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Publication #ENH1272

Biology and Management of Pilea microphylla (Artillery weed) in Ornamental Crop Production1

Debalina Saha, Chris Marble, Shawn Steed, and Nathan Boyd2

Species Description

Class: Dicotyledonous plant

Family: Urticaceae

Other Common Names: rockweed, gunpowder plant, artillery weed

Life Span: annual or short lived-perennial

Habitat: Artillery weed occurs primarily in moist, disturbed areas but can tolerate dry conditions once established. In landscapes, artillery weed is typically found growing in rock mulch beds, in cracks in driveways and hardscapes, and in planting beds. In nurseries, it can be problematic in containers, container drain holes, ground cloth (Figure 1), walkways, aisles, and in greenhouses. It prefers a partially shaded environment but can grow in full sun.

Figure 1. 

Artillery weed growth through nursery ground cloth.


Credit:

Annette Chandler, UF/IFAS


[Click thumbnail to enlarge.]

Distribution: Artillery weed is thought to be native to South America, but some authors claim it is native to parts of North America (USDA 2016). It is found throughout Florida, but is most problematic in the central and southern parts of the state. It has also become widely naturalized in subtropical and tropical areas in many parts of the world.

Growth Habit: It is a low growing succulent herb that can spread 1 to 2 feet in width and grows 0.5 to 1.5 feet in height.

Seedling: Cotyledons are green with opposite, obcordate (they start at a point and then widen until they form a notch at the apex) leaves with smooth margins (Figure 2). First true leaves are green. Stems are whitish in color.

Figure 2. 

Artillery weed seedlings.


Credit:

Annette Chandler, UF/IFAS


[Click thumbnail to enlarge.]

Shoots: Stems are fragile, green, nearly translucent, succulent, and spreading (Figure 3). Plants have opposite to subopposite, simple, obovate leaves with short petioles and with most but not all leaves having three primary veins originating from the leaf base. Leaves are evergreen and are 3.64–1.13 mm wide and 8.25–1.14 mm long.

Figure 3. 

Artillery weed stems.


Credit:

Annette Chandler, UF/IFAS


[Click thumbnail to enlarge.]

Roots: Fibrous roots or rarely a short taproot.

Inflorescence: dense cyme (a group of flowers where the growing points end in a flower), branched clusters from leaf axils

Flowers: small, whitish to greenish, unisexual; sepals 4 mm in male flowers, 3 mm in female; no petals; 4 stamens; year round flowering occurs (Figure 4).

Figure 4. 

Artillery weed in flower.


Credit:

Annette Chandler, UF/IFAS


[Click thumbnail to enlarge.]

Fruits: achene, light brown, length less than 0.5 mm, smooth (Gilman 1999).

Similar Species: Pilea pumila (Canadian clearweed) is a similar species to Pilea microphylla. The distinguishing character of Pilea pumila is that it has broad, ovate to elliptic leaves, that grow to 4 in. long and 2.5 in. wide, and leaf margins are toothed (serrated). Artillery weed has small smooth edged leaves, usually less than 8.25 mm long and 3.64 mm wide.

Plant Biology: Year-round flowering occurs, and the pollen grains are ejected forcefully (hence the common name artillery weed). Artillery weed grows primarily in shaded areas and can tolerate extended flooding, alkalinity, clay, sand, and acidic loam (Gilman 1999). This plant shows moderate drought tolerance and poor tolerance in soils with high salinity (Gilman 1999).

Management

Cultural and Physical Control

Cultural control involves proper sanitation practices, such as using weed-free growing media and clean pots. Inspect plant material for artillery weed seedlings when bringing in new plant material into the nursery or landscape. In landscapes, remove artillery weed seedlings from potted plants before transplanting to prevent future infestations.

Hand weeding effectively controls artillery weed in small areas. Because small seedlings are not easily removed by hand and artillery weed can regrow from roots or stem fragments, hand weeding must be thorough. Large and densely infested areas may be too labor intensive for such thorough hand weeding. Applying organic mulch (pinestraw, pinebark, etc.) at a depth of at least 2 inches has been shown to provide control in research trials.

