Mexican Prickly Poppy: Biology and Control1

Brent Sellers 2

Mexican prickly poppy (Argemone mexicana), also known as goatweed, Mexican thistle, prickly poppy, and yellow thistle, is a member of the poppy family (Figure 1). It is an annual or biennial plant that is found east of the Rocky Mountains, Hawaii, and Puerto Rico. In Florida, it is typically recognized as 'some type of thistle' until it flowers.

Figure 1. Mexican prickly poppy in a bahiagrass pasture.
Figure 1.  Mexican prickly poppy in a bahiagrass pasture.

Young Mexican prickly poppies are often mistaken for thistles because their toothed and prickly leaf margins give them a thistle-like appearance (Figure 2). One recognizable difference is that Mexican prickly poppy exudes a yellowish milky sap when its stem is broken, while thistles do not. Another difference is that Mexican prickly poppy flowers are relatively large and yellow with 4 to 6 petals (Figure 3), while thistles have inflorescences composed of many small flowers. Like thistles, Mexican prickly poppy reproduces only through seed production. Its seeds are enclosed in a relatively large spiny capsule (Figure 4). Approximately 400 seeds can be produced in one capsule.

Figure 2. Mexican prickly poppy plants have a thistle-like appearance prior to flowering.
Figure 2.  Mexican prickly poppy plants have a thistle-like appearance prior to flowering.

Figure 3. Flowers of Mexican prickly poppy are approximately 2.5 inches in diameter and have 4 to 6 yellow petals.
Figure 3.  Flowers of Mexican prickly poppy are approximately 2.5 inches in diameter and have 4 to 6 yellow petals.

Figure 4. Seed capsules of Mexican prickly poppy are approximately 1.5 inches long with numerous spiny prickles.
Figure 4.  Seed capsules of Mexican prickly poppy are approximately 1.5 inches long with numerous spiny prickles.

Key Characteristics

Stem: Cylindrical, whitish in color and with scattered prickles. The broken stem exudes a yellowish milky sap.

Leaves: Leaves can be up to 8 inches long, and are silvery-green with white veins and deep regular lobes. The upper surface of the leaf is smooth, while the underside has a few prickles along the midrib. The edges of the leaf are often lined with many prickles.

Flowers: Flowers are yellow and approximately 2.5 inches in diameter.

Seed: Seeds are produced inside a prickly capsule measuring about 1.5 inches in length. Approximately 3 to 6 openings in the capsule allow the seeds to disperse, but many seeds can remain inside the capsule for weeks until wind or animals shake the plant. Up to 400 seeds can be enclosed by a single capsule and they may stay dormant in the soil for many years.

Mexican prickly poppy is poisonous to livestock, but is not readily eaten, so there are relatively few cases of poisoning from this plant. However, plants in hay have caused poisoning. Seeds are its most toxic part, but the entire plant contains toxic elements.

Although this weed is common throughout Florida, it is typically not a wide-spread problem. For those who wish to control it, 2,4-D, WeedMaster (or other products containing 2,4-D + dicamba), or GrazonNext HL are the most effective and economical herbicides for control. The application rate for 2,4-D or WeedMaster is 3 pt/acre for broadcast applications, and 24 oz/acre for GrazonNext HL If spot spraying is necessary, a 3% solution of either 2,4-D or 2,4-D + dicamba or a 1% solution of GrazonNext HL in water is appropriate. Remember, it is best to treat a younger plant, especially before seed set occurs, to ensure that seeds are not added to the soil seedbank.

Footnotes

1. This document is SS AGR 304, one of a series of the Agronomy Department, UF/IFAS Extension. Original publication date May 2008. Revised February 2014 and April 2017. Reviewed April 2020. Visit the EDIS website at https://edis.ifas.ufl.edu.
2. Brent Sellers, assistant professor; UF/IFAS Range Cattle Research and Education Center, Ona, FL 33865.

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication does not signify our approval to the exclusion of other products of suitable composition. All chemicals should be used in accordance with directions on the manufacturer's label.

Publication #SS AGR 304

Date: 2020-10-11
Sellers, Brent A
Agronomy

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  • Brent Sellers