Know the signs

Preemergence herbicides should be recommended as the “backbone” of a landscape weed control program. Achieving satisfactory weed control generally requires repeated applications of one or more herbicides. Two applications, one in the spring (March) and one in the fall (August) of preemergence herbicides are recommended. Using less than two, in most situations, will result in more frequent use of postemergence herbicides and, as a result, increased potential injury to ornamental plants. The use of postemergence herbicides versus preemergence herbicides is always a high-risk behavior.

Preemergence herbicides commonly used around deciduous trees and in established woody plant beds 15-20 years ago included Princep (simazine), Goal (oxyfluorfen) and Gallery (isoxaben). Although, all are great herbicides, Goal and Princep were known to cause injury to certain woody plant materials. Goal is very safe around coniferous materials but should be used with caution around deciduous plants. Gallery has become one of the most widely used preemergence herbicide in landscape beds, as it is one of two active ingredients (ai) in the granular formulation, Snapshot (isoxaben + trifluralin) (Table 1).

SureGuard 51 WDG or BroadStar G both contain flumioxazin as their active ingredient. These preemergence herbicides were released in 2003/2004 in the nursery trade and later in the landscape industry. Both provide excellent control in woody beds. Many new preemergence herbicides were researched and released between 2008 and 2014 for the ornamental market. These new products are capable of dealing with heavy weed pressures and are generally less phytotoxic to woody plants than their older counterparts. These new products include FreeHand 1.75 G (dimethamid-p + pendimethalin) by BASF; Biathlon 2.75 G (oxyfluorfen + prodiamine) by OHP; Marengo G (indaziflam) or Marengo SC by OHP for nurseries or Specticle (indaziflam) by Bayer for landscapes; Tower (dimethamid-p) by BASF which when combined with Pendulum Aqua Cap (BASF) provides excellent control of most broad leaf and grassy weeds (Table 1). Dow AgroScience has also come out with a newer formulation of Gallery DF vs. the old SC. The Gallery 75DF is easier to pour and mix and has improved safety around landscape plants compared to the Gallery SC. Marengo is the first active ingredient for ornamentals from the Group 29 mode of action (MoA). Group 29 herbicides inhibit cellulose biosynthesis. Trials in 2013 found indaziflam caused no injury on a wide variety of plants applied at normal rates including: Green Velvet boxwood, Knock Out rose; Crimson Pygmy barberry, Little Henry sweetspire, Viburnum plicatum ‘St. Keverne,’ Viburnum × ‘Juddi,’ Home Run Red rose and Euonymus alatus ‘Compactus.’

Use rates have also declined in these new herbicides, further increasing safety around ornamental plants and in the environment (Table 1). The lowest use rate in the ornamental market today is with (indaziflam) (Table 1). The attributes of indaziflam are long-lasting, soil degradation not occurring until 150 days after treatment (DAT), non-volatile, non-mobile in soil once watered in and less required applications. Marengo also requires only 0.25 inches of irrigation or rainfall to activate, half that required for other products and it is effectively incorporated up to 21 DAT. The Verge granule used for Marengo G and Biathlon 2.75G further reduces injury to desired plants while maintaining exceptional weed control (efficacy) and residual control.
 

Group 3 herbicides

lthough preemergence herbicides are preferred due to their lower phytotoxicity, all herbicides are plant-killers and as such can cause plant problems. Some of the most commonly used preemergence herbicides in ornamentals are inhibitors of microtubule assembly. The MoA of Group 3 herbicides is mitosis inhibitors (Mi). These Mi herbicides include the dinitroaniline herbicide family which are root inhibitors. Barricade 65WG (Prodiamine); Pendulum 2G, Pendulum 3.3 EC, Pre-M 60DG, Pre-M 3.3 EC, or Corral (Pendimethalin); Surflan T/O (Oryzalin); Treflan 5G or Trifluralin EC (Trifluralin) are all dinitroaniline herbicides. Dimension (Dithiopyr) or Dacthal (DCPA) are also Mi herbicides in the pyridine and benzoic acid herbicide families, respectively. Mi herbicides also include shoot inhibitors such as the chloroacetanilide herbicide family. For Mi herbicides to be effective, the germinating seed must contact the top half inch of the media surface where the herbicide has been incorporated. If the seed is not in contact with the incorporated layer germination will not be inhibited. Any seed that has already germinated will be unaffected by the Mi herbicides.

