Effects of Repeated Applications of Roundup Pro® Over the Top of Container-Grown Nursery Crops

Significance to the Nursery Industry

Over the past several years, many growers have had to lower their prices of container-grown plants in order to remain competitive in an industry hit hard by a major downfall in the nation's economy. Increased pressures including rising costs of fuel and chemicals, paralleled with a decrease in the availability of seasonal labor, have caused growers to have to make decisions about weed control that, at one time, may have been considered drastic.

Our research on repeated applications of Roundup Pro® over-the-top is intended to provide data for emergency or salvage weed control in nurseries and landscapes when labor is unavailable or when hand weeding would exceed budget limitations.

As growers have increased their use of over-the-top glyphosate (varied formulations) applications, they are beginning to ask about how often they can apply glyphosate over the top. The objective of the experiment was to determine crop tolerance of container-grown nursery crops to repeated Roundup Pro® applications at 1.12 kg ai·ha⁻¹ (1.0 lb ai·A⁻¹), a rate found to be adequate for control of most weeds found in container production.

Introduction

Between 1975 and 1980, glyphosate (various formulations) was evaluated over-the-top of numerous container-grown crops, with many exhibiting some degree of tolerance. In one study, glyphosate (Round-Up®) was applied either once, twice, or three times at 0.56, 0.84, 1.12 and 1.68 kg ai·ha⁻¹ (0.50, 0.75, 1.00, and 1.50 lb ai·A⁻¹) on 7, 14, and 21 April over 18 ornamental cultivars (6). Total amounts of Round-Up® applied ranged from 0.56 to 5.00 kg ai·ha⁻¹ (0.5 to 4.5 lb ai·A⁻¹). Of the 18 species evaluated, nine were not injured, including saucer magnolia (Magnolia × soulangeana), shore juniper (Juniperus conferta), cypress (Cupressus sp.), ‘Burford’ holly (Ilex cornuta ‘Burfordii’), ‘Yellow Top’ holly (Ilex cornuta ‘Yellow top’), Fraser photinia (Photinia × fraseri), pittosporum (Pittosporum tobira), podocarpus (Podocarpus sp.), and windmill palm (Trachycarpus fortunei). ‘Radicans’ gardenia (Gardenia jasminoides ‘Radicans’) was injured at 3.36 and 5.04 kg ai·ha⁻¹ (3.0 and 4.5 total lb ai·A⁻¹). Of the remaining species, ‘Hinodegiri’ Kurume azalea (Rhododendron obtusum ‘Hinodegiri’) and ‘Fashion’ azaleas [Azalea × ‘Fashion’ (Girard Hybrid)] were the most sensitive, with injury occurring from as little as two applications of the 0.56 kg ai (0.5 lb) rate.

In another study, glyphosate (neither formulation nor product was identified) was applied at 0.28, 0.84, and 1.12 kg ai·ha⁻¹ (0.25, 0.75, and 1.0 lb ai·A⁻¹) over the top of 10 species, once on August 3 and again on August 17, 1978 (5). Following two applications, no phytotoxicity was observed on Mentor barberry (Berberis × mentorensis), camellia (Camellia japonica), border forsythia (Forsythia × intermedia), and Golden privet (Ligustrum vicaryi) at any rate. Temporary slight yellowing was observed on wintergreen barberry (Berberis julianae), Japanese euonymus (Euonymus japonicus) and ‘Dwarf Burford’ holly (Ilex cornuta ‘Dwarf Burford’) at higher rates. Damage was more severe on ‘Hinodegiri’ Kurume azalea, ‘Helleri’ holly (Ilex crenata ‘Helleri’), and ‘Hetzii’ japanese holly (Ilex crenata ‘Hetzii’) at rates ≥ 0.84 kg ai·ha⁻¹ (0.75 lb ai·A⁻¹). All species overwintered well, but evaluation of root systems in late February indicated slightly less root density on plants treated with 1.12 kg ai·ha⁻¹ (1.0 lb ai·A⁻¹).

In a 1985 study, ligustrum (Ligustrum japonicum) responded with a linear decrease in susceptibility between March and November to glyphosate [Carbon-14-labeled N-(phosphonomethyl)glycine] applications (4). ‘Blue Pacific’ shore juniper (Juniperus conferta ‘Blue Pacific’) sustained tip necrosis on young elongating tips from early summer applications, but applications at other growth stages resulted in no significant absorption of ¹⁴C-glyphosate.

