 120 ppb to mature plants grown in 7.5-inch-diameter nursery containers. Pots were filled with washed masonry sand amended with one of five carbon contents: 0%, 0.3%, 0.6%, 1.5%, and 4.0%. Plants were irrigated twice weekly for 4 weeks with topramezone-containing water and grown out for 12 weeks after the final topramezone treatment to evaluate possible recovery from any herbicide damage. Plant material was clipped as needed for a total of eight harvests and each harvest was dried and weighed. EC₁₀ values for ‘Palmetto’ st. augustinegrass grown in substrates with 0%, 0.3%, 0.6%, 1.5%, and 4.0% carbon were 3.7, 7.3, 10.1, 28.1, and 25.7 ppb, respectively. These experiments revealed that substrate carbon content has a noteworthy effect on the susceptibility of ‘Palmetto’ st. augustinegrass to topramezone in irrigation water. However, regular irrigation with water containing high concentrations of topramezone is likely to cause damage to ‘Palmetto’ st. augustinegrass in Florida's sandy soils.

The foundation of most aquatic weed management programs in Florida is synthetic herbicides because many of these U.S. Environmental Protection Agency (USEPA)-registered products are effective, selective, and inexpensive compared with other strategies such as mechanical harvesting. However, stakeholders have expressed concern regarding their use and managers are interested in exploring alternative methods for aquatic weed control. To that end, we evaluated the efficacy, selectivity, and costs of the “natural” products acetic acid and d-limonene (alone and in combination with each other and citric acid) on the invasive floating plants waterhyacinth (Eichhornia crassipes) and waterlettuce (Pistia stratiotes), and the native emergent plants broadleaf sagittaria (Sagittaria latifolia) and pickerelweed (Pontederia cordata). These products, plus an industry-standard synthetic herbicide (diquat dibromide), were applied once as foliar treatments to healthy plants, which were grown out for 8 weeks after treatment to allow development of phytotoxicity symptoms. A 0.22% concentration of diquat dibromide eliminated all vegetation, but neither “natural” product alone provided acceptable (>80%) control of floating weeds, even when applied at the maximum concentrations under evaluation (20% acetic acid, 30% d-limonene). Citric acid (5% or 10%) had no effect on the activity of acetic acid or d-limonene, but some combinations of acetic acid and d-limonene controlled floating weeds effectively without causing unacceptable damage to native plants. However, these treatments are much more expensive than the synthetic standard and managers would realize a 22- to 26-fold increase in product cost alone without factoring in other expenses such as additional labor and application time. Combinations of acetic acid and d-limonene may have utility in some areas where the use of synthetic herbicides is discouraged, but broad-scale deployment of this strategy would likely be prohibitively expensive.

Most lake, canal, and pond management programs in the United States use herbicides labeled for aquatic use because many of these products, which are registered by the US Environmental Protection Agency, are relatively inexpensive and can effectively control undesirable plants without excessive damage to desirable species. Managers of these resources have expressed an interest in alternative methods for aquatic weed control that could reduce the use of traditional synthetic herbicides. We studied the effects of acetic acid and d-limonene on growth of the invasive aquatic species rotala (Rotala rotundifolia) and crested floatingheart (Nymphoides cristata), as well as on the native wetland plants spatterdock (Nuphar advena) and giant bulrush (Schoenoplectus californicus). We applied acetic acid and d-limonene (alone and in combination) once as foliar treatments to healthy plants, then grew out the plants for 8 weeks after treatment to observe damage resulting from treatments. We also evaluated diquat dibromide at three concentrations as “industry-standard” synthetic treatments for comparison. A 0.22% concentration of diquat dibromide eliminated most or all vegetation of rotala, crested floatingheart, and giant bulrush, but was much less damaging to spatterdock. Single-product applications of acetic acid or d-limonene had little effect on any of the four species evaluated. Some combinations of acetic acid and d-limonene provided acceptable control of rotala and selectivity on spatterdock and giant bulrush, but no treatments reduced crested floatingheart growth by more than 40%. Treating rotala with acetic acid and d-limonene instead of diquat dibromide would result in a 25-fold increase in material costs, which would make this option unaffordable for most aquatic system managers.