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Post-harvest best time to apply herbicide on dandelion


March 31, 2008
By Fruit & Vegetable

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New research shows that the best time to apply herbicide on dandelion is post-harvest. “We now have more information on this pesky weed,

dandilionsNew research shows that the best time to apply herbicide on dandelion is post-harvest.

“We now have more information on this pesky weed,” says Nick Underwood, an Alberta-based agronomist who credits Kristin Hacault, a recent Master’s graduate from the University of Manitoba, with the finding. “She did her thesis on the emergence timing and control of dandelion in spring wheat.”

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In some parts of Canada, the amount of dandelion infestation in field crops has increased from 7.3 per cent of fields in the 1980s to 16.7 per cent in the 2000s. “Some farmers blame reduced tillage for this increase, but it isn’t that simple,” says Underwood.

In the past, dandelions and other weeds were partly controlled by producers tilling post-harvest. Since this is no longer practiced, the timing of herbicide application to control dandelion is critical.

“Dandelion populations are spread to new locations by seed alone, but populations can be increased by seed and from rootstock,” says Underwood. “Tillage can actually increase the reproduction from rootstock.”

Hacault’s research aimed to gain a greater understanding of dandelion biology. The objectives were to:

determine the emergence timing of dandelion from rootstock and seed investigate the efficiency of pre-seeding and post-harvest herbicide treatments as influenced by dandelion’s emergence timing.

“We can be thankful that dandelion seed doesn’t remain viable in the soil for long,” says Underwood. “In the study, less than four per cent of the seedlings were from the seed bank in the soil, while the majority were from seed that was disseminated a few weeks earlier. The optimum temperature for dandelion seed germination is relatively high, and seedling emergence did not occur until after the peak flowering period of the existing dandelions that were the seed source.”

As dandelion seedlings are the source of the population growth, controlling them is key. But in-crop herbicide applications occur too early to control seedlings and too late to control mature flowering plants. In the study, dandelion shoots from rootstock emerged early in the season. But because of re-growth, pre-seed herbicide applications were less effective at controlling dandelions than were fall herbicide applications.

The length and timing of the flowering period of dandelions can vary. Underwood says this is because of interactions among temperature, precipitation, day-length and dandelion biotype.

“I remember being surprised to learn that there are at least 60 different biotypes of dandelions in Canada,” notes Underwood. “In general, cooler temperatures lengthen the dandelion flowering period. Moist conditions after seed dispersal favour seedling emergence, especially if there is frequent rain. These seedlings will not compete significantly with the annual crop in which they are growing, because they are about two months behind. Instead, they are getting ready to be a major competitor next year. They will do a significant amount of growing after the August or September harvest. Like most perennial weeds, they become a significant competitor with annual crops after their first year.”

Hacault’s research looked at controlling the dandelions with:
• glyphosate alone
• glyphosate plus florasulam
• glyphosate plus tribenuron

The treatments were applied either in the spring prior to seeding the wheat, or in the fall post-harvest at various rates. All plots, except the untreated control, were sprayed with Horizon, Buctril M and Refine Extra when the wheat crop was at the three-leaf stage. The results showed that post-harvest application of the herbicides was much more effective in reducing the dandelion population than the pre-seeding application. The low rate of glyphosate (0.5 L per acre) by itself sometimes gave reasonable control when applied in the fall. It performed better at higher rates and when combined with florasulam or tribenuron.

“By applying herbicide post-harvest, you can control established dandelion plants, emerged seedlings, and plants that emerged from rootstock earlier in the year,” says Underwood. “This is also the time of year that the plant is transporting reserves to the root so it is a good time to add some herbicide to those reserves!”