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