Chemical Control

Preemergence Control

Most of the common preemergence herbicides provide excellent control of artillery weed. Oxyfluorfen has been shown to provide good control (Freitas et al. 2004, Freitas et al. 2007a, Freitas et al. 2007b). Research trials at UF/IFAS have shown the following preemergents to provide excellent control: oxadiazon (Ronstar® G), dimethanamid-p (Tower®), indaziflam (Marengo® or Specticle®), pendimethalin + dimethanamid-p (FreeHand®), oxyfluorfen + pendimethalin (OH2®), oxyfluorfen + prodiamine (Biathlon®), prodiamine (Barricade®), pendimethalin (Pendulum®), s-metolachlor (Pennant Magnum®), isoxaben (Gallery®), and prodiamine + isoxaben (Gemini™). Table 1 lists preemergence herbicides labeled for use in ornamental plant production and landscapes and that can be used for the control of artillery weed.

Postemergence Control

Due to prolific seed production, it is important to tank-mix a preemergence herbicide in with a postemergence herbicide when attempting to control artillery weed. Herbicides, including oxadiazon (Ronstar® FLO) (Conover and Stamps 1994), flumioxazin (SureGuard®), and glufosinate (Finale®), have provided excellent postemergence control in research trials. Oxadiazon and flumioxazin are both primarily used as preemergence herbicides but do provide some early postemergence control. In greenhouse trials, these two herbicides have provided greater than 95% control of mature artillery weed. Contact herbicides, including pelargonic acid (Scythe®) and diquat (Reward®), can provide temporary control but re-treatment is typically needed. Glyphosate (RoundUp® and many others), sulfosulfuron (Certainty®), and sulfentrazone (Dismiss®) typically only provide suppression or poor control. If artillery weed is growing in containers, directed applications can be made using a contact action herbicide such as pelargonic acid or diquat but care must be taken to ensure ornamental foliage is not contacted. If the ornamental growth habit does not allow for directed applications to be made, granular formulations of oxadiazon (Ronstar® G) and liquid formulations of dimethenamid-p (Tower®) can provide some suppression and are labeled for over-the-top use in many crops. A list of postemergence herbicides, including their labeled sites, are included in Table 2.

References

Conover, C.A. and R.H. Stamps. 1994. Controlling artillery plant (Pilea microphylla) with herbicides. University of Florida Research Report RH-94-7. http://mrec.ifas.ufl.edu/foliage/resrpts/rh_94_7.htm

Freitas, F. C. L. et al. 2004. “Chemical weed control in bromeliários”. Congresso Brasileiro Da Ciência Das Plantas Daninhas São Pedro 24 CD-ROM.

Freitas, F. C. L., J.A.S Grossi, A.F. Barros, E.R. Mesquita, F.A. Ferreira, and J.G. Barbosa. 2007a. “Chemical control of Pilea microphylla in Orchid cultivation”. Planta Daninha 25(3): 589–593.

Freitas, F. C. L., J.A.S Grpsso, A.F. Barros, E.R. Mesquita, and F.A. Ferreira. 2007b. “Weed control in ornamental plant seedling production”. Planta Daninha 25(3) 595–601.

Gilman, E. F. 1999. Pilea microphylla Artillery Plant. FPS479. Gainesville: University of Florida Institute of Food and Agricultural Sciences. http://edis.ifas.ufl.edu/fp479

Mallory-Smith, Carol A. and E. James Retzinger, Jr. 2003. “Revised Classification of Herbicides by Site of Action for Weed Resistance Management Strategies.” Weed Technology 17, no. 3: 605-19. http://www.jstor.org/stable/3989198.

USDA Plants Database. 2016. Pilea microphylla (L.) Liebm. http://plants.usda.gov/core/profile?symbol=PIMI2.

Tables

Table 1. 

Preemergence herbicides labeled for use in ornamental plant production and landscapes and their efficacy for control of artillery weed.

Common Name (active ingredient)

Example trade name and formulation

Labeled Use Rate (Product/Acre)

WSSA Herbicide Group1

Efficacy2

Container production

Field production

Greenhouse or fully-enclosed structures

Landscape

dithiopyr

Dimension® 2EW

1 to 2 pts.

3

G

YES

YES

NO

YES

pendimethalin

Pendulum® 2G

100 to 200 lbs.

