Gary Bachman, Chris Wilson, and Ted Whitwell
Department of Horticulture, Clemson University
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
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
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.
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.