Postemergence herbicides: the basics


I have always stressed that weed control in nursery production should be preventative.  Nonetheless, there will always be weeds that escape preventative sanitation practices and preemergence herbicides.  Hoeing and use of postemergence herbicides are the primary means of controlling escape weeds in nursery fields.  This article will address basic information on how postemergence herbicides work.  A subsequent article will address issues of weed biology that affect herbicide performance and will highlight specific weeds that are problematic in Oregon fields.  

There are two ways we can categorize postemergence herbicides: contact vs. translocated herbicides, and selective vs. non-selective herbicides.  

Contact vs. Translocated

Contact herbicides only kill the part of the plant contacted by the spray.  When using contact herbicides, thorough coverage of the weed is essential.  It's possible to spray and kill just half of a weed and watch the other half live happily ever after.  Contact herbicides will not affect root systems or other underground storage tissue (rhizomes, tubers, corms, bulbs, taproots, etc.).  Translocated herbicides are absorbed by the foliage (and in some cases green stems), and moved throughout the plant to stems, roots, and underground storage tissues.  Translocated herbicides are ideal for killing plants with well-developed perennial root systems.  

When possible, use contact herbicides around nursery crops because they are generally safer.  If accidental contact is made with a nursery plant, you will only injure the leaf or leaves that are contacted.  At worst you will cause minor and localized foliar burn that the plant should outgrow.  If using a translocated herbicide, minor contact with the nursery crop could cause severe injury, although it may not be immediately noticeable.  Small amounts of translocated herbicides will be moved throughout the plant and accumulate in root and shoot tips.  This can result in a sub-lethal dose, which may not kill the plant but could cause stunting, malformed foliage, poor root growth, or foliar rosetting.  Injured root systems could reduce plant vigor and cause stress that leaves the plant increasingly susceptible to insect and disease attack.  

I recommend using contact herbicides as a first choice; however, if the weeds requiring control are established perennials, then translocated herbicides are more effective.  Contact herbicides only burn back weed foliage and thus are only a short-term solution to killing perennial weeds.  Translocated herbicides, when used properly under ideal conditions, will kill roots and foliage to provide complete control of perennial weeds.  

In summary, use the following guides to select herbicides for weed control:

  • Use contact herbicides for annual weeds, or for recently germinated perennial weeds (which presumably do not have established a significant root system yet).
  • Use translocated herbicides to kill established perennial weeds, which have developed a significant underground root system.

Selective vs. Non-selective

Selective herbicides only kill a certain type of plant, while not harming others.  For example, Fusilade selectively controls grasses, but will not harm most broadleaf plants.  This and similar products (Table 1) are commonly used to eradicate grasses from nursery crops because of their demonstrated safety when sprayed directly over the top of a broad spectrum of woody and herbaceous nursery crops.  Non-selective herbicides kill all plants, regardless of type.  Roundup is the most commonly used non-selective herbicide.  

Herbicide coverage

Thorough coverage of plant foliage is important for postemergence herbicides.  For contact herbicides, parts of the plant not contacted by the spray will likely survive.  Plants that develop a large or thick canopy can shield themselves from herbicides.  Weeds sprayed with a contact herbicide may have the outer canopy burned off while the inner canopy and stems remain relatively unaffected.  Most plants quickly grow out of this type of injury, especially vigorous weeds.  This is the biggest reason why contact herbicides are most effective on small weeds.

Contrary to what many text books say, thorough coverage is also essential for translocated herbicides.  The more foliage contacted, the more herbicide will be absorbed, and the more it will be moved to underground storage tissue.  Remember, large weeds will likely have large root systems so thorough coverage is paramount for obtaining control.  

Factors affecting herbicide movement

Climate greatly affects postemergence herbicide efficacy.  This is especially true with translocated herbicides.  In a nutshell, factors that promote plant growth generally improve herbicide effectiveness.  Stressed plants are more difficult to kill than healthy and actively growing plants.  This may seem counter-intuitive.  Our instinct when we see a stressed plant is to say to ourselves, “Let's kick it while it's down!,” and then proceed to bombard it with herbicides.  

Weed stress is directly related to local environment and climate.  Let's take a few minutes to recap some plant biology to understand how stress affects herbicide efficacy.  Recall that plants have a vascular system for moving food and water from one part to the other.  The vascular system in plants is made of two components, xylem and phloem, which are kind of like the arteries and veins in the human circulation system.   Xylem transports water (with some dissolved nutrients) from roots upward to shoots, with virtually no movement downward.  Phloem transports sugar, water, metabolites, and other substances (like herbicides!) in all directions throughout the plant.  

Translocated herbicides move with the flow of sugar.  Sugars are generally moved to ‘sinks’, or areas that require sugar for rapid growth.  Sugars generated in the upper foliage of stems will move to that stem's growing tip or developing fruit and flowers.  Sugars generated in lower foliage are moved to developing roots.  Thorough coverage of the plant (inner and outer canopy) will result in greater amounts of translocated herbicides moved throughout the entire plant, and thus more complete control.

During times of stress, photosynthesis rates are slowed, plants become less metabolically active, and movement of sugar throughout the plant is reduced.  Therefore, stressed plants are less likely to move translocated herbicides to the root system, and control will be reduced.  

