Third Installment of the Understanding Herbicides series
Herbicides must overcome biological and environmental barriers to control a target plant. In this article, learn the general processes by which herbicides control invasive plants and environmental factors that can influence herbicide performance.
Herbicide Uptake, Translocation, and Mode of Action
To be effective, systemic herbicides must: 1) adequately contact plants; 2) be absorbed by plants; 3) move within the plants to the site of action without being deactivated; and 4) reach toxic levels at the site of action. Herbicides can enter a plant through roots, shoots, stems, foliage, or buds. The primary site of entry is specific for each herbicide and may be influenced by the method of application. Some herbicides can enter a plant through multiple sites such as leaves and roots (e.g., Milestone® and Transline®).
HERBICIDE UPTAKE (Absorption)
Foliar Applied Herbicides
Leaves are the primary sites of entry for most foliar-applied herbicides. To enter the leaf, herbicides must first penetrate the waxy cuticle (primary barrier to absorption) or other specialized cells on the leaf surface. The cuticle is a layer of overlapping waxy platelets. Not all leaf components absorb herbicide; guard cells around stomates, cells surrounding hairs, and cells overlying veins are primarily responsible for herbicide absorption. Once inside the leaves, systemic herbicides move within the plant either cell to cell, or in phloem.
Soil Applied Herbicides
Soil-applied herbicides can be absorbed (taken up) by seeds, roots, and shoots. Herbicides are absorbed into plant roots in the same way as plant nutrients and water, and are moved through the plant initially in the xylem.
HERBICIDE TRANSLOCATION (movement in plant)
Translocation is the process of moving an herbicide from the site of uptake to the site of action where it kills the plant. Some herbicides (e.g., glyphosate, Milestone, Transline, Opensight®) are mobile within the plant (systemic), whereas others are immobile (contact) (Diquat). Movement within the plant is primarily by xylem and phloem vascular systems.
The xylem is a non-living, hollow, thick-walled, pipe-like conduit that moves water and nutrients from the roots up to all plant parts. Translocation in the xylem is the primary means of moving root-absorbed herbicides to above-ground plant parts. The rate of movement of herbicides in the xylem is determined by soil moisture and the transpiration demands of the plant.
The phloem is comprised of living cells that carry organic nutrients, particularly sucrose (sugar) to all parts of the plant. Translocation in the phloem is the primary means of moving foliar-absorbed herbicides to newly forming roots and shoots.
The figure at left shows the site of uptake (absorption) and movement (translocation) of some common herbicides. Foliar applied, contact herbicides (diquat) do not translocate within the plant, compared to foliar applied herbicides such as glyphosate (Roundup) that translocate in the phloem, killing plant shoots and roots. Commonly used range and pasture herbicides such as aminopyralid (Milestone), clopyralid (Transline), and others are absorbed by leaves and roots and are translocated throughout the plant in both phloem and xylem. Uptake of tebutiuron (Spike® 20P) is by roots with transport in the xylem.
SITE OF ACTION / Mode of Action
Following absorption and translocation to the site of action, an herbicide must remain active and at toxic levels to kill the target plant. Mode-of-action is the manner in which herbicides affect plants at the tissue or cellular level. Herbicides with the same mode-of- action may have similar translocation patterns and produce similar injury symptoms. For example Milestone® (aminopyralid) and Transline® (clopyralid) herbicides are both auxin growth regulators and plant injury symptoms include twisting, bending of leaves and stems, and swelling of roots.
Knowing and understanding each herbicide’s mode of action is an important step in selecting the proper herbicide for the target plant, diagnosing herbicide injury, and designing a successful weed management program. Over-reliance on a single herbicide active ingredient or mode of action places heavy selection pressure on a weed population and may eventually select for resistant individuals. Detailed information regarding herbicide mode of action is available at http://www.extension.purdue.edu/extmedia/ws/ws-23-w.html.
A plant must be actively growing when an herbicide is applied in order for the chemical to upset or stop certain plant processes. Therefore, herbicides are often applied during the spring and early summer. Some herbicides such as Milestone and Tordon® 22K are effective when applied in the fall to perennial weeds such as Russian knapweed (Acroptilon repens) or Canada thistle (Cirsium arvense). With fall applications, uptake is mainly by roots rather than foliage but underground parts are still actively growing and producing root buds for the following spring.
