Controlling Artillery Plant (Pilea microphylla) with Herbicides

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C.A. Conover, Ph.D.and R.H. Stamps, Ph.D.*

University of Florida/IFAS
Central Florida Research and Education Center
CFREC-Apopka Research Report RH-94-7

Weed control in growing containers and nursery areas should be an important part of pest management programs. Weeds in nurseries are not only unsightly; they can hinder growth of ornamental crops; increase production costs due to weeding labor or diminish value of containerized plants if not removed, and they can harbor insects or diseases, which may be passed on to the crop. Pilea microphylla (L.) Liebm, commonly called artillery plant, is a creeping herb that thrives under the humid conditions typical of many nursery areas. By freely re-seeding itself, this plant spreads rapidly, and can quickly become a major problem.

For this reason, two experiments were performed to test six herbicides (Table 1) for control of artillery plant. The tests are identical except for time of the year performed, with experiment 2 being a repeat of experiment 1. Artillery plant growing as a weed in pots containing foliage plants or in cracks in shadehouse ground pack were transplanted into 6-inch tubs using Florida sedge peat as the growing medium. Plants were placed on black plastic ground pack located outside under full sun or in a shadehouse under 60% shade provided (by polypropylene shadecloth) and allowed to grow undisturbed until growing medium surface of each container was completely covered by artillery plant foliage.

Both experiments were 7 x 2 factorial tests, with 7 replications per treatment. Artillery plants were sprayed with a 0.25% non-ionic surfactant solution or one of six herbicide solutions. Herbicides tested were Barricade® 65 WG, Gallery® 75 DF, Image® 1.5 EC, Premier® 1.2 EC, Ronstar® 50 WP and Roundup®. Herbicide solutions contained a 0.25% non-ionic surfactant except for the solution with Roundup®, which contained a 0.50% by volume non-ionic surfactant. Herbicide solutions were prepared and applied following label instructions.

In experiment 1, artillery plant was transplanted into 6-inch containers on June 21, 1993. Each container was top-dressed with 4 g/6-in pot 19-6-12 Osmocote® (Grace/Sierra Horticultural Products, 1001 Yosemite Dr., Milpitas, CA 95035) on July 22, 1993. On September 29, 1993, plants were sprayed with a surfactant solution or a solution containing surfactant and one of the six herbicides. Plants were assigned a phytotoxicity rating (based on a scale of 1 = no visible effects, artillery plant still healthy looking, 5 = visible damage with moderate control, 10 = artillery plant completely eradicated) two, four and eight weeks after herbicide application (October 13, October 27 and November 24, 1993, respectively). Percent of artillery plant foliage covering the growing medium surface in containers was also determined four and eight weeks after spray application. Research was terminated on November 24. 1993.

For experiment 2, artillery plant was transplanted into 6-inch tubs on September 15, 1993. Each container was top-dressed with 3.3 g/6-inch pot 14-14-14 Osmocote® on March 8, 1994. On March 22, 1994, artillery plants were sprayed with a surfactant solution or a solution containing surfactant and one of the six herbicides. Plants were assigned a phytotoxicity rating based on the scale described in experiment 1, two, four and eight weeks after herbicide application (April 7, April 21 and May 17, 1994, respectively). Percent of artillery plant foliage covering the growing medium surface in containers was also determined four and eight weeks after spray application. Research was terminated on May 17, 1994.

Results

In both experiments, best results were obtained from Ronstar® 50 WP (Tables 2 through 5). Artillery plants growing in full sun or 60% shade were almost dead two weeks after Ronstar was applied and no new growth had appeared eight weeks after plants were sprayed. Two weeks after spraying, artillery plant treated with Roundup exhibited significant phytotoxic symptoms. However, eight weeks after treatments were made, seed germination and plant regrowth had occurred in Roundup® sprayed containers and artillery plant was covering nearly 50% of the growing medium surface in containers under full sun or 60% shade.

Other treatments tested, including the control, were not very phytotoxic and artillery plant covered over 60% of the growing medium surface in containers eight weeks after herbicides were applied.

