Three Seeded Mercury Weed

Finally, I know who this fellow is, sort of. Acalypha…somebody, probably rhomboidea, commonly known as three-seeded mercury. He and his brothers are everywhere in my garden. This annual member of the Euphorbiaceae family starts as a thin, erect reddish stem with narrow leaves, about an inch to an inch and a half long, arranged opposite… OMAFRA Crops OMAF Crops

Know thine enemy: Three-seeded mercury

Finally, I know who this fellow is, sort of. Acalypha…somebody, probably rhomboidea, commonly known as three-seeded mercury. He and his brothers are everywhere in my garden.

This annual member of the Euphorbiaceae family starts as a thin, erect reddish stem with narrow leaves, about an inch to an inch and a half long, arranged opposite along the stem. As it grows, it branches, and leaves are arranged alternately. Here is where my ignorance of botany is exposed:There appear to be small yellow flowers at the leaf axils, but those yellowish bits I see could be bracts, or technically it might be an inflorescence …anyway, if you care to read details about the plant’s structure, you can read the description from the University of Guelph extension, or the Wikipedia site. For me, right now, I know I’m fairly close to identifying the plant.

This summer annual weed is not a nuisance, except that there’s a lot of it in my garden. It doesn’t reseed aggressively like hairy bittercress and it’s not difficult to eradicate. Despite the taproot, the plants are easy to pull. They are also said to be browsed by deer (not if there are phlox and hosta to eat, they’re not).

According to the Southern Living Garden Problem Solver, which may or may not have misclassified this as Acalypha virginica (my plant definitely doesn’t look like the one shown by Illinois Wildflowers.info), many insects love to feed on the leaves. Thus, my sample, with its raggedy, chewn leaves should be pretty typical.

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The seeds are supposed to be choice food for mourning doves, whom I would gladly welcome to my garden because I love their call. The buffet is open!

Ministry of Agriculture, Food and Rural Affairs

General Description: Annual, reproducing only by seed.

Photos and Pictures


Three-seeded mercury.
A. Plant. B. Portion of fruiting stem showing 1 three-seeded fruit developing from 1 of 4 flowers in a leaf axil.

Stems & Roots: Stems erect, 7.5-100cm (3-40in.) high, simple or branched, slightly hairy; leaves green to bronze-green, 1-9cm (2/5-3½in.) long, lance- to rhombic-ovate on petioles that are 1/3 the length to almost as long as the leaf blade; margins with irregular, rounded teeth.

Habitat: Three-seeded mercury occurs in dry or moist soil in open woods, fields, waste places, ditches and roadsides throughout south-central Ontario.

Similar Species: 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.

Related Links

. on general Weed topics
. on weed identification, order OMAFRA Publication 505: Ontario Weeds
. on weed control, order OMAFRA Publication 75: Guide To Weed Control

Ministry of Agriculture, Food and Rural Affairs

A member of the spurge family. Three-seeded mercury has become 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.

Life Cycle

Annual, reproducing only by seed.

Distinguishing Characteristics

In the seedling stage this weed is often confused with redroot and green pigweed. It is distinguished from pigweed species by its glossy bronze-green leaf colour, leaf margins with irregular, rounded teeth and clusters of greenish flowers at each axil. In addition Three-seeded mercury has round cotyledons, whereas green and redroot pigweed has very long and slender cotyledons.

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Three-seeded Mercury Pictures

Each thumbnail image links to a larger image

Herbicide Control in Field Corn

Pre-Emergent Control in Corn

A pre-emergent herbicide containing the active ingredient atrazine will provide very good control of three-seeded mercury (Table 1).

Various Trade Names Exist
Post-Emergent Control in Corn

Again, products containing the active ingredient atrazine as well as Distinct, PeakPlus and Summit provide effective control of emerged three-seeded mercury (Table 2). Pardner or Koril (applied alone) and Banvel II do not control three-seeded mercury.

Various Trade Names Exist PARDNER or KORIL + atrazine

Source: Dr. Peter Sikkema and Dr. Clarence Swanton, University of Guelph.

Number of Trials:

Table 1 is based on 4 field trials in Ontario, 2 conducted by Dr. Peter Sikkema and 2 conducted by Dr. Clarence Swanton.

Table 2 is based on 3 field trials in Ontario conducted by Peter Sikkema.

Herbicide Rates: Rates used in this trial are listed in OMAF Publication 75 – Guide to Weed Control.

Weed Stage: Three-seeded mercury had not emerged at the time of any pre-emergent applications and was in between the cotyledon to 8 leaf stage at the time of all post-emergent applications.

What has been your experience?

We want your feedback. Let us know what you have experienced with these or other products, as well as any other effective management strategies.

Herbicide Control in Soybeans

Pre-emergent Control in Soybean

If growing no-till soybeans, a pre-plant glyphosate burndown should control any emerged seedlings. FirstRate or the high rate of Sencor should provide excellent residual control of three-seeded mercury. Broadstrike Dual Magnum has proven to be inconsistent as it has offered excellent control in some trials, while performing poorly in others.

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FIRSTRATE (High Rate) SENCOR (High Rate) SENCOR (low Rate) BROADSTRIKE DUAL MAGNUM LOROX L (High Rate) DUAL II MAGNUM
Post-emergent Control in Soybean

Control of three-seeded mercury has been fairly inconsistent with many of the post-emergent herbicides. FirstRate and Classic would appear to be the best option for post-emergent control of this weed.

FIRSTRATE (High Rate) glyphosate (Roundup Ready Soybeans only) CLEANSWEEP BASAGRAN FORTE

Source: Dr. Peter Sikkema and Dr. Clarence Swanton, University of Guelph.

Number of Trials:

Table 1 is based on a summary of 1 field trial in no-till soybean.

Table 2 is based on a summary of 6 field trials in Ontario, 4 conducted by Dr. Peter Sikkema and 2 conducted by Dr. Clarence Swanton.

Herbicide Rates: Rates used in this trial are listed in OMAF Publication 75 – Guide to Weed Control.

Weed Stage: Three-seeded mercury had not emerged at the time of all pre-emergent applications and was at the cotyledon to 8 leaf stage for all post-emergent applications.

What has been your experience?

We want your feedback. Let us know what you have experienced with these or other products, as well as any other effective management strategies.

Herbicide Control in Winter Wheat

Not a very common weed in winter cereals due to its emergence being in the spring when many winter cereals are beginning to fill in and “shade out” many annual species. A limited number of products have been tested, of which MCPA and 2,4-D appear to offer adequate control (Table 1).