Prof Discovers Secret to Reducing Dandelion Population

May 15, 2008 - News Release

If you think spring is the best time to try and rid your lawn of those dreaded dandelions, you're about six months too late.

Research by a University of Guelph plant scientist has found that gardeners need to be attacking these weeds in the fall if they want to prevent them from reappearing.

"You can put a huge dent in the dandelion population if you deal with it in the fall because that way you're killing the next generation before it has a chance to flower and spread its seeds," said Prof. Rene Van Acker.

After conducting a number of studies looking at the proliferation of dandelions in field crops, Van Acker discovered it's actually easier and more effective to kill these weeds in the fall when they have not yet flowered.

"People tend to concentrate on getting rid of the dandelions when they see those yellow flowers in the spring, but they are much more difficult to kill at this point. Pulling them out will work, but they can often withstand any herbicide."

Spraying at this time of year is also ineffective in killing the seedlings that will appear as flowers next year because these young plants don't emerge from your lawn until the middle of the summer, he said.

Van Acker began studying the life cycle of dandelions in 2000 when farmers in Western Canada found their wheat and canola crops were suffering from dandelion infestations.

Over a five-year span, he worked with master's students Kristen Hacault and Nathan Froese to determine why the weed proliferates so successfully. His research was published in Weed Science and Weed Technology.

What he found was that farmers were spraying the dandelions in the spring in an attempt to kill the mother flower, but many still flowered, set seeds and then spread these seeds, he said.

"When the farmers changed to spraying in the fall, the results were like night and day. The dandelion is much more physiologically susceptible to spray in the fall, so it's easier to stop them from populating."

Ultimately, the best strategy is to pull the flowered weeds in the spring and spray or pull the young plants in the fall as well, he said.

"It's the only way you can stay on top of the dandelion issue."

Prof. Rene Van Acker
Department of Plant Agriculture
519-824-4120, Ext. 53386

For media questions, contact Communications and Public Affairs: Lori Bona Hunt, 519-824-4120, Ext. 53338, or, or Deirdre Healey, Ext. 56982 or

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