Glyphosate (Roundup Pro®) at 0.45 kg ai·ha⁻¹ (0.4 lb ai·A⁻¹) can be used as a cleanup treatment for effective control of spurge (Euphorbia maculata) (96%) in ‘Big Blue’ liriope (Liriope muscari ‘Big Blue’) with no short-term or long-term injury (1). In separate experiments, glyphosate (Roundup Pro®) at 1.79 kg ai·ha⁻¹ (1.6 lb ai·A⁻¹), the maximum rate tested, was applied to recently-divided liners of ‘Variegata’ liriope (Liriope muscari ‘Variegata’) and ‘Big Blue’ liriope infested with mature and flowering spurge. Effective control of spurge (93 and 100% respectively) with no short-term or long-term injury to ‘Variegata’ liriope was reported. ‘Big Blue’ liriope showed slight initial injury which was outgrown at 60 days after treatment (DAT). In another study, glyphosate (neither formulation nor product was identified) applied on September 6, 2005, in a 1% solution of 1.82 kg ai in 378 liters (4.00 lb ai in 100 gal) caused no injury on either liriope or asiatic jasmine (Trachelospermum asiaticum) (9).

Minimal injury has been reported on ‘Blue Pacific’ juniper, ‘Blue Star’ juniper (Juniperus squamata), and ‘Parsons’ juniper (Juniperus davurica ‘Parsonii’) when glyphosate (applied as RoundupPro®; 4.0 lb·gal⁻¹ glyphosate in the form of its isopropylamine salt) was applied at rates up to 2.8 kg ai·ha⁻¹ (2.5 lb ai·A⁻¹) on May 29, 2004, and May 13, 2005 (2). Injury ratings and dry weights were similar to non-treated controls at 4, 8, and 12 weeks after treatment (WAT) in both experiments when 1.12 kg ai·ha⁻¹ (1.00 lb ai·A⁻¹) was applied.

Nine species of ornamentals in containers were treated over-the-top with glyphosate [N-(phosphonomethyl)glycine] at four rates 0.28, 0.56, 1.12 and 2.24 kg ai·ha⁻¹ (0.25, 0.50, 1.00 and 2.00 lb ai·A⁻¹) once in June 2007, August 2007, or February 2008 (3). A fourth group was treated on all three dates. Dwarf mondo grass (Ophiopogon japonicus ‘Nana’), mondo grass (Ophiopogon japonicus), ‘Cleopatra’ liriope (Liriope muscari ‘Cleopatra’), and ‘Variegata’ liriope were tolerant to all rates and application dates. ‘Blue Pacific’ juniper was slightly injured by February applications but recovered quickly. Asiatic jasmine and ‘Stoke's Dwarf’ dwarf yaupon (Ilex vomitoria ‘Stoke's Dwarf’) suffered significant injury after February applications. ‘Blue rug’ juniper (Juniperus horizontalis) was tolerant in August but injured at ≥ 1.12 kg ai·ha⁻¹ (≥1 lb ai·A⁻¹) for all other dates. ‘Pink Gumpo’ azalea (Rhododendron eriocarpum ‘Gumpo Pink’) was injured at all rates and application dates. Growth indices were taken in March and June 2008. Mondo grass, dwarf mondo grass, ‘Cleopatra’ liriope, ‘Variegata’ liriope and ‘Blue Pacific’ juniper were not affected by over-the-top glyphosate applications except at 2.24 kg ai·ha⁻¹ (2.0 lb ai·A⁻¹) applied 3 times. The remainder of the species had reduced growth as glyphosate rate increased.

In the current economic conditions, many growers have had to lower prices to remain competitive in the nursery industry, while production costs have increased due to increased costs for substrates, chemicals, and labor. While over the top applications of glyphosate should not replace a solid program of weed management consisting of monitoring, hand weeding, and application of pre-emergence herbicides, growers are turning towards using glyphosate on hardy ornamental species in emergency situations. Earlier research evaluated 13 ornamental container species for their response to single over the top applications of Roundup Pro® (in June 2007, September 2007, or February 2008) and multiple over the top applications (June 2007 + September 2007 + February 2008) (7). Species evaluated included dwarf mondo grass (Ophiopogon japonicus ‘Nana’), mondo grass (O. japonicus), liriope (Liriope muscari ‘Cleopatra’), variegated liriope (L. muscari ‘Variegata’), ‘Blue Pacific’ Juniper (Juniperus rigida subsp. conferta ‘Blue Pacific’), ‘Blue Rug’ Juniper (J. horizontalis ‘Blue Rug’), Asiatic jasmine (Trachelospermum asiaticum), dwarf yaupon (Ilex vomitoria ‘Stoke's Dwarf’), ‘Pink Gumpo’ azalea (Rhododendron eriocarpum ‘Gumpo Pink’), ‘Hardy Daisy’ gardenia (Gardenia jasminoides ‘Hardy Daisy’), Sky Pencil holly (Ilex crenata ‘Sky Pencil’), purpleleaf wintercreeper euonymus (Euonymus fortunei ‘Coloratus’), and wintergreen boxwood (Buxus sempervirens ‘Wintergreen’). Most plants were reported to be tolerant to single applications in June after the spring growth flush through September. Injury from February single applications was mostly gone by mid to late June. ‘Blue Rug’ juniper was the exception to this rule, having exhibited injury from the June and September single applications, but no injury from the February single application.

