Tolerance of Nineteen Container Grown Landscape Plants to Postemergence Applications of Basagran

Chris Wilson and Ted Whitwell
Department of Horticulture, Clemson University

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Introduction

Basagran (bentazon) controls certain broadleaved weeds and yellow nutsedge in turf and other crops when applied postemergence. In the late 1980's, Basagran's label was expanded to include use around many landscape plant species as a directed spray on certain ground covers as a topical spray. While there has been much research reported on the use of Basagran in agronomic crops, little has been documented about its use aroung landscape plants.

Landscape plants differ in their tolerance to Basagran. Hosta, Daylily, Ban Hoote's Spirea, Rose of Sharon, Mock Oral Redbud, and Golden Raintree are reported to be tolerant to Basagran applications. Basagran causes severe injury to liners of Cotoneaster, Privet, Spirea, and Dogwood. 'Rosebud' and 'Tradition' azalea cultivars are very tolerant to Basagran but other cultivars have substantial foliar injury from Basagran.

Plant tolerance to Basagran appears to be realated to the ability of certain species to detoxify the herbicide in a relatively short time period. The mode of action of Basagran involves the inhibition of the photosynthetic process.

There are very few selective postemergence herbicides approved for use with landscape plants and the only one effective on yellow nutsedge is Basagran. Since directed applications of Basagran are not always possible or feasible due to plant density and form, information on species tolerance to topical applications would be very useful. The objective of this study was to evaluate a diverse group of nineteen container-grown landscape species for tolerance to topical applications of Basagran.

Materials and Methods

Experiments were initiated in May 1989, and repeated in June, 1991. The landscape species (Table 1) were established in 1 gallon containers in an 80% pine bark, 12% peat, and 8% sand media. They were grown in full sun and overhead sprinkler irrigated daily, except for the treatment day, and fertilized with 17N-3P-8.3K a week before starting the experiment.

Basagran was applied at 1 and 2 lb ai/acre a crop oil concentrate at 1.25 % (by volume) over-the-top of landscape plans using a CO2 pressurized sprayer. The sprayer was calibrated to deliver 40 psi, and was equipped with 11002 flat fan nozzle tips. Herbicide applications were made on May 11 and June 12, respectively.

Visual injury ratings were made weekly for 5 weeks after treatment on a scale 0 = no injury and 100 = complete plant death. Growth measurements were taken at the end of the two experiments to determine any differences in growth. A growth index was calculated by measuring the (height + minimum width + maximum width) / 3. Analysis of variance was used for data analysis and means were seperated using a protected least significant difference at P =0.05. There was a significant year effect so data are presented by years.

Results and Discussion

Species response varied greatly to Basagran applications. Injury symptoms observed included yellowing of the leaf margins, necrosis from the leaf apex toweds the base and stunting. At 14 days after treatment, both rates of Basagran were phytotoxic (9 to 63%) to Azalea, Barberry and Nandina both years (Table 2). Contoneaster and Arborvitae were also injured by both rates in 1991 and by the high rate in 1989. Helter Holly, and Crape Myrtle were injured with the high rate in 1989 while Armstrong Juniper, Parsons Juniper and Grey Owl Juniper were injured with only the higher rate in 1991. Japanese Pieris was not damaged by any treatment in 1989 but significant injury (21 to 33%) occurred from both rates in 1991. This may be due to the time of application relative to the initiation of new growth for this species. The May treatment date in 1989 could have been prior to new growth while the June application date in 1992 was likely after new growth initiation.

Azalea, Barberry, Arborvitae, and Nandina remained stunted and chlorotic 37 days after treatment (Table 3). Visual injury from the 2.2 kg aiMa (2 lb ai/A) rate was evident on Burford Holly, Heller Holly, Parsons Juniper, and Pachysandra in 1989 and on Armstrong Juniper and Parsons Juniper in 1991. Japanese Pieris also remained damaged from both Basagran rates at the 37 day evaluation in 1991.

Growth measurements made 37 days after treatment revealed similar results to those of the visual evaluations except for the Junipers (Table 4). Both rates of Basagran reduced the growth of Azalea and Barberry both years and Armstrong Juniper, Nandina, and Heller Holly in 1989. The growth index of Parsons Juniper, Blue Pacific Juniper, Grey Owl Juniper and Pieris was also reduced by both rates in 1991. The high rate caused stunting of Rotunda Holly, Grey Owl Juniper and Ligustrum in 1989 and Armstrong Juniper and Ligustrum in 1991. In general, visual injury was not severe for the Junipers, however, the growth was reduced for several species indicating the visual ratings for these species may not be the best measure of Basagran's effect on these plants.

Camellia, Burford Holly, Rotunda Holly, Yaupon Holly, Hetzi Juniper, Blue Pacific Juniper and Pachysandra were not visually injured by either rate any year. However, growth was reduced with the high rate for Rotunda Holly and Blue Pacific Juniper one of the two years. It appears that growth can be slowed by Basagran applications even though injury may not be obvious for some species. This growth reduction may be more critical in container production of landscape plants than when the plants are in the landscape. Variable results were obtained for the remaining species in the two studies.

Several species in this study were tolerant to topical applications of Basagran without damage or only temporary injury symptoms. Basagran is considered a contact herbicide with little translocation in plants. Therefore, when applied as a directed spray Basagran may cause minimal injury for most of the species evaluated. Cotoneaster, Blue Pacific Juniper, Nandina, and Azalea are approved for applications of Basagran T/O as a directed spray. However, caution should be used when directed applications are made to these very sensitive plants or in areas where visible damage can not be tolerated.

Significance to the Nursery Industry

Basagran effectively reduces yellow nutsedge and certain broadleaf weed infestations which interfere in the production and maintenance of landscape plants. This research indicates that Cotoneaster, Nandina, Barberry, Holmstrup Arbovitae and Azalea are very sensitive to Basagran topical applications and caution should be used when directed sprays are made around these plants. Camellia, Burford Holly, Rotunda Holly, Yaupon Holly, Hetzi Juniper and Pachysandra were not visibly injured from topical application of Basagran either year.

Last Updated 2/1/97