1.1.97
Virtual WeedPatch
Weed ID
quackg.html
Grass Family
Quackgrass (Elytrigia repens)
Quackgrass is a troublesome perennial grass in many agricultural fields,
as well as many other habitats. It thrives in relatively cool, moist climates.
As such, it is one of the worse weeds in the northeastern part of North
America, and is less so in the drier midwestern and prairie states of the
US. In Iowa it is a problem sometimes, but our drier prairie environment
doesn't allow it dominate.
Quackgrass reproduces in two ways. The least important
way it reproduces is by seed, as seen in the picture left. Often
seed produced by quackgrass plants are sterile.
The most important way quackgrass reproduces
is vegetatively, from rhizomes in the soil, or from the bud
bank. The picture (left) shows the entire quackgrass plant, including
leaves of the shoot, and rhizomes which are also shoot tissue. The
root system grows from nodes along the rhizome axis.
Notice the white rhizomes on the right side of
the picture (left). They have no roots yet. These rhizomes are the growing
points of the plant, exploiting new soil in order to send up more nasty
shoots. The tip is pointed and curves upward. The French call quackgrass
"chiendont", meaning dog's tooth. Don't let this one bite you.
Arf, arf.
Left is a fragment of rhizome, notice the branching
that occurs at the nodes. In studies we did in Ontario we discovered
that these branch rhizomes would "self-fragment", meaning
they would form an abscission zone at the node and the two pieces would
naturally separate in the soil. This is exactly what seed-producing plants
do, but this is vegetative propagation. New abscissed fragments form new
"seeds" or propagules to ensure their survival in the bud bank,
and to bother growers. Notice also the remaining roots coming out from the
nodes.
Another interesting thing about quackgrass rhizomes
we observed in Ontario bud banks is pictured left. If you look closely at
the junction of these two rhizomes you will notice that the more vertical
one is going through the more horizonal one, not around each other.
This is definitely a dog's tooth, but it's biting itself. Rhizomes are very
aggressive, and they have the ability to exert strong physical force to
get where they want, even to puncture a neighbor. Competition among species
can be violent, even in plants. Another important implication of this puncture
is that it destroys the vascular system of the rhizome, preventing herbicides
from translocating and killing buds. This is an important form of herbicide
resistance. Several years ago Phil Westra at the University of Minnesota
(now Colorado State) discovered some nasty little weevils that bored into
rhizomes, also preventing herbicide translocation. Weed biology can be very
weird sometimes.
This is another picture (left) of a quackgrass
rhizome. It and its cohorts litter the surface of this Ontario field. Although
the rhizome is dried out, it still has the ability to germinate and grow.
Tolerance to drying is an important weedy adaptation in buds of perennial
weeds.
Above ground, the pointed tip (dog's tooth) pierces
the soil surface. A green shoot arises (left) when sunlight stimulates
chlorophyll development in the rhizome end. Next to this shoot are
some other emerged and established shoots. Quackgrass shoot emergence from
the bud bank is staggered over time, and this makes precise timing of
herbicide applications difficult if they must be sprayed at a particular
stage of development.
Left is a three leaf quackgrass shoot in the sandy
Ontario soil it so loves. In the backround are some unsuspecting soybean
plants. Notice the dried rhizome litter on the right.
Leaves of quackgrass (left) take many different
forms. This biodiversity is a consequence of "somatic polymorphism",
an important adaptation to maximize available resources. Many single plants
produce different types of leaves (e.g cotyledons and true leaves). Quackgrass
is no exception, each different leaf type allows precise exploitation of
local resources and conditions.
Quackgrass leaves, culms
(stems) can be seen (left). The junction of the leaf and culm is called
the collar region.
Quackgrass collars (left) have auricles,
elongated extensions of the leaf base. They curl around the culm as if holding
on tight to the stem to avoid being blown away. Hold on.
Left is a variant, or biotype, of
quackgrass. Notice that it is growing close to the ground, a prostrate
habit. When competition for light is not a problem, being prostrate
allows a plant to exploit more surface area. This allows it to capture more
light, and allows it to spread more rhizomes over a larger area of the bud
bank. Prostrate habit is also important in terms of herbicide control.
Having larger surface area of land also exposes more leaf area to herbicide
spray and uptake from overhead applications.
This is a closeup of the culm of the prostrate
quackgrass biotype (left). Notice that is has a horizontal habit, and progressively
angles upward at each node, gradually pointing upward.
Another variant of quackgrass is featured
left, an upright shoot biotype. The culms of this plant are vertical,
and the plant covers less surface area. This upright habit also has important
implications for its ability to resist herbicides. The more densely arranged
leaves intercept less herbicide spray and therefore get a smaller dose.
Research and on-farm experience have shown the upright plant has a better
chance of surviving foliar herbicide treatment than the prostrate type.
Upright and prostrate growth habits are examples of genotypic polymorphism,
an essential ingredient in biodiversity.
Left is the base of a quackgrass shoot, sometimes
called the crown region. Above this region the shoots spread upward to intercept
sunlight; below the rhizomes and roots spread to intercept moisture and
nutrients. Herbicide applications that translocate in quackgrass often fail
to kill dormant buds in this region. As a result, regrowth often occurs
from these buds sometime later.
Herbicide injured shoots and leaves:
These quackgrass shoots (left) are yellow, chlorotic,
from treatment by the old experimental ACCase inhibitor RO 13-8895.
The quackgrass shoot (left) is dark and necrotic
at its base, the shoot has been easily pulled from the culm. This injury
was caused by the ACCase inhibitor fluazifop.
This quackgrass leaf (left) has lesion, necrosis
caused by bentazon, a herbicide that quackgrass normally isn't injured by.
The perennial quackgrass plant ends its annual life cycle every year with
flowering and seed production, of lesser importance to its
survival than rhizome production. Left are two spikes (seedheads) of quackgrass.
This seedhead on the left is young and its slender
shape indicates it is just starting its sexual romp.
This one on the left is at anthesis, the pollen
sacs hang heavy with sexual energy, waiting for an opportunity to impregnate
any female organ it can spread its haploid essence on.
Considerable variation (genotypic polymorphism)
exists in quackgrass spikes. Left is a variant with a single floret
(seed) per spikelet (spike branch).
The variant left has a spike with multiple florets
per spikelet.