During recent economically difficult times, growers have increased use of over-the-top glyphosate applications, and now they are asking how often they can apply glyphosate over-the-top. The objective of the experiment was to determine crop tolerance of container-grown nursery crops to repeated glyphosate (Roundup Pro®) applications at 1.12 kg ai·ha⁻¹ (1.0 lb ai·A⁻¹), a rate reported to be adequate for control of most weeds found in container production (8).

Materials and Methods

Three separate experiments were conducted; one in 2008 2009 (Experiment 1 in Auburn, AL) and two in 2009 2010 (Experiment 2 in Auburn, AL, and Experiment 3 in Mobile, AL). All test plants were obtained from local nurseries and were grown in 3.8 liter (1 gal) containers in a 6:1 (v:v by vol) pinebark:sand substrate. Plants received 1.0 cm (0.4 in) of daily overhead irrigation. The species included in each experiment varied. All experiments included both single and multiple applications of Roundup Pro®. All treatments were applied at 1.12 kg ai·ha⁻¹ (1.0 lb ai·A⁻¹) with a CO₂-pressurized backpack sprayer, calibrated to deliver 280 liters·ha⁻¹ (30 GPA). Treated plants were allowed to dry for at least 6 hours after herbicide application. Plants were grouped by species in a completely randomized block design with 8 single-pot replications. A non-treated control (NTC) was included in all three experiments. Plant injury ratings (1 = no injury, 10 = dead) were taken at 11–19 day intervals after test initiation and continued until the end of the growing season. Growth indices (GI) [(height + width1 + width2) / 3] were collected twice after completion of all treatments; once during dormancy before spring growth had begun (January 2009 for Experiment 1, January 2010 for Experiment 2, February 2010 for Experiment 3), and once after spring growth flush (June 2009 for Experiment 1, June 2010 for Experiment 2, and May 2010 for Experiment 3). Plant vigor (1 = healthy, 5 = chlorotic) and marketability (1 = marketable, 3 = not marketable) was also rated. Data was analyzed using the SAS® statistical software package (SAS® 9.1.3, SAS Institute, Cary, NC). Treatment means were separated using the Waller-Duncan k-ratio t-test (p ≤ 0.05). Data for each sampling date were analyzed separately.

Experiment 1. Experiment 1 was initiated in July 2008 in Auburn, AL (Hardiness zone 7b 8a). The following three species were evaluated: ‘Big Blue’ liriope (Liriope muscari ‘Big Blue’), ‘Radicans’ gardenia (Gardenia jasminoides ‘Radicans’), and ‘Shishigashira’ camellia (Camellia sasanqua ‘Shishigashira’). Single Roundup Pro® applications were applied in either July, August, September, or October 2008. Multiple applications were applied in July and August (J+A), July, August, and September (J+A+S), July, August, September, and October (J+A+S+O), and July and September (J+S) of 2008. Plant GI measurements were recorded in January and June 2009. Plant vigor was evaluated in May 2009, and marketability in June 2009.

Experiment 2. Experiment 2 was initiated in July 2009 in Auburn, AL. The following six species were evaluated: liriope and gardenia (same as Experiment 1), ‘Martha Sims’ camellia (Camellia sasanqua ‘Martha Sims’), ‘Blue Pacific’ juniper (Juniperus conferta ‘Blue Pacific’), ‘Dwarf Burford’ holly (Ilex cornuta ‘Dwarf Burford’), and common cleyera (Ternstroemia gymnathera). Single Roundup Pro® applications were applied in July, August, September, or October 2009. Multiple applications were applied in either J+A, J+A+S, J+A+S+O, and J+S of 2009. Plant GI measurements were recorded in January and June 2010. Injury ratings were collected a total of 17 times; i.e., every 11–19 days beginning July 24, 2010, and ending November 13, 2010.

