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.
culms (stems) can be seen (left). The junction of the leaf and culm is called the collar
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.
Th equackgrass 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.