3

G

YES

YES

NO

YES

Pendulum® 3.3EC

2.4 to 4.8 qt.

YES

YES

NO

YES

prodiamine

RegalKade 0.5G

132 to 300 lbs.

3

G

YES

YES

NO

YES

Barricade® 4FL

21 to 48 fl. oz.

flumioxazin

Broadstar™ 0.25G

150 lbs.

14

G

YES

YES

NO

YES

SureGuard® 51WDG

8 to 12 oz.

YES3

YES3

NO

YES4

oxadiazon

Ronstar® 2G

 

14

G

YES

YES

NO

YES

dimethenamid-p

Tower® 6EC

21 to 32 fl. oz.

15

G

YES

YES

NO

YES

s-metolachlor

Pennant Magnum® 7.6 EC

1.3 to 2.6 pts.

15

S-G

YES

YES

NO

YES

isoxaben

Gallery® 75DF

0.66 to 1.33 lbs.

21

G

YES

YES

NO

YES

Gallery® 4.16SC

16 to 31 fl. oz.

indaziflam

Marengo® 0.622 SC

7.5 to 15.5 fl.oz.

29

G

NO5

YES

YES6

NO7

Marengo® 0.0224G

100 to 200 lbs.

YES

YES

NO

NO7

pendimethalin+dimethenamid-p

FreeHand® 1.75G

100 to 200 lbs.

3 + 15

G

YES

YES

NO

YES

1Herbicide groups are based according to primary sites of action and can be used to select herbicides that have differing sites of action (Weed Technology 17:605-619 [2003]).

2P = poor control; S = suppression, G = good control.

3Can only be used in selected conifer and deciduous tree species. Check manufacturer's label for a complete list of species and recommended application methods.

4Can be applied as a directed application around established woody landscape ornamentals.

5Marengo® 0.622 SC can be used in pot-in-pot container ornamentals as a directed application only.

6Labeled for use on greenhouse floors prior to plant production. Plants can be placed back inside greenhouse 24 hrs after application.

7Indiziflam is also available by the trade name Specticle which can be applied to turf and landscape sites.

Table 2. 

Postemergence herbicides labeled for use in and around ornamentals for control of artillery weed1.

Active Ingredient

Example trade name

Labeled Use Rates (product/Acre)

WSSA Herbicide Group

Container production2

Field production

Greenhouse or fully-enclosed structures

Landscape

Notes:

Diquat

Reward®

1 to 2 qt.

22

YES

YES

YES

YES

Thorough coverage and repeated applications may be needed.

Flumioxazin

SureGuard®

8 to 12 oz.

14

YES3

YES3

NO

YES3

Higher rate provides better postemergence control. Primarily used as a preemergence herbicide.

Glufosinate

Finale®

2 to 6 qt.

10

YES

YES

YES

YES

Thorough coverage is needed.

Oxadiazon

Ronstar® FLO

80 to 122 fl. oz.

14

YES4

YES4

NO

YES

Thorough coverage is needed. Primarily used as a preemergence herbicide.

1Postemergence control is highly dependant upon the growth stage at the time of application. Many factors can impact herbicide performance. Repeat applications and use of preemergence herbicides may be needed for complete control.

2Herbicide groups are based according to primary sites of action and can be used to select herbicides that have differing sites of action (Mallory-Smith and Retzinger 2003).

3Labeled for use as a directed application or over-the-top only to certain trees and conifers. Consult product label for details.

4Labeled for use as a directed application or over-the-top only to certain conifers and a small list of other crops. Consult product label for details.

Footnotes

1.

This document is ENH1272, one of a series of the Environmental Horticulture Department, UF/IFAS Extension. Original publication date December 2016. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Debalina Saha, graduate research assistant, Environmental Horticulture Department, Mid-Florida Research and Education Center; Chris Marble, assistant professor, Environmental Horticulture Department, Mid-Florida REC; Shawn Steed, environmental horticulture production Extension agent, UF/IFAS Extension Hillsborough County; and Nathan Boyd, associate professor, Department of Horticulture, Gulf Coast REC; UF/IFAS Extension, Gainesville, FL 32611.


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U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.