Experiment 3. Experiment 3 was initiated in July 2009 in Mobile, AL (Hardiness zone 8b). The following six species were evaluated: liriope, gardenia, juniper, ‘Martha Sims’ camellia, and cleyera (same as Experiment 2), and ‘Carissa’ holly (Ilex cornuta ‘Carissa’). Single Roundup Pro® applications were applied in either July, August, September, or October 2009. Multiple applications were applied in J+A, J+A+S, J+A+S+O, or J+S and 2009. Plant GI measurements were recorded in February and May 2010. Plant vigor and marketability were rated in late May 2010.

Results and Discussion

Experiment 1. Injury ratings for liriope treated with single applications in July or September, and multiple applications in J+S were similar to the NTC through the end of October (Table 1), concurring with other research (1, 8, 9). However, at the end of October, slight injury was observed characterized by 1 to 4 yellow leaf blades per plant. All treatments with a single application in August had the highest percentage of plants with yellow leaf blades. Plants were rated for vigor on May 14, 2009, and no differences were observed. GI taken on January 21, 2009, showed that single application treatments in September and October and multiple application treatments in J+A were similar to the NTC. Single application treatments in July and August, and multiple application treatments in J+A and J+S resulted in smaller plants than the NTC plants. Liriope receiving three or more applications tended to be smallest. GI taken on June 9, 2009, followed a similar trend to the January 21 ratings. Comparison of the differences in GI between January 21and June 9 showed no significant difference in spring growth from the NTC for any single or multiple application treatments except for the J+A multiple application treatment and the four successive applications (J+A+S+O) (data not shown). Although some treatments received up to 4.48 kg ai·ha⁻¹ (4.0 lb ai·A⁻¹) total, regrowth was similar to the NTC. All plants were similar to NTC in plant vigor ratings on May 14, 2009. Plants were rated for marketability on June 6, 2009. All treatments except the four application treatment (J+A+S+O), or a single application in August treatment, were similar to the NTC; however, all plants were marketable.

Table 1. ‘Big Blue’ liriope and ‘Shishigashira’ camellia responses to repeated Roundup Pro® applications at 1.0 lb ai·A⁻¹, Exp 1.

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No injury was noted on ‘Shishigashira’ camellia in any treatment (Table 1). Plant GI was similar in camellia for all treatments and the NTC on both January 21 and June 9, 2009. There was no difference in plant vigor or color on May 14, 2009. Differences in GI between January and June were similar as well, except for the J+A+S multiple application treatment. Plant vigor for all treatments was similar to the NTC. All camellia treatments were rated as marketable. Our data concurs with previous research where no phytotoxicity was reported when 1.12 kg ai·ha⁻¹ (1.0 lb ai·A⁻¹) of glyphosate was applied once on August 3, and again on August 17 over camellia (5).

Injury on gardenia from single application treatments appeared as chlorotic leaves about 7 days after treatment (Table 2). However, within three weeks, treated plants were similar to the NTC. Chlorosis and stunting was more evident with the multiple application treatments. Single application treatments applied in July were significantly different from the NTC when rated on August 8. Within one month, all plants recovered from the single application July treatment and were visually similar to the NTC. On September 12, the single application July treatment was similar to the NTC and three J+A multiple application treatments (September and October not applied yet) were similar to each other and had significantly greater injury than all other treatments. On September 29, the day of the September application, all gardenia treated in J+A had the most injury while plants treated with a single application in July only were similar to the NTC plants. Plants treated only with a single application in August exhibited slight injury. October ratings were similar to the September ratings, with all plants that received J+A multiple applications exhibiting the most injury. Injury among all other treatments was minimal, with the single application in August treatment having slightly greater injury than the NTC. Single application treatments in September were similar to the NTC.

Table 2. ‘Radicans’ gardenia response to repeated Roundup Pro® applications at 1.0 lb ai·A⁻¹, Exp 1.

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GI for gardenia on January 21, 2009, were similar for NTC and the single application treatments in either August, September, or October. All gardenia treated with a single application in July were smaller than NTC. Those receiving two Roundup Pro® applications were smaller than all other plants. GI taken after the spring flush on June 9, 2009, indicated that the NTC and single applications in August, September, and October were similar. All gardenia receiving multiple applications were smaller than the NTC. As previously noted, gardenia treated any time in July were smaller in size. Ratings for plant vigor on May 14 showed that all single applications were similar to the NTC, while gardenia receiving multiple treatments were rated lower due to slight leaf stunting. Marketability ratings showed that all single application treatments were similar to the NTC, while plants receiving multiple application treatments had slightly higher ratings due to smaller leaf size; however, all plants were rated as marketable.