Symptoms include clubbed or swollen root tips, yellowing or purpling of leaves and stem often associated with nutrient deficiencies. Whitening may also occur due to inability to produce chlorophyll. Weak stems are common due to poor cell wall development. Callus proliferation is another symptom. Callus is usually formed from structural tissue, not individual cells. The disruption of mitosis leads to the proliferation of callus. Injury with Mi herbicides usually occurs when they are applied too frequently or at too high a rate. Species tolerance also plays a significant role. Root damage is usually limited to the top 5 cm of the soil since the chemical has limited water solubility. Long-term injury to established trees is unlikely, however, the lack of lateral production may have longer-term consequence. Shoots of callused stems wilt readily when conditions are dry and the long-term impact of this growth is unknown.
 

Group 14 & 21

Some non-Mi preemergence herbicides used in landscaping are Goal (oxyfluorfen), SureGuard (flumioxazin) and Ronstar (Oxadiazon) which are Group 14, protoporphyrinogen oxidase (PPO) inhibitors and Gallery (isoxaben) a Group 21, cell wall synthesis inhibitor (CWSi). PPO inhibitor damage can appear as mottling, leaf crinkling, malformation and puckering from the rapid destruction of the contacted tissue. Cell walls are found in all plant cells. CWSi herbicides prevent new cells in the root and shoot apices from producing cell walls causing plant growth to cease. When absorbed by roots, Gallery moves primarily in the xylem and causes severe root and shoot stunting. When absorbed by leaves, however, there can be slow basipetal or phloem translocation which results in mottling and random leaf chlorosis and necrosis of contacted tissue.
 

Group 5

Healthy established plants show considerable tolerance to all the MoA’s cited above; therefore, injuries occurring with these MoA’s need to occur early in the life of the plant. Injuries from Group 5 preemergence herbicides can have longer-term consequence, especially at high concentrations. Group 5 includes the triazine family of: Princep Liquid; Simazine 4L, 90 DF, 90 WDG; Atrazine 4L, Atrazine 90DF, 90 WDG (simazine). In the foliage the effects of the triazines are yellowing or leaf chlorosis, veinal, interveinal, marginal or overall chlorosis. Injury appears first in the new growth, since the chemical is translocated to the growing point. Plants may outgrow injury, which is the result of low concentrations. New leaves will eventually become greener. The whole leaf may become chlorotic at high concentrations. Leaves may turn brown and die. Excessive root absorption of the chemical is the major cause of plant injury and long-term effects. Repeated applications over several years will cause soil buildup. Plants transplanted into soils with significant triazine accumulations may be damaged or killed by the carry-over in the soil.

Although the preemergence herbicides listed above caused some short- and long-term problems, the incidence of herbicide injury is much higher with postemergence herbicides. Postemergence products can cause a larger variety of short-term injuries the consequences of which have long-term effects in the tree.
 

Group 22

The short-term injuries of postemergence herbicides are usually seen in the foliage of woody plants. Gramoxone (Parquat), a contact postemergence herbicide, causes contact burn of plant foliage. Brown leaf spots result from spray drift. Overall leaf death will result if the entire plant is sprayed. Injury appears within several hours when sprays occur on hot, humid days. Gramoxone causes contact, not systemic injury, new growth will not be damaged. Plants may outgrow injury if only a few branches affected. However, paraquat, when sprayed on tree trunks, repeatedly, can cause injury, looking like a stem weakness or constriction, similar to a graft incompatibility. This trunk injury will cause long term, severe, problems for the tree.
 

Group 4

The long-term impact of growth regulator herbicides such as the phenoxy family (WSSA Group 4) have been well-documented in forestry, reducing tree survival long-term. Herbicides such as 2, 4-D, MCPA and MCPB are members of this family and are severely phytotoxic to woody dicot plants. In the foliage 2, 4-D will causes tip chlorosis, tip dieback, epinasty, twisting and abnormal appearance. This chemical causes the largest percentage of documented herbicide injury to plants in the landscape. Injury may be the result of spray drift, volatilization, or root uptake. Symptoms depend on the concentration to which the plant has been exposed. Low concentrations cause shoot tips to twist, leaves become cup-shaped with margins curling up or down. Leaf petioles may bend down giving the plant a wilted look. Cracked calluses appear on longer stems. At low concentrations leaves which develop after contact may be long, strap shaped, darker green in color with prominent veins. At higher concentrations, shoot tip and leaves turn chlorotic and die, eventually leading to the death of the entire plant. Long-term use of even low concentrations of 2, 4-D around older trees will cause significant growth deformations of stems. Phenoxy herbicides should only be used around trees with extreme caution and when all other options have been exhausted.

