Three-Seeded Mercury
(Acalypha rhomboidea Raf.).COMMON NAMES: Copper-leaf, rhombic copperleaf, ricinelle rhomboide.
DISTRIBUTION: Nova Scotia, southwest Quebec, southwestern and western Ontario, central Maine, Minnesota, Florida, Alabama, Arkansas, Oklahoma, North Dakota.
OCCURRENCE: Common in river flats and low fields, in dry or moist soils, roadsides, waste places, ditches, borders of woods and in open woods.
BIOLOGY: Three-seeded mercury falls under the sub-family Acalyphoideae of the family Euphorbiaceae. With about 450 species, Acalypha is the fourth largest genus of the family Euphorbiaceae. The three-seeded mercury gets its name from its characteristic 3-lobed seed pods containing 3 seeds. They are annuals reproducing only by seed and have erect stems ranging from 7.5-100 cm (3-40 in.) in height. The stems are high, simple or branched and slightly hairy. The leaves are green to bronze-green ranging in length from 1-9 cm (2/5 -3 ½ in.); long, lance- to rhombic-ovate in shape growing on petioles that are 1/3 the length to almost as long as the leaf blade. The leaf margins are irregular with rounded teeth. The flowers are in greenish clusters in axils of leaves. Each cluster is composed of one or more palmately cleft bracts with 5 to 9 lobes. The flowers consist of one or more stalked male spikes ranging in length from 4-15 mm (1/6-2/5 in.) and one or more shorter female flowers. The seedpods are 3-lobed (similar to those of leafy spurge) and contain 3 seeds. The seeds are tan coloured and range in length from 1.6-1.8 mm (1/15 in.). The plant flowers from July to September and it resembles young plants of Redroot pigweed but is distinguished by its flowers borne in axillary clusters with bracts having 5-9 lobes and its leaves occasionally a bronze-green colour.
IMPORTANCE IN AGRICULTURE: Three-seeded mercury is becoming a serious weed in agricultural fields where group 2 herbicides (e.g., Pinnacle, Pursuit etc.) have been repeatedly used. This weed can cause serious crop losses in cereal, corn and soybean fields.
CONTROL/MANAGEMENT OPTIONS FOR THREE-SEEDED MERCURY: Very few research articles exist on this species as a weed. As a result, very little is known about the control options for three-seeded mercury in agricultural fields. The Weed Research group at the University of Guelph conducted an experiment for 2 years at various locations in Ontario from 1995 to 1997 to test the effectiveness of several herbicides for the control of three-seeded mercury. The results (average of 2 years) are as follows:
Control of three-seeded mercury in herbicide tolerant transgenic crops: The effectiveness of Roundup and Liberty was tested in Roundup Ready soybean and Liberty Link corn. It was found that Roundup provided 80% control of three-seeded mercury whereas Liberty provided only 9% control (Table 1).
Table 1: Non-selective control of three-seeded mercury in herbicide tolerant crops.| Herbicide | Dose (g ai/ha) | Application | Percent (%) control |
| Roundup | 900 | Post | 80 (± 6) |
| Liberty | 400 | Post | 9 (± 6) |
Control of three-seeded mercury in corn: Pre or postemergence application of herbicides such as Atrazine and 2,4-D amine provided excellent control of three-seeded mercury in corn (Table 2).
Table 2: Effectiveness of some herbicides in controlling three-seeded mercury in corn.| Herbicide | Dose (g ai/ha) | Application | Percent (%) control |
| Atrazine | 1000 | Pre or Post | 98 (± 2) |
| 2,4-D amine | 500 | Post | 90 (± 5) |
| Banvel | 300 | Post | 68 (± 2) |
| Striker | 236 | Post | 61 (± 1) |
| Pardner | 300 | Post | 13 (± 0) |
Control of three-seeded mercury in soybean: Of the several Pre and Post emergence application of several herbicides tested none provided excellent control of three-seeded mercury in soybean. Lexone/Sencor applied PRE provided the best control of three-seeded mercury in soybean (Table 3).
Table 3: Effectiveness of some herbicides in controlling three-seeded mercury in soybean.| Herbicide | Dose (g ai/ha) | Application | Percent (%) control |
| Lexone/Sencor | 600 | Pre | 75 (± 8) |
| Broadstrike Dual | 2200 | Pre | 57 (± 14) |
| Lorox/Afolan | 1000 | Pre | 53 (± 11) |
| Reflex | 250 | Post | 55 (± 27) |
| Pursuit | 75 | Post | 42 (± 11) |
| Blazer | 150 | Post | 19 (± 1) |
| Pinnacle | 6 | Post | 13 (± 7) |
| Bentazon | 1100 | Post | 9 (± 1) |
RECOMMENDATION: Adequate control of three-seeded mercury can be obtained by a postemergence application of Roundup in Roundup Ready soybean. In corn, the best control can be obtained by a pre or postemergence spray of treatments with Atrazine, for example, Primextra, Marksman or Atrazine alone. In soybean, a preemergence application of Lexone/Sencor may be the best treatment.