Fighting tulip weeds, diseases with cover crops

By Cathy McKenzie, WSU Mount Vernon

Skagit-Valley-tulipsMOUNT VERNON, Wash. – Rotating cover crops in tulip fields shows promise for fighting disease in the economically important flower bulb, according to early research findings at the Washington State University research center in Mount Vernon.

In the first round of testing, the occurrence of tulip fire disease was cut by growing a rye-pea mix in August before planting tulip bulbs in October. If second-year results are similar, producers may have a new strategy for fighting the disease, which is caused by the fungus Botrytis tulipae.

“The break cover crops provide may be enough to get rid of the disease in the soil,” said Ph.D. student Yushan “Sherry” Duan. “If we can manage this disease with cover crops, then tulip growers could potentially continue to plant the same crop each year – and not have to alternate between tulips and daffodils.”

Top crop for sale and tourism

Ph.D. student Yushan “Sherry” Duan with emerging tulips from a WSU Mount Vernon greenhouse trial. (Photo by Kim Binczewski, WSU Mount Vernon)

This time of year in the Skagit Valley, the annual Tulip Festival in April attracts hundreds of thousands of visitors from around the world to this small farming community an hour north of Seattle. Commercial and small-scale growers reap a share of the state’s $12 million-per-year ornamental bulb industry during the short growing and harvest season.

Though weeds and diseases take their toll, Northwest growers may someday have an added weapon as a result of cover crop research conducted by weed scientist Tim Miller, Duan and WSU Puyallup plant pathologist Gary Chastagner.

Duan was enlisted in 2012 to study the impacts of plow-down and cover crops on field production of tulip – one of 25 research projects funded under the U.S. Farm Bill through Washington State Department of Agriculture Specialty Crop Block Grants totaling $3 million.

Replacing fumigation, pesticides

“Ornamental bulb farmers are limited to fumigation and a small assortment of highly regulated fungicides and pesticides, which can be costly to their bottom line if multiple applications are needed each year,” said Duan. “For the small-scale growers, fumigation is not an option due to restrictions on many of the chemicals that were previously available for agricultural purposes.”

“Our research has been twofold,” Miller said. “First, we wanted to see whether cover crops or green manure plow-down crops can reduce competition from weeds and soil-borne pathogens, and perhaps reduce the need for pesticide applications. Second, we wanted to find out whether growing these crops immediately before tulips negatively affects flower or bulb production by creating field conditions that are unfavorable for tulip.”

Collaboration on many fronts

Cut-flower tulip grower Roger Knutson helped Duan establish her half-acre plots on his land near Sumner, Wash., where she planted two cover crops – a wheat-and-pea mixture and a two-variety blend of mustard. Cover crops were planted in mid-summer, grown for four weeks and then plowed into the soil about a month prior to planting tulip bulbs.

“We planted the cover crops and he planted the tulips,” Duan said. “It was a real team effort.”

Miller and Duan are also conducting companion trials in tulip at the WSU Mount Vernon research station. In a controlled experiment, Duan is infesting soil with the fungi that cause tulip fire and tulip-gray bulb rot prior to planting cover crops to determine if the cover crops can help reduce the incidence of these diseases in the tulip crop.  She is working closely with WSU Puyallup Plant Pathologist Gary Chastagner.

“We planted cover crops either in July or in August to see how planting date affects cover crop growth or their ability to control soil-borne diseases,” said Duan. “We are also evaluating how quickly cover crops break down in the soil before planting tulip. Cover crop residue left on the soil surface should help reduce weed seed germination in the tulip crop.”


Tim Miller, WSU Mount Vernon weed scientist, 360-848-6138,
Yushan “Sherry” Duan, WSU Mount Vernon doctoral student, 360-416-5211,