Quackgrass, sometimes called witchgrass in my neck of the woods, is a common weed on many organic vegetable farms in the Northeast and other temperate growing areas. A perennial, this weed stores energy in its underground stems, or rhizomes, which can spread widely underground. New tops then sprout up from the nodes of the rhizomes, creating new plants that can survive on their own if separated by mechanical cultivation. That makes quackgrass especially hard to control.
The following information is taken from the excellent fact sheet titled “Quackgrass Management on Organic Farms” by Heather Westwood, Kara Cox and Eric Gallandt of the University of Maine Sustainable Agriculture Program.
How quackgrass grows
Although seeds contribute to long-distance quackgrass dispersal, local infestations spread by prolific rhizome growth. These rhizomes, in addition to the fibrous root system, comprise 60 to 70 percent of total plant weight. Rhizomes may grow more than an inch per day, extending 10 feet or more from the mother plant.
Quackgrass stores sugar in roots and rhizomes in the late summer and fall to ensure winter survival. The following spring, the plant has a ready supply of energy, giving it a competitive advantage over many crops that start from seed. It is most vulnerable just as the shoots are emerging, when sugar from the roots is being expended in the new growth, but the leaves are not photosynthesizing at full capacity, so no sugar is being sent back to the roots.
Because it is a cool-season grass, the annual cycle of quackgrass starts early in the spring. During the hot part of the summer its aboveground growth is limited, but belowground rhizome production is greatest in summer. In the fall the tops of the parent plants die off, and the rhizomes stop growing horizontally and instead form new shoots that emerge from the soil. Even following repeated disturbance throughout the summer, if soil moisture is adequate, quackgrass may begin to flourish in the fall after most crops have been harvested or have died back.
Quackgrass management requires an integrated approach, involving prevention, tillage, grazing or mowing to deplete the sugar stores in the rhizomes, followed by a competitive crop to keep surviving rhizome buds from flourishing.
Till to kill
For a small, recently established patch of quackgrass, hand digging, a mulch and a fabric weed barrier, and careful monitoring may be sufficient. Disruption of the root system will cause the grass to resprout profusely, because dormant rhizomes have been activated. The new plants must be dug again before they have more than four leaves, but several repetitions of this process will usually kill the stand.
For larger stands of quackgrass, repeated tillage with a spring or spike-tooth harrow beginning in the hottest, driest part of the summer is recommended. The goal is to expose the roots and rhizomes to the sun, causing them to dry out. Rhizomes that have adapted to drought-like conditions will be harder to kill in this manner. The disruption will separate rhizome buds from their parent plants and cause them to sprout, so tillage must be repeated when the new plants put out three leaves.
Rhizome fragments buried in the top 4 inches of soil tend to sprout at the same time, making the shoots easier to kill at the correct stage, so tillage methods should be designed to keep rhizomes as close to the soil surface as possible. Tillage should be repeated every time new shoots come up and develop three to four leaves. If young quackgrass plants are allowed to grow any more leaves, they will begin to send out new rhizomes, and they will also begin accumulating new stores of sugar in the roots. Each tillage operation removes the new shoot growth and forces the plant to sprout again, until its belowground food reserves are completely exhausted.
While a significant number of quackgrass rhizomes can be killed in the first year of management, at least two years of tillage may be required for good control. If a quackgrass stand remains relatively thick after the first year, tillage must be continued until the middle of the next summer. The amount of cultivation needed to eradicate quackgrass can vary with soil type. On lighter soils, two to three rounds of cultivation may be sufficient, while it may take as many as six on heavier clays. Once your perennial weed problems are under control, the length of the summer fallow can usually be shortened.
Mowing or intensive grazing can also be used to manage quackgrass. Mow as close as possible to the soil level to reduce new rhizome growth. Intensive grazing that encourages the animals to eat the shoots down to the soil level would have a similar effect.
Following a sequence of repeated tillage throughout the summer, a fall cover crop should be planted. Some examples of overwintering cover crops include winter rye or rye plus hairy vetch. Another option is to plant a winterkill cover crop like oats or oats and peas. To prevent quackgrass reinfestation in years without a summer fallow, competitive fall cover crops such as rye or oats should be planted soon after harvest of the cash crop. The key with fall cover crops is to establish a good, competitive stand. This means you would want a seeding rate of 2 to 2.5 bushels per acre of oats, for example.
Quackgrass tends to spread from weedy field edges into fields, where occasional tillage actually encourages it to spread by separating buds from the parent plant. Field margins should be mowed regularly, because this causes the same sort of reduction in energy reserves as repeated digging.
Because it is important to prevent reestablishment, tillage equipment should be thoroughly cleaned following work in infested fields, as rhizomes may be easily moved from an infested area to a clean one.
Keep in mind that several strategies need to be employed if you want to be successful in the battle against quackgrass. One is well-timed tillage, repeated when regrowth reaches three to four leaves. Thus, summer fallowing is critical to controlling quackgrass if it is well-established in a field. If possible, take your land out of production every other year for a summer fallow, or at least try doing a summer fallow every four years. Periodic fallowing of field edges should also be performed to reduce movement of rhizomes into fields. Harrows with S or C-shaped spring shanks are particularly useful for lifting quackgrass rhizomes to the soil surface where they will dry out and die (weather permitting). Crop rotations that include vigorous fall cover crops, both after summer fallowing and summer cash crops, can help reduce quackgrass infestations.
The author is Vegetable and Berry Specialist with the University of Vermont Extension based at the Brattleboro office.