In experiment 1 (September 29 - November 24), four weeks after treatment there was a slight tendency for phytotoxicity and growth inhibition to be greater under 60% shade than full sun for some of the herbicides (data not shown). At eight weeks, however, even for Roundup® the effects were not great enough to benefit commercial growers. In experiment 2 (March 22 May 17), the tendency was reversed but again phytotoxic effects were not great enough to be of value commercially.

Conclusions

Of the six herbicides tested in the two experiments described above, Ronstar® WP 50 was by far the most effective in controlling artillery plant for up to eight weeks. For maximum benefit and safety when using agricultural chemicals, please follow label directions regarding use and disposal.

Table 1. Herbicides sprayed on artillery plant growing in 6-inch containers under full sun or 60% shade.

Table 2. Effects of herbicide sprays on artillery plant growing in 6-in containers under full sun. Experiment 1.

Results significant at P = 0.001. Mean separation in columns by Duncan's multiple range test, 5% level. Means in same columns with same letters are not significantly different.
^zOn September 29, 1993, artillery plants were sprayed with a surfactant solution (the control) or a solution containing surfactant and one of the six herbicides.
^yPlants were assigned a phytotoxicity rating (based on a scale of 1 = no visible effects, artillery plant still healthy looking, 5 = visible damage with moderate control, 10 = artillery plant completely eradicated) two, four and eight weeks after herbicide application (October 13, October 27 and November 24, 1993, respectively).
^xPercent of container growing medium surface covered by artillery plant foliage was also determined four and eight weeks after spray application.

Table 3. Effects of herbicide sprays on artillery plant growing in 6-in containers under 60% shade. Experiment 1.

Results significant at P = 0.0001. Mean separation in columns by Duncan's multiple range test, 5% level. Means in same columns with same letters are not significantly different.
^zOn September 29, 1993, artillery plant were sprayed with a surfactant solution (the control) or a solution containing surfactant and one of the six herbicides.
^yPlants were assigned a phytotoxicity rating (based on a scale of 1 = no visible effects, artillery plant still healthy looking, 5 = visible damage with moderate control, 10 = artillery plant completely eradicated) two, four and eight weeks after herbicide application (October 13, October 27 and November 24, 1993, respectively).
^xPercent of container growing medium surface covered by artillery plant foliage was also determined four and eight weeks after spray application.

Table 4. Effects of herbicide sprays on artillery plant growing in 6-in containers under full sun. Experiment 2.

Results significant at P = 0.0001. Mean separation in columns by Duncan's multiple range test, 5% level. Means in same columns with same letters are not significantly different.
^zOn March 22, 1994, plants were sprayed with a surfactant solution (the control) or a solution containing surfactant and one of the six herbicides.
^yPlants were assigned a phytotoxicity rating (based a scale of 1 = no visible effects, artillery plant still healthy looking, 5 = visible damage with moderate control, 10 = artillery plant completely eradicated) two, four and eight weeks after herbicide application (April 7, April 21 and May 17, 1994 respectively).
^xPercent of container growing medium surface covered by artillery plant foliage was also determined four and eight weeks after spray application.

Table 5. Effects of herbicide sprays on artillery plant growing in 6-in containers under 60% shade. Experiment 2.

Results significant at P = 0.0001. Mean separation in columns by Duncan's multiple range test, 5% level. Means in same columns with same letters are not significantly different.
^zOn March 22, 1994, plants were sprayed with a surfactant solution (the control) or a solution containing surfactant and one of the six herbicides.
^yPlants were assigned a phytotoxicity rating (based a scale of 1 = no visible effects, artillery plant still healthy looking, 5 = visible damage with moderate control, 10 = artillery plant completely eradicated) two, four and eight weeks after herbicide application (April 7, April 21 and May 17, 1994 respectively).
^xPercent of container growing medium surface covered by artillery plant foliage was also determined four and eight weeks after spray application.

*Professor of Environmental Horticulture and Center Director (retired 7/96), and Professor of Environmental Horticulture, respectively, Central Florida Research and Education Center, 2807 Binion Road, Apopka, FL 32703-8504.