Environmental Impacts on Foliar Herbicide Performance
Environmental conditions before, during, and after application influence herbicide coverage on the plant or soil surface, absorption, and translocation within the plant. This can ultimately affect the level of control achieved from an herbicide.
Temperature, Moisture, Humidity
Prolonged exposure to low moisture and humidity, and high temperatures reduces herbicide penetration by increasing the thickness of the leaf cuticle and closing stomata. Late season application to invasive plants may also be less effective since mature leaves tend to have thicker cuticles that are more resistant to herbicide penetration. Addition of a surfactant may aid movement of the herbicide through plant cuticles and especially under stress conditions.
Both high and low temperature extremes slow plant metabolism and can reduce the effectiveness of herbicides. The best temperatures to kill weeds are between 65 and 85 degrees F. High temperatures, low humidity, and wind can dry spray droplets before the chemical is absorbed. Under these adverse conditions, the herbicide may either vaporize (if the herbicide is volatile) or crystallize on the leaf surface where it may then be removed by wind or rain, or degraded by sunlight.
Plant response to foliar applied herbicides is usually more rapid on sunny days since light enhances photosynthesis. When photosynthesis is reduced other metabolic pathways in a plant slow or stop completely reducing both translocation and action of herbicides.
Some herbicides may photodegrade in sunlight if they remain on the leaf surface for extended periods. Photodegradation (breakdown by sunlight) on the leaf surface is negligible for some herbicides including Transline and Milestone.
Rain Following Application
Rain following application of herbicides can influence control of the target plant. Some herbicides that are absorbed rapidly such as 2,4-D Ester may be “rainfast” in as little as 15 minutes after application. However, some herbicides require as long as eight hours free of rain following application for best performance, so rain too soon after application can cause variable results. Be sure to read the herbicide label to determine the “rainfast (rain-free)” period for the herbicide you are applying.
Table 1: The approximate number of hours required between herbicide application and rainfall (rainfast) for some commonly used natural area herbicides.
Foliar Canopy Factors
Foliage area determines the amount of herbicide intercepted by a plant. Weeds with small leaves may intercept an inadequate amount of herbicide to obtain control. Also, plants with hairy leaves, such as common mullein, do not absorb foliar herbicides readily because hair holds spray droplets away from the leaf surface. An excessive amount of old growth from the previous growing season may also intercept herbicides, preventing contact with new growth.
Herbicides such as Milestone, Transline, and Tordon 22K that are taken up by both roots, stems and leaves can perform better than herbicides that are mainly taken up by foliage such as 2,4-D. Surfactants can be added to the spray solution that may increase penetration of hairy or waxy leaf surfaces.
EVALUATING EFFECTIVENESS OF HERBICIDE TREATMENTS
How long does it take to determine if an herbicide treatment was effective? Plants treated with herbicides during the spring and summer may show symptoms of herbicide activity within a few days following treatment depending on the herbicide applied. Milestone, Tordon 22K and Transline are growth regulator type herbicides. Injury symptoms will usually occur within several days during optimum growing conditions. Control results are most evident 6 to 8 weeks following application. Herbicide treatments made in late summer or fall should be evaluated the following spring. It may take days for some herbicides to be translocated throughout the plant, so cultivation or digging should be delayed for 10 days to 2 weeks to increase control.
Active ingredients for herbicide products mentioned in this article: Accord® XRT II (glyphosate), Vastlan™ and Garlon® 4 Ultra (triclopyr), Milestone® (aminopyralid), Opensight (aminopyralid and metsulfuron), Roundup (glyphosate), Tordon® 22K (picloram), Transline® (clopyralid).
*Label precautions apply to forage treated with Milestone and to manure from animals that have consumed treated forage within the last three days. Consult the label for full details.
®™Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow
State restrictions on the sale and use of Accord XRT II, Garlon 4 Ultra, Opensight and Transline apply. Consult the label before purchase or use for full details.
Milestone and Vastlan are not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state.
Opensight: When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. See the product label for details.
Tordon 22K is a federally Restricted Use Pesticides.
Always read and follow label directions.