Transcript Weed Biology

Herbicide Classification
Herbicides are classified according to timing and
method of application and chemistry.
Select a proper herbicide based on weed sensitivity
as well as turfgrass tolerance, and also when
and how you want to control the weed
population.
Herbicide Classification
Foliar Absorbed Herbicides
Absorption of the active ingredient occurs in leaves.
Postemergence (POST) control only
Examples: glyphosate, 2,4-D, dicamba
Root Absorbed Herbicides
Absorption of the active ingredient occurs in roots.
Preemergence (PRE) or sometimes POST control
Examples: DNA crabgrass herbicides, atrazine
Herbicide Classification
Selective vs. Nonselective Herbicides
Selective herbicides control or suppress certain
plants without negatively affecting the growth
of other plants.
Selectivity may be due to differential absorption,
translocation or physical (morphological)
differences between species.
Herbicide Classification
Selective vs. Nonselective Herbicides
Selective herbicides may be detoxified in plants or
just not converted into its active ingredients.
Examples: 2,4-D, mecoprop, dicamba control
many broadleaf weeds but many turfgrasses are
tolerant. PRE crabgrass herbicides are safe on
many turfgrass species also.
Herbicide Classification
Selective vs. Nonselective Herbicides
Nonselective herbicides control plants regardless of
species.
Examples: Glyphosate, glufosinate and paraquat
are nonselective, foliar absorbed herbicides.
(Note – glyphosate is selective to certain turf
species at low rates)
Herbicide Classification
Contact vs. Systemic Herbicides
Contact herbicides kill only the portion of green
tissue that is contacted. Therefore, uniform
spray coverage and particle size are essential for
adequate control. (the entire plant must be
sprayed)
Example: If a drop of paraquat (a nonselective,
contact herbicide) were placed on a leaf tip,
only the leaf tip would show injury symptoms.
Coverage with Granular vs Spray
Herbicide Classification
Contact vs. Systemic Herbicides
Contact herbicides do not kill perennial
underground structures such as rhizomes, tubers
and bulbs. Repeat applications to regrowth may
eventually drain the plants’ underground
resources, but it is better to try to control
perennial weeds with systemic herbicides.
Use contact herbicides on annual weeds.
Herbicide Classification
Contact vs. Systemic Herbicides
Systemic herbicides are extensively translocated in
a plant’s vascular system from point of
absorption (leaf or root) to sites of action.
Example: If a drop of glyphosate (nonselective,
systemic herbicide) were placed on a leaf tip,
the entire plant would express injury symptoms
due to translocation throughout the plant.
Herbicide Classification
Contact vs. Systemic Herbicides
Systemic herbicides are transported throughout the
vascular system along with nutrients, water and
organic materials necessary for plant growth.
Systemic herbicides require days to weeks for total
control. (herbicide must travel throughout plant)
Systemic herbicides are more effective on perennial
weeds than contact herbicides.
Herbicide Classification
Contact vs. Systemic Herbicides
Contact herbicide examples
MSMA for POST grass control
Bentazon for POST yellow nutsedge control
Paraquat for nonselective weed control
Herbicide Classification
Contact vs. Systemic Herbicides
Systemic herbicide examples
Glyphosate and glufosinate for nonselective weed
control
2,4-D, mecoprop, dicamba for broadleaf weed
control
Atrazine and simazine for annual bluegrass and
broadleaf weed control
Herbicide Classification
Herbicide Timings
Herbicides can also be characterized as…
Preplant
Preemergence
Postemergence
Herbicide Classification
Herbicide Timings
Preplant herbicides (soil fumigants) are used in
areas where a new turf stand is to be
established. They are nonselective herbicides
that kill desirable and undesirable plants!
Soil fumigants kill seeds.
Herbicide Classification
Herbicide Timings
Preplant herbicide examples include…
Methyl bromide (to be completely banned in 2005
because it is an ozone depleter)
Metam-sodium
Dazomet
Herbicide Classification
Herbicide Timings
PRE herbicides should be applied and activated
prior to weed seed germination (either annual or
perennial).
PRE herbicides form a thin barrier at or right below
the soil surface. Emerging seedlings contact the
treated zone, absorb the herbicide, then die.
PRE herbicides in turf are selective herbicides.
Weed Control With Preemergence
Herbicides
Herbicides do not prevent weeds from germinating, they
kill weeds as they grow through the herbicide treated zone.
Weed Control With Preemergence
Herbicides
Large gaps in herbicide barriers result in weed escapes.
Herbicide Classification
Herbicide Timings
Weeds that have already emerged before application
or activation are not affected by PRE herbicides
because their primary growing point has
escaped treatment.
PRE herbicides must be activated with
approximately 0.5 inch of irrigation or rainfall.
Herbicide Classification
Herbicide Timings
PRE herbicides are relatively immobile, persistent
and nonvolatile in soils. However, if left on the
soil surface for an extended period of time
without activation by water, these herbicides
can volatilize or photodecompose.
PRE herbicides do not control perennial weeds
emerging from vegetative structures.
Herbicide Classification
Herbicide Timings
PRE herbicides (especially crabgrass and
goosegrass herbicides) are very effective as
granular applications.
Particle sizes are uniform and small enough to
ensure adequate soil coverage as long as your
sprayer is properly calibrated and you’re
applying the correct amount!!!
Herbicide Classification
Herbicide Timings
PRE herbicides DO NOT kill seeds. They don’t
keep seeds from germinating. They kill
seedlings as they germinate!
PRE herbicides kill weeds before they are visible.
This lack of dead and dying weeds is more
aesthetically pleasing.
