Quackgrass has many adaptations to survive in agricultural fields. These include
rhizome, growth habit and herbicide resistance adaptations.
Rhizome adaptations to agricultural practices
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. Growth habit adaptations
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. Herbicide resistance adaptations
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, an important adaptation in quackgrass conferring herbicide
resistance.
