Nursery and Landscape - Ornamental Plant Sensitivity to Different Herbicides

Herbicides are important weed management tools in nurseries and landscapes. However, certain herbicides can damage landscape plants. The following is an abstract of a talk by Jeff Derr of Virginia Tech on this subject.

Preememergence and postemergence herbicides are effective tools for managing weeds in nursery production. These chemicals can injury certain nursery crops, depending on the specific herbicide and formulation, specific nursery crop, ornamental growth stage, soil type, and weather conditions. This article will focus on ways to reduce the potential for injury associated with herbicide application.

Bedding plants and herbaceous perennials:

Certain herbicides cannot be used on most annual bedding plants and herbaceous perennials. This list includes the oxyfluorfen-containing products Goal, Rout, OH2, and Regal O-O. Dichlobenil (Casoron, Barrier), simazine (Princep, others), flumioxazin (BroadStar, SureGuard) and oxadiazon-containing herbicides (Ronstar, RegalStar, and Pre Pair) also cannot be used on most herbaceous ornamental species. Isoxaben-containing products (Gallery, Snapshot) can only be used on certain herbaceous perennials. Do not apply isoxaben to Danes rocket, oxeye daisy, the mustard family, sedum, ajuga, lambsear or Veronica. As with all herbicides, check the label for specific use restrictions.

Sprayable formulations of the dinitroaniline herbicides (pendimethalin, prodiamine, oryzalin, and trifluralin), especially oil-based (EC) formulations, can stunt bedding plants and reduce flowering. It is best to use granular forms of these products in bedding plants and herbaceous perennials. Pennant Magnum, an emulsifiable concentrate form of metolachlor, can burn tender foliage, especially in herbaceous ornamentals. Use directed sprays when possible and avoid applications during high temperature/high humidity conditions. Avoid herbicides altogether on Phlox paniculata. Currently no preemergence herbicides are registered for use in greenhouses or other enclosed structures such as over-wintering houses. Herbicide vapors could be trapped around ornamental foliage, resulting in nursery crop damage. Do not apply preemergence herbicides in enclosed structures; this applies to both herbaceous and woody ornamentals. The last application for the year should be applied at least 2 weeks prior to covering over-wintering houses.

Woody nursery crops:

The granular products containing oxyfluorfen, oxadiazon, or flumioxazin should not be applied to plants with wet foliage since they can cause a contact burn. Wet foliage causes the granules to stick and then release the herbicide, resulting in spotting of foliage. Since these chemicals are contact herbicides, these granules should not be applied to plants that could catch and funnel granules to their base, such as yucca. Avoid applications during budbreak since tender foliage is more susceptible to damage. Another concern with these products is injury following splashing of treated soil onto foliage. Applying a layer of mulch after application could make these products safer when applied to young plants growing in field soil. This could also be beneficial with herbicides that could cause injury through volatilization, such as oxyfluorfen.

Use lower rates when applying preemergence herbicide to sandy soils low in organic matter. Generally higher preemergence herbicide rates are needed in clay soils higher in organic matter. Emulsifiable concentrate formulations should not be applied overtop nursery crop foliage, especially during hot, humid weather. This applies to products such as Pendulum EC, Pennant Magnum, and the postemergence grass herbicides (Envoy, Fusilade/Ornamec, and Vantage). Avoid adding oil adjuvants to overtop applications during summer – use nonionic surfactants instead. For Fusilade/Ornamec, check the label for juniper, azalea and other cultivar restrictions Do not apply oryzalin (Surflan) to Douglas fir, hemlock, or true firs, especially on seedbeds, liner beds, and young plants. Isoxaben (Gallery) can injure dwarf burning bush (Euonymus alata compacta), hydrangea, and lilac. Simazine can injure dwarf burning bush, lilac, and mock orange.

BroadStar can injure wax myrtle, privet, butterfly bush, hydrangea, spiraea, and viburnum, although there may be differences in cultivar sensitivity and newly planted liners probably are more susceptible than older plantings. Do not apply SureGuard overtop broadleaf ornamentals; only conifers have tolerance to overtop application, and then primarily after new growth has hardened off or when plants are dormant. It is preferable to apply SureGuard or Goal to dormant shade trees. Dichlobenil (Casoron) can injure hemlock, fir, spruce or pines, especially if treated when young. Clopyralid (Lontrel, Stinger) can severely injure members of the aster, legume, and nightshade families, including such species as asters, mums, coreopsis, redbud, and locust, along with damaging English ivy. Certain species tend to be sensitive in general to herbicides. Test cultivar sensitivity using a few plants prior to widespread use for herbicide application to azalea, barberry, hydrangea, and dwarf burning bush.

From "You Better Watch Out!! – Herbicide/Ornamental Combinations to Avoid" by J.F. Derr, Virginia Tech in the 2006 Proceedings of the Annual Meeting of the Northeastern Weed Science Society meeting.