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weed seeds and rhizomes

Quackgrass

Quackgrass is difficult to control. The long running rhizomes can weigh up to 6.7-9 tonnes per hectare (3-4 tons per acre). The seeds are a serious contaminant in cultivated forage seed.

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Biology

This perennial grass spreads by seed and underground rhizomes. The stems are smooth and 35-140 cm (1-4.5 ft.) high. The leaves are flat and slightly hairy on the upper surface. There are clasping auricles at the base of the leaf blade. The spikelets have 3-7 florets, attached broadside to the flowering stem. There are 2 horizontal rows on each spike.

The seeds are yellow or white and can remain in the soil for more than one year before germinating. However, quackgrass propagates and spreads mainly by rhizome formation. These rhizomes also secrete a toxic substance that suppresses the growth of surrounding plants and enhances the competitiveness of quackgrass (known as an allelopathic effect). Quackgrass thrives under cool, moist conditions.

Scouting Techniques

Inspect fields frequently to locate infestations early. Take a minimum of 20 weed counts across the field. Quackgrass usually occurs at high densities in localized patches within a field but can spread rapidly by its underground rhizomes. Look carefully for patches that are beginning to encroach into the field from field borders or headlands.

Effects On Crop Quality

Quackgrass can cause reductions in seed filling and the weed seeds can be a contaminant in forage seed. The rhizomes often get tangled in harvesting equipment (e.g., in potatoes).

Threshold/Yield Loss

When present in dense patches (more than 900 shoots per square metre or sq. yd.), quackgrass can reduce wheat yields by up to 100%. For example, 500 shoots per square metre (sq. yd.) cause about a 50% loss in yield. As a rule of thumb, for every 100 shoots per square metre (sq. yd.), the wheat yield will drop by about 10%. The economic threshold in canola is about 20-25 shoots per square metre (sq. yd.).

Control Tips

A multi-year integrated control plan must be used for control of quackgrass. This could include tillage, patch mowing, in-crop herbicides, and pre-harvest and post-harvest herbicides. Proper crop rotation is essential. As with some of the other weeds, the keys to control are recognizing the problem and being persistent.

Tillage may decrease quackgrass density or may spread the rhizomes increasing spread of the weed. Quackgrass patches should be tilled into the center of the patch to limit the spread of the rhizomes. In the spring, repeat tillage whenever the top-growth approaches 5 cm (2 in.). Remove rhizomes from tillage and seeding equipment before leaving quackgrass infested fields.

Herbicides are available in broadleaf crops for suppressing quackgrass. Glyphosate applied pre-seeding or post-harvest will control quackgrass, but pre- harvest applications are the most effective.

Invasive weeds in remote locations can be killed with solar tents

Research by Jim Stapleton, a UC Cooperative Extension advisor based at the Kearney Agricultural Research and Extension Center, was published as Feasibility of solar tents for inactivating weedy plant propagative material in the March 2012 issue of the Journal of Pest Science.

Stapleton, who specializes in plant pathology and integrated pest management, was inspired to conduct the study when a fire crew came upon a patch of Iberian starthistle growing along a stream in the Sierra foothills near Mariposa. Iberian starthistle is a robust, spiny weed native to the Middle East that, left unchecked, can dominate entire landscapes.

“Crews were going through and cutting dried plants and stacking them, but the seeds survived,” Stapleton said. “If you start moving plant material around with viable seeds, seeds are liable to spread, making the problem worse instead of better.”

Iberian starthistle is only one of many exotic, invasive plants that are capable of transforming California’s open areas into useless and unsightly tracts of land. On rangeland, for example, such weeds diminish desirable annual rangeland feed for cattle and wildlife. Weeds can shade out native wildflowers, make recreational areas inaccessible and, in dense infestations, become a fire laddering fuel.

In the past, such weeds may have been stacked and burned, but fire danger and air quality regulations have forced land managers to find alternatives.

“You wouldn’t want to try this on a 40-acre area,” Stapleton said. “Eradication of weeds with solar tents is best suited for small-scale weed infestations in warm climates.”

For the research project, Stapleton constructed three replicate solar tents with concrete rubble, mulberry shoots and clear plastic tarps. He placed johnsongrass rhizomes inside black trash bags along with about one cup of water. The sample bags were left inside the solar tents for 72 hours.

“Regardless of where you are, regardless of financial resources, you should be able to construct a solar tent,” Stapleton said. “Most of the materials needed – rocks and sticks – are easy to find on site.”

Air temperature inside the sample bags rose to 158 degrees Fahrenheit. Over the three days of the experiment, the rhizomes were exposed to temperatures 140 degrees and higher for 10 hours. None of the rhizome segments treated for three days in the solar tents sprouted. In contrast, rhizomes maintained in clear vegetable storage boxes and kept indoors for comparison all sprouted.

UC Cooperative Extension farm advisor Carl Bell, a San Diego area weed expert, tested the process in Lakeside, east of the San Diego metropolitan area, where a group of volunteers were working on restoration of the San Diego River.

“There are a lot of sites in California where volunteer groups are going into canyons and other remote spots to clean up weeds,” Bell said. “They’re going to places with no roads or trails, scrambling over rocks to clean up these areas. They could construct one of these tents and return in a week to find everything in the bags overheated to a point where seeds won’t germinate and rhizomes are dead. They only have to carry out the plastic bags and tarps.”

For the demonstration, volunteers pulled weeds and built solar tents on a parking lot. They invited the public to a workshop at the site a week later.

“When we pulled out the bags of treated material after a week of cooking, it was a gooey mass of vegetative material incapable of regenerating the weeds,” Bell said.