Herbicide Classification
Herbicide Timings
POST herbicides are applied directly to the
emerged weed, and are usually more effective
on seedlings. Plants have two growth stages,
vegetative (more easily killed when rapidly
growing) and reproductive.
POST herbicides CAN be used on grass and
broadleaf weeds, but MUST be used on sedge
and kyllinga species, wild garlic, and many
other perennial weeds. (Yellow nutsedge is an
exception; metolachlor provides PRE control).
Herbicide Classification
Herbicide Timings
POST herbicides usually require multiple
applications for adequate control. They can be
foliar or root absorbed, selective or
nonselective, contact or systemic.
Example: 2,4-D is a POST, selective, systemic,
foliar absorbed herbicide.
Herbicide Classification
Herbicide Timings
POST herbicides are more effective when applied
as liquid formulations rather than granular
formulations (liquids stick to the weeds).
If granular products are chosen, apply to wet foliage
so the herbicide will stick to the weeds and not
roll onto the soil where they are ineffective.
Herbicide Classification
Herbicide Timings
POST herbicides are more effective when…
•
•
•
•
•
Applied to actively growing weeds
Temperature range from 60 to 85 degrees
Good soil moisture
Applied BEFORE mowing
Mowing is delayed 1-2 days after application
Herbicide Classification
Herbicide Timings
POST herbicides must penetrate a waxy leaf cuticle
to get inside of the plant. When plants are
actively growing, the cuticle is thinner allowing
for easier penetration.
When plants are drought-stressed, the cuticle
becomes thicker to conserve plant moisture and
herbicide penetration is more difficult.
Herbicide Classification
Herbicide Timings
POST herbicides usually should be avoided when
temperatures exceed 90 degrees. The weeds
could be drought-stressed and also the herbicide
could become volatile and drift to nontargeted
sensitive plants.
Example: 2,4-D injures tomatoes, cotton, tobacco.
Herbicide Classification
Herbicide Timings
Delay mowing for several days prior to a POST
application to allow for more available foliage
for herbicide absorption and to ensure that the
weeds are actively growing.
Delay mowing for several days after a POST
application to allow for herbicide translocation
throughout the plant.
Herbicide Classification
Herbicide Timings
Do not apply POST herbicides in the rain or irrigate
immediately after application. Depending on
the individual label, herbicides can be rain-fast
(inside the plant) in as little as one hour or as
long as 12 hours. If a POST herbicide gets
washed off to the soil, it becomes ineffective.
Herbicide Classification
Herbicide Timings
Herbicide retention on a leaf is affected by…
Leaf size
Leaf shape
Leaf surface area
Leaf angle
Herbicide Classification
Herbicide Timings
Good herbicide retention on a leaf occurs when…
Leaf size is LARGE
Leaf shape is WIDE
Leaf surface area is NON-WAXY
Leaf angle is HORIZONTAL (FLAT)
Herbicide Classification
Herbicide Timings
Poor herbicide retention on a leaf occurs when…
Leaf size is SMALL
Leaf shape is NARROW
Leaf surface area is WAXY
Leaf angle is UPRIGHT
Herbicide Classification
Herbicide Timings
Wild garlic and nutsedge species are examples of
weeds that have narrow, waxy, upright leaves.
Many herbicide particles tend to miss the plants
entirely, or bounce or slide off the leaves.
For POST herbicides to be effective on these types
of weeds, spray adjuvants are needed.
Herbicide Classification
Herbicide Timings
Adjuvants are chemicals added to a pesticide
formulation or tank-mix to improve mixing and
application, or enhance pesticide performance.
Check the herbicide label to determine if spray
adjuvants are needed.
Herbicide Classification
Herbicide Timings
Spray adjuvants include…
Surfactants
Methylated seed oils
Crop oil concentrates
Herbicide Classification
Herbicide Timings
Surfactants consist of several classes of chemicals
that reduce the interfacial tension between water
and solids (plant material) or other liquids.
Herbicide Classification
Herbicide Timings
Classes of surfactants
Wetting Agents – used to increase the wetting
capacity of water (make water wetter)
Spreaders – allow pesticides to form a uniform
layer or coating over the treated surface
Stickers – cause pesticides to adhere to plant foliage
and reduce spray runoff or washoff
Herbicide Classification
Herbicide Timings
Types of surfactants
Anionic – negatively charged, readily leach from
soil
Cationic – positively charged, binds
electrostatically to soil causing waterproofing
Nonionic – no charge, best type of surfactant to use
Herbicide Classification
Herbicide Labels
It is important to read and understand the label of
any pesticides that you use, not just
herbicides!!! For example…
PRE and POST herbicides generally should not be
applied to newly seeded, sodded or sprigged
turfgrass until it becomes well established.
Herbicide Classification
Herbicide Labels
Immature turfgrass seedlings can be affected by
supposedly tolerant herbicides. Tolerance
ultimately depends on root growth. This and
other important information will be stated in the
label. Individual labels will give their definition
as to what is an established turfgrass.
Herbicide Classification
Herbicide Labels
Some PRE crabgrass and goosegrass herbicide
labels state that applications should not be made
to turfgrass seeded the previous fall.
Examples: benefin, oryzalin, benefin + trifluralin
Trimec Classic – POST Broadleaf Herbicide
Fescue seed can be sown 3 to 4 weeks after application
Do not apply to newly seeded turf until 2nd or 3rd mowing
Confront – POST Broadleaf Herbicide
Do not reseed until 3 weeks after application
Newly seeded turf should be mowed 2 to 3 times before
application
Millennium Ultra – POST Broadleaf Herbicide
Do not apply to newly seeded turf until well established