While phenoxy herbicides may cause the most documented injuries to trees in the landscape, I speculate glyphosate (N-(phosphonomethyl glycine) causes the most long-term, undocumented injuries. Glyphosate is the most widely used herbicide in the world, and results in the accumulation of shikimic acid, depletion of aromatic amino acids, and a reduction in phenolic compounds.
 

Glyphosate injury: hard to diagnose

Glyphosate is readily broken down within the soil, once in the phloem of a plant; however, it may take years to break down. The advent of increased and improved surfactants, also increased the potential loading of glyphosate in nursery and landscape trees receiving repeated exposure, limited breakdown and increased uptake. Non-metabolized glyphosate is rapidly translocated to the roots. Presumably, it is stored along with sugars flowing to the roots in the summer and fall. In the spring when the flow of sugars is reversed, glyphosate can be translocated to meristematic regions to continue its effects. This reversal shows up as glyphosate carry-over injury which is characterized by witches broom, cupped, puckered, strap-like, stunted growth, chlorosis, and/or death and potentially bark splitting. Glyphosate carry-over injury can occur one, two or three years after application. Because of the spiral pattern of the vascular system in many conifers, glyphosate carry-over in spruce appears as a spiral in the tree. White spruce (Picea glauca) is particularly susceptible to glyphosate injury.

Shikimic acid is frequently used as a highly sensitive, real time biomarker to determine if a plant has been exposed to glyphosate. Shikimic acid levels begin to rise within hours of glyphosate exposure and can remain elevated for several days to several years, depending on the dose and species. A positive correlation has been found between shikimic acid levels immediately following glyphosate dosing on one and two year old Prunus pensylvanica and Populus tremuloides Michx. Shikimic acid levels in two year old Betula papyrifera seedlings were increased 10-fold in trees treated with glyphosate, even at a rate of 1.848 pounds per acre compared to the control. One year after application these two year birch seedlings still had significant higher shikimic acid levels in leaves compared to controls, suggesting prolonged effects of glyphosate in the plant.

In 2008, I surveyed the nursery usage of glyphosate products, and identified five high-risk practices such as over use of glyphosate, glyphosate exposure to green-bark, sucker removal with glyphosate, glyphosate applications immediately following sucker removal and elimination of preemergence herbicides for reliance on glyphosate. One nursery surveyed was using glyphosate as frequently as eight times per season at one quart per acre.
 

Sucker removal

The use of glyphosate to remove suckers is a non-registered use. Scythe (pelargonic acid), a contact herbicide is the only herbicide labelled for sucker removal. The use of glyphosate shortly after mechanical removal of suckers is a high-risk practice. Applying postemergence systemic herbicides after sucker removal is similar to basal stump applications performed to control root and stump sprouts.

The best way to conduct sucker control is with a sprout inhibitor such as Tre-Hold Sprout Inhibitor A-112, Tre-Hold RTU or other formulations containing naphthaleneacetic acid (NAA) to inhibit the development of adventitious shoots. These products replace the apical dominance of the tree, which is why the tree is suckering. Mechanical removal of suckers, which is another common practice, has been shown to increase sprouting, as this creates a wound response in the plant and initiation of more adventious shoots. Paint or spray the NAA products onto pruning cuts after sprouts are removed. A 1 percent solution applied to the lower 30 inches of trunk in March or April when growth has just begun and all leaves are removed in the area to be sprayed is recommended.

Our research supports certain glyphosate formulations are disrupting the SA pathway, reducing cold hardiness and increasing frequency of bark cracking in susceptible species; wounding (of any kind) does increase cracking; and, the influence of glyphosate applied near basal bark wounds is experienced in current year’s shoot growth. High-risk practices with glyphosate, exposure to green-bark, sucker removal, applications immediately following sucker removal and elimination of preemergence herbicides for reliance on glyphosate are considered contributory to stock loses from bark cracking and mortality in landscapes.

Dr. Hannah Mathers is a contract researcher and independent nursery/landscape crop consultant with Mathers Environmental Science Services, LLC. She has more than 23 and 17 years of industry and weed science experience, respectively.