Diagnosing and Preventing Herbicide Injury to Trees Bruce R. Fraedrich, Ph. D., Plant Pathologist Herbicides have become an integral part of TYPES OF HERBICIDES landscape maintenance because chemical weed control often is more economical and Pre-Emergence Herbicides For Turf - This efficient than hand or mechanical group of herbicides is widely used in lawns to cultivation. Chemical weed control also is prevent growth of annual grasses, such as crabgrasses and certain annual broadleaf a primary method of vegetation weeds. The principal herbicides included in this management in industrial areas, utility group are: DCPA (Dacthal), benslide rights-of-way, along highways and other (Betasan), siduron (Tupesan) and benefin non-cropland areas. This increased (Balan). These herbicides also are mixed with dependence on herbicides in landscape fertilizers to form the so-called "weed and feed" maintenance has caused concern and products for turf. Since pre-emergence confusion among arborists regarding the herbicides for turf have little or no root activity, effects of herbicides on non-target woody they are not phytotoxic to woody plants when plants. Much of this confusion stems from properly applied. the multitude of herbicides, formulations Pre-Emergence Herbicides For Landscape and products presently on the market. Plantings - The following herbicides are Herbicide injury also is difficult to diagnose registered for pre-emergent control of annual based on symptoms alone because other weeds around woody ornamentals in the environmental or cultural adversities, as landscape and in commercial nurseries: well as infectious diseases and insects, can produce similar symptoms. Detection of Chloramben (Ornamental Weedar) herbicide residues by chemical analysis is oxyzalin (Surflan) expensive and often unfeasible. dichlobenil (Casoron) pronamide (Kerb) This report presents a listing of herbicides diphenamide (Enide) simazine (Prenceep) (low rates) frequently used in the landscape and their napropamide (Devrionol) potential toxicity (or lack thereof) to woody trifluralin (Treflan) plants. A description of herbicide injury oxadizon (Ronstar) symptoms is presented along with suggestions to aid diagnosis and The herbicides generally are not absorbed by recommendations for remedial treatment of roots of woody plants and are safe around most herbicide-injured plants. species. Exceptions exist and labels of specific 2 pre-emergence herbicides should be consulted 2,4-D may volatilize and cause minor leaf injury concerning precautions. Drift contacting the when applied near desirable woody plants. foliage may cause injury on some ornamental Using amine formulations of 2,4-D minimizes species especially if leaves are wet. Some pre- the possibility of injury from volatilization. At emergence herbicides may cause injury on high rates, the hormone herbicides are tolerant woody ornamentals if mis-applied at absorbed through the roots of woody plants, exceptionally high rates. Injury is usually which can cause severe injury. Shallow-rooted evident as foliage chlorosis followed by leaf species and deciduous hardwoods are most browning and defoliation. Plants usually sensitive to high rates of the hormone recover from injury from these herbicides. herbicides while conifers are more resistant. Post-Emergence Grass Killers - This group of Dicamba and picloram have a different herbicides is composed of arsonates such as chemistry than the hormone herbicides, but MSMA and DSMA which are applied to turf for they produce similar injury symptoms. Dicamba post-emergent control of crabgrass and other is used for controlling hard-to-kill broadleaf annual grasses. Arsonates are not root- weeds in turf and to control woody vegetation in absorbed and are safe around woody rights-of-way, woodlots, fence lines, and other vegetation. Drift contacting the foliage of non-cropland areas. Dicamba is absorbed desirable woody plants may cause leaf burn, through the roots of woody plants and can but injury is confined only to treated foliage. severely injure or kill ornamentals if applied Plants usually refoliate and recover from injury within their root zone. This herbicide is caused by drift from arsenate herbicides. persistent in soil for three months or more and will leach downward in most soil types. The Post-Emergence Broadleaf Herbicides - The persistence and mobility of dicamba increases following herbicides selectively kill broadleaf the likelihood of the herbicide contacting plants but are safe on most grass species: 2, 4- absorption roots of trees. Injury on woody D 2, 4-DP (dichlorprop), MCPP (mecoprop), plants can range from leaf distortion and dicamba (Banvel) and picloram (Tordon). defoliation to branch dieback and complete 2,4-D, 2,4-DP and MCPP commonly