For perennial weeds with extensive root systems, fall may be the best time for application.  When many weeds first set flower buds, plants begin to shuttle sugars and carbohydrates back to the root system for storage over the dormant season.  Translocated herbicides are moved with these sugars, and thus increased levels of herbicide will be moved to roots during late summer or early fall.  However, waiting this long may not be practical in some situations.

Commonly used herbicides

Herbicides listed in Table 1 are the most commonly used among nursery crops, which I discuss briefly below.  There are many other herbicides used by nursery growers in non-cropland areas, too many to list here (perhaps another article).  Always read labels before using any herbicide.

Fusilade, Vantage, and Envoy.  These products are grass-selective translocated herbicides effective at controlling grasses without harming broadleaf plants.  They only kill grasses, or more precisely, plants in the family Poaceae.  Some plants resemble grasses, but are not in the grass family and thus are not controlled by these products!  Most notable is nutgrass (or nutsedge, Cyperus esculentus in the family Cyperaceae) and toad rush (Juncus bufonius in the family Juncaceae).

Roundup and other products containing glyphosate.  Glyphosate is the active ingredient in Roundup.  Since the patent for glyphosate was lost, many companies have introduced new products containing glyphosate.  As of 2002, there were at least 20 different products containing glyphosate.  Roundup is a nonselective, translocated herbicide.  It is one of the most widely used and effective herbicides.  For annual weeds less than 6 inches tall, glyphosate should be applied as a 1% solution; for weeds greater that 6 inches tall, use a 2% solution; and for perennial weeds use a 3 to 5% solution.  

Finale.  The active ingredient in Finale is glufosinate, and is chemically similar to Roundup.  However, unlike Roundup, Finale is poorly translocated in plants.  In fact, it is primarily considered a contact herbicide.  Finale and Gramoxone (discussed below) have similar uses; however, Finale is much safer to humans than Gramoxone.

Gramoxone and Reward.  Paraquat and diquat are the active ingredients in Gramoxone and Reward, respectively.  These products are similar in chemistry and mode of action.  Both are non-selective contact herbicides that cause rapid injury to contacted plants.  Diquat can be used in greenhouses.  Paraquat is highly toxic to humans and should be used with caution.

Goal.  Goal uses the active ingredient oxyfluorfen.  It is commonly used as a preemergence herbicide, although it can also be used as a non-selective contact herbicide for small weeds (less than 4 inches tall).  In our research, it provided effective control of many common weeds including redroot pigweed (Amaranthus retroflexus) and common lambsquarter (Chenopodium album) to name a few.  Goal is commonly used in conifer production, where it can be safely applied over the top of many dormant conifers.  Caution should be used around deciduous plants where directed sprays are necessary.

Scythe.  Scythe is a nonselective, contact herbicide using naturally occurring pelargonic acid as the active ingredient.  Scythe works better on warm sunny days when the temperature is above 70º F.

Basagran and Manage.  These products are in different chemical families and utilize different modes of action; however, both are used primarily for the same purpose: killing nutsedge.  While both herbicides are effective at killing a range of weed species, they are most utilized for their effectiveness on yellow nutsedge.  Both herbicides are nonselective and translocated.  Both products can cause injury to nursery crops, and therefore should be applied as directed sprays.

Lontrel.  Lontrel uses the active ingredient clopyralid.  Clopyralid is unique among herbicides in its selectivity.  It is a auxin-type growth regulator or phenoxy herbicide.  These herbicides mimic plant hormones and cause rapid development of undifferentiated cell masses, epinasty of plant stems (twisting of plant stems), and eventually death.  Clopyralid is most effective on plants in the families Asteraceae and Leguminaceae.  Plants in Asteraceae include all the thistles, common groundsel, horseweed, and many others.  Most notable and difficult to control of plants in Leguminaceae are the clovers.  Clopyralid causes sporadic and sometimes no injury to plants in other families.  It can be safely applied to Christmas trees, strawberries, and mint, for example.  It may be safe applied directly to some nursery crops, but destroy others.  So use caution when applying this herbicide around nursery plants.  It can also be root absorbed, so do not drench plants with the herbicide but only apply enough to obtain thorough coverage of the weed.  For additional information on how to use this chemistry for controlling Canada thistle, see another article on this website.

Note:  Due to issues surrounding residual clopyralid in compost, the ODA developed new restrictions for using the herbicide in turf areas.  Generally this does not apply to agricultural sites, but first read the ODA rules or check with your local Dow Agrosciences rep to be sure you are permitted to use these products.

Disclaimer:  This article is for educational purposes only.  Mention of a specific product should not be interpreted as an endorsement, nor should failure to mention a product be considered a criticism.  Always read the product label prior to using any herbicide.

Field workers hoeing weeds
Hoeing is an important component of a postemergence weed control program, however, an effective preemergence program will alleviate this burden.
Holly injured with Roundup
Sublethal concentrations of Roundup can cause foliar rosetting among other types of injury. (Photo by Carey Simpson)









Annual and perennial weeds
For annual weeds like common groundsel and chickweed (above) use contact herbicides. For perennial weeds like field bindweed and Canada thistle (below) use translocated herbicides.
Lactuca serriola
Prickly lettuce (above) was sprayed with a contact herbicide and poor coverage. It will continue to grow and produce seed. Thorough coverage is essential with contact herbicides.



























Toad rush is not a true grass, and is thus not controlled by grass-selective herbicides.






















Canada thistle rosettes
Lontrel is most effective against composites and legumes. It has been shown to be very effective against Canada thistle (above), especially when applied at the stage shown in this photo.

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