Experiment 2. All GI values before and after the spring flush the following year were similar to the NTC for ‘Big Blue’ liriope, ‘Radicans’ gardenia, ‘Martha Sims’ camellia, ‘Blue Pacific’ juniper, and ‘Dwarf Burford’ holly (Tables 3, 4). Temporary foliar injury on juniper as reported by Neal et al. (1985) and Perry and Knowles (1979) was not seen (data not shown). Our results concurred with Czarnota (2008), who reported no significant injury at the 1.12 kg ai·ha⁻¹ (1.0 lb ai·A⁻¹) rate. Occasional slight chlorosis from single application treatments in July was seen on holly, similar to temporary symptoms reported by Perry and Knowles (1979). Cleyera exhibited chlorosis, necrosis, and stunting of shoot tips for all treatments (data not shown). January and June GI showed that cleyera treated with multiple applications in J+A+S, J+A+S+O, and J+S were significantly smaller than the NTC (Tables 3, 4). Cleyera injury ratings for single application treatments indicated less injury as plants were treated later in the growing season (data not shown). Multiple application treatments began with a July treatment and thus showed similar patterns of injury and stunting for each treatment of two, three, or four applications (Fig 1). For J+S treatments, there was some recovery in August from the July application treatment. This recovering tissue was then treated again in September, causing additional injury not seen on plants treated 2, 3, or 4 months in a row. All injury was restricted to immature tissue. Mature tissue was not affected and remained completely healthy while immature leaves and stems became necrotic and died back. Spring growth revealed significant branching and normal growth, with plants treated multiple times appearing fuller and more compact than the NTC and much more desirable in appearance. All plants were vigorous and marketable.

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Table 3. Growth indices^z of six species treated with Roundup Pro® at 1.0 lb ai·A⁻¹; measured on 1/25/10, Exp 2.

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Table 4. Growth indices^z of six species treated with Roundup Pro® at 1.0 lb ai·A⁻¹; measured on 6/2/10, Exp 2.

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Experiment 3. Liriope, gardenia, juniper, ‘Carissa’ holly, and cleyera GI were similar to the NTC in February (Table 5) and May (Table 6). Spring growth was similar to or larger than the NTC for all species. All plants were vigorous and marketable (data not shown). Camellia GI indicated that all treatments were similar to the NTC. The tolerance of liriope to single Roundup Pro® applications concurs with data in Experiment 2 and previous research (1, 8, 9). No injury on juniper was seen in this study, similar to a report of no significant injury at the 1.12 kg ai·ha⁻¹ (1.0 lb ai·A⁻¹) rate (2), but contrary to other reports (4, 8). Injury on cleyera was much lower in Experiment 3 than in Experiment 2. Cleyera in Experiment 2 grew constantly throughout the test period, presenting new tissue which was affected by Roundup Pro® applications. Cleyera in Experiment 3 did not grow as much, thus presenting less new tissue.

Table 5. Growth indices^z of six species treated with Roundup Pro® at 1.0 lb ai·A⁻¹; measured on 2/8/10, Exp 3.

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Table 6. Growth indices^z of six species treated with Roundup Pro® at 1.0 lb ai·A⁻¹; measured on 5/28/10, Exp 3.

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This research demonstrates that multiple applications of Roundup Pro® at 1.12 kg ai·ha⁻¹ (1.0 lb ai·A⁻¹) are relatively safe when applied over the top of selected nursery crops. Our research shows that several ornamentals are very tolerant, especially when applications are applied from July through the end of the growing season. After application to some species, growth was slightly reduced and slight visual differences could be detected as compared to the NTC. However, re-growth was similar for all treatments except some plants treated four times in successive months.

Our research shows that ‘Big Blue’ liriope, ‘Radicans’ gardenia, ‘Shishigashira’ camellia, ‘Martha Sims’ camellia, ‘Blue Pacific’ juniper, ‘Carissa’ holly, and ‘Dwarf Burford’ holly are tolerant to repeated applications 28 days apart starting in July. Plants that were not affected by fall applications had normal spring growth and marketability similar to the NTC. Individual species should be tested for tolerance before large groups of plants are treated. Our research is intended to provide data for emergency measures for weed control in nurseries and landscapes when labor is unavailable or when hand weeding would exceed budget limitations. It should not replace a solid program of weed management consisting of monitoring, hand weeding, and application of preemergence herbicides.