are death, depending on the rate and frequency of referred to as growth regulator or hormonal type application and the species, age and vigor of herbicides because their chemistry resembles the plant. naturally occurring growth regulators in plants. The hormone herbicides are the most widely Picloram (Tordon) is applied primarily for used chemicals for broadleaf weed control in broadleaf weed and brush control in non- turf and are the principal herbicides that are cropland such as utility rights-of-way. Picloram mixed with turf fertilizers to form the "weed and is absorbed through the foliage and roots of feed" products. plants and will severely injure or kill trees if applied within their root zone. At rates applied At rates recommended for broadleaf weed for brush control, picloram may have a residual control in turf, the hormone herbicides are not effectiveness of a year or more in most soils. root active and their persistence in soil is less than one month. Subsequently, 2,4-D, 2,4-DP, Post-Emergence, Non-Selective (foliar and MCPP seldom will injure woody plants absorbed) Herbicides - This group of herbicides when applied at their labeled rates for weed which includes paraquat, glyphosphate control in turf. Granular formulations are safest (Round-Up), aminotriazole (Amitrole), and near trees because there is less likelihood of cacodylic acid controls both broadleaf weeds drift than from spray formulations. and grasses. These herbicides are absorbed through the foliage of plants and have little or Drift from these herbicides, which contacts the no root activity. These materials are safe foliage of desirable plants, will cause leaf around woody plants as long as drift does not distortion and browning; however, most woody contact the foliage, green bark or suckers of plants will tolerate the injury and recover. On desirable species. warm days certain forms (ester formulations) of BTRL TR-43 3 Non-Selective Soil Sterilant Herbicides - Soil sterilants are used for total vegetation control in Hormonal (Growth Regulator) Herbicides - (2,4- rights-of-way, pipelines, industrial facilities and D, 2,4-DP, MCPP, dicamba, picloram.) other non-cropland areas. Symptoms are most pronounced on new growth. Leaf distortion including cupping, curling, abnormal elongation of leaf margins (epinasty) and parallel leaf venation are key Herbicides included in this group are: symptoms. Current year's shoots may be twisted and flattened rather than round or linuron (Lorox) angular. Leaf necrosis, dieback and mortality hexazinone (Velpar) may be evident in severe instances especially bromacil (Hyvar) with dicamba and picloram. Late spring frosts diuron (Karmex) may cause leaf and twig distortions similar to tebuthiuron (Spike) injury by growth-regulator herbicides. karbutilate (Tandex) atrazine (Aatrex) (high rates) Triazine Herbicides (simazine, prometon, prometon (Pramitol) atrazine) - The initial symptom is chlorosis of monuron (Urox) leaf blade between the veins. Leaf browning, princep (Simazine) (high rates) dieback and death may follow. Nutrient deficiencies, air pollution injury, and sucking The soil sterilant herbicides are extremely toxic insects may produce leaf chlorosis similar to to woody plants and misapplications of this triazine herbicide injury. group account for the majority of herbicide- related deaths of landscape plants. Soil Contact Herbicides (glyphosphate, paraquat, sterilants are absorbed readily by tree roots and cacodylic acid) - Light drift may appear as translocated throughout the crown. Many of small, brown spots where droplets contacted these herbicides are water-soluble and may be the foliage. Leaf browning and dieback usually leached or carried by runoff into the root system is restricted to treated areas of the plants. of a desirable plant. These materials also have Foliage, such as root suckers, accidentally a long residual of a year or more. treated late in the growing season may cause Subsequently, plants may be injured long after severe crown-dieback the following year, application if roots grow into a treated area or if especially on shrubs and small trees. a tree or shrub is planted in contaminated soil. Amino-traizole causes severe chlorosis and Injury and mortality of landscape plants is may bleach treated foliage white. Amitrole may particularly common when soil sterilants are be translocated to untreated portions of the used for total vegetation control for sidewalks plant where similar symptoms are evident. In and driveways around homesites before severe instances, dieback and decline may pavement is installed. Soil sterilants often are result. formulated with other herbicides and sold under a variety of trade names. Homeowners and Urea and Uracil Herbicides (Soil Sterilants: inexperienced