Tolerance of Containerized Landscape Plants to the Postemergence Herbicides Stinger, Manage and Basagran

Gary Bachman, Chris Wilson, and Ted Whitwell

Department of Horticulture, Clemson University

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Introduction

Selectively controlling a wide range of emerged weeds in container grown landscape plants is a goal for many in the nursery industry. Presently there are few postemergence herbicides labeled for use in landscape plants and some of these require directed spray application which is not always possible. There is a need for additional postemergence herbicides and the first phase of development of these herbicides is determining landscape plant tolerance.

Stinger (clopyralid) is extensively used in turf, Christmas trees, and agronomic crops. It controls broadleaf weeds with little effect on grasses. Stinger (clopyralid) moves throughout the plant after being absorbed by botyh roots and shoots accumulating in the meristematic regions and interfering with cell differentiation in susceptible plants.

In a study evaluating the effects of turf herbicides on landscape plants, the greatest injury from Stinger (clopyralid) occurred in the absence of mulch at the base of deciduous trees. Bradford callery pear (Pyrus calleryana 'Bradford') and red maple (Acer rubrum ) were moderately sensitive with mulch present with 8% and 26% injury, respectively, from 0.25 lb ai/A Stinger (clopyralid). Redbud (Cercis canadensis ) was very sensitive (89% injury) without mulch.

Over the top application of Stinger (clopyralid), up to 0.5 lb ai/A, in early of late summer caused no observable injury to yew (Taxus cuspidata ), arbovitae (Thuja nigra 'Dark American'), juniper (Juniperis horizontalis ) or rhododendron (Rhododendron 'Roseum Elegans') (1). Slight injury was observed on fraser fir (Abies fraseri ) and hemlock (Tsuga canadenaia ) in the form of curled needles on new growth.

Manage (MON 12000) is being developed for the control of purple nutsedge (Cyperus rotundus ) and yellow nutsedge (Cyperus esculentus ) in turfgrass and agronomic crops. Nutsedge control in a variety of warm and cool season turfgrass at rates up to 0.063 lb ai/A was greater than 85% with no turf phytootxicity or growth reduction. Manage (MON 12051) was also evaluated for yellow and purple nutsedge nontrol in container grown landscape plants. Over the top applications caused no observable damage to green liriope (Liriope muscari ) 28 days after treatment (DAT) with rates up to 0.016 lb ai/A. However, new growth of 'Macrantha Orange' azalea (Rhododendron x hybrida ) was injured at rates of 0.008 and 0.016 lb ai/A (24% and 44% respectively) at 28 DAT.

Basagran (bentazon) controls yellow nutsedge and broadleaf weeds with postemergence applications and is labeled as a directed spray around many landscape plants. However, tolerance to Basagran (bentazon) is variable among landscape plant species (11) with azalea (Rhododendron satsuki 'Amargasa'), nandina (Nandina domestica ), barberry (Berberis thunbergei 'Crimson Pygmy') and pieris (Pieris japonica ) injured at rates of 1 and 2 lb ai/A and cotoneaster (Cotoneaster dammeri 'Coral Beauty') at 2 lb ai/A. No significant injury was observed from topical application to camellia (Camellia sasanqua), juniper (Juniperus conferta 'Blue Pacific' and J. virginiana 'Grey Owl'), crape myrtle (Lagerstroemia indica ) and holly (llex vomitoria 'Schellings' and I. crenata 'Helleri').

The objective for this experiment was to determine tolerance of container grown landscape plants to Stinger (clopyralid) and Manage (MON 12051) postemergence herbicides. Basagran (bentazon) was included for comparison purposes.

Materials and Methods

Phytotoxic evaluation on container grown species was conducted during 1992 at the South Carolina Botanical Gardens, Clemson, SC and in 1993 at Carolina Nurseries, Monck's Corner, SC. Plant species evaluated both years were Juniperus chinensis L 'Hetzi Glauca', Juniperus chinensis L. 'Parsoni', Juniperus conferta Parl. 'Blue Pacific', Juniperus chinensis procumbens Endl. 'Nana', Cotoneaster dammeri C.K Schneid -'Coral Beauty'. Lagerstroemia indica L. 'Carolina Beauty' was evaluated only in 1992. Plants were grown in pine bark:sand (17:3 by vol) medium in 1 gal containers. Lagerstroemia, Cotoneaster, and Juniperus chinensis 'Hetzi Glauca' were pruned to uniform heights 2 weeks prior to treatment. All plants were fertilized 4 weeks before herbicide application in 1992 and 1 week before treatment in 1993.

The herbicides were applied with a CO2 backpack sprayer delivering 25 gal/acre at 20 psi through a single 9504E flat fan spray tip in 1992 and two 8002 flat fan spray tips in 1993. Treatments included a control (untreated), Manage (MON 12051) at 0.063, 0.125, 0.25 lb ai/A, Stinger (clopyralid) at 0.125, 0.25, 0.50 lb ai/A, and Basagran (bentazon) at 1 lb ai/A. A surfactant was added to the Manage (MON 12051) treatments at 2.5 ml/liter in 1993. The treatments were applied on August 25, 1992, and September 15, 1993. Experimental design consisted of a randomized complete block design with 4 single plant replicates per herbicide treatment.

Phytotoxicity was visually evaluated 1,2,3 and 6 weeks after treatment (WAT) on a scale of 0-100 with 0 representing no damage and 100 being dead plants. Growth index was determined at 6 WAT using the following formula, (maximum width + width perpendicular to maximum width + height)/3.

Results and Discussion

Stinger (clopyralid) at all three rates (0.125,0.25,0.50 lb ai/A) injured the Crape Myrtle in 1992 with symptoms observed as leaf discoloration. There was no visual injury associated with Stinger (clopyralid) applications on any of the other landscape plants evaluated in this study. Although there were no visual injury symptoms on three of the junipers, plant growth was influenced (Tables 1 and 2). Stinger (clopyralid) reduced the growth index of Dwarf Japanese Juniper at the highest rate (0.50 lb ai/A) in 1992 (19.4%) (Table 3). Parson's juniper growth index actually increased with the 0.25 lb ai/A rate in 1993.

Symptoms of phytotoxicity caused by topical application of Manage (MON 12051) included leaf discoloration followed by necrosis of the new shoot tips. Visual injury was observed on the cotoneaster 1 week after treatment (WAT) for all three rates of Manage in 1992 and 1993 (Table 1). Due to similarity of results only 3 and 6 WAT data will be presented. At 6 WAT injury was less apparent as the plants were beginning to recover from the damage by generating new lateral growth below the killed shoot tip (Table 2). The crape myrtle also had foliar discoloration from all three Manage treatments in 1992. Dwarf Japanese juniper was injured on the new shoot growth. These symptoms were observed 6 WAT (Table 9) and 3 WAT (Table 1) in 1992 and 1993, respectively. The other juniper species did not have any observable foliar injury at 6 WAT when treated with Manage.

Manage reduced the growth of Cotoneaster with 0.063 and 0.25 lb ai/A in 1992 and 0.125 lb ai/A in 1993 (Table 3). Hetzi Glauca growth was reduced in 1992 for all three rates of Manage. We earlier observed reduced plant growth without visual injury on several landscape plants treated with Basagran (bentazon). Though this may not be of concern once plants are planted in the landscape, it is essential for commercial nurseries that containerized plants achieve maximum growth.

Basagran (bentazon) application caused foliar injury on Crape Myrtle in 1992, however, there was not an accompanying reduction in growth index. Cotoneaster was injured by Basagran (bentazon) in 1992 and 1993. The injury was similar to that of the Manage with new shoots discolored and killed on the Cotoneaster. In this study Basagran (bentazon) did not effect growth index of these species.

Parsons and Blue Pacific junipers were the only species in the study that were not injured or had reduced growth index. The two deciduous species, lagerstroemia and cotoneaster, were the most susceptible to foliar discoloration and necrosis when compared to the evergreen Juniperus species. This could be possibly due to the thicker cuticle of the junipers and thus having more of a physical barrier to the absorption of the herbicides.

Cuticle thickness has been shown to be an effective barrier to foliar absorption of herbicides, where many studies have shown plants with thicker citicles absorb less herbicide. Junipers absorbed little Roundup (glyphosate) during dormancy, budbreak or elongation. However newly formed leaves of Ligustrum japonicum absorbed significantly greater amounts of Roundup than did the harder and thicker leaves that had overwintered.

Summary

Additional selective postemergence herbicides are needed for weed management by the nursery industry. Stinger (clopyralid) and Manage (MON 12051) are new herbicides that will effectively control problem weeds including nutsedge, and information about the tolerance of landscape plants to these postemergent herbicides is needed. The deciduous species, Crape Myrtle and Cotoneaster, were most sensitive to all three herbicides and topical applications should be avoided. The junipers evaluated displayed a higher degree of tolerance, however, there were growth reductions in some cases without accompanying foliar injury. None of the herbicides injured or reduced the growth of Parson's or Blue Pacific juniper. Other investigations, including directed sprays, are needed to ascertain the safety and utility of these herbicides for the nursery and landscape industry.

Last Updated 2/1/97