Agronomy Library > Annual Weeds

Dandelions in Annual Crops (Based on information from a paper by K.M. Hacault & R.C. Van Acker)
Author: Nick Underwood, Reduced Tillage Linkages Agronomist. Peace Region
Date Created: June 14, 2006
Last Reviewed: February 14, 2007
An article based on the Hacault, Van Acker paper in Weed Science, 54:172-181. 2006   We now have more information on this pesky weed, thanks to Kristin Hacault, M.Sc. who recently completed her master’s thesis on the emergence timing and control of dandelion in spring wheat. In the Northern Great Plains the abundance of dandelion, a simple perennial species, in field crops has increased from 7.3% of fields in the 1980s to 16.7% in the 2000s. Some farmers blame reduced tillage for this increase, but it is not that simple. When we were tilling post harvest we were controlling dandelions, and other weeds, to some degree. Now we are not doing post harvest tillage and Kristin’s research has shown that the timing of herbicide application to control dandelion is critical and post harvest is the best time to do it. Dandelion populations are spread to new locations by seed alone, but populations can be increased by seed and from rootstock. Tillage can increase the reproduction from rootstock. Kristin’s research was conducted to gain a greater understanding of dandelion biology. The objectives were (1) to determine the emergence timing of dandelion from rootstock and seed, and (2) investigate the efficacy of pre-seeding and post-harvest herbicide treatments as influenced by the emergence timing of this species. We can be thankful that dandelion seed does not remain viable in the soil for long. In Kristin’s study less than 4% of the seedlings were from the seed bank in the soil, 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 existing dandelions, which were the seed source. Controlling dandelion seedlings is key, as they are the source of population spread. But in-crop herbicide applications occur too early to control seedlings and too late control mature flowering plants. In the study, dandelion shoots from rootstock emerged early in the season, but control from pre-seed applications was reduced when compared to the fall herbicide applications because of re-growth. The length and timing of the flowering period of dandelions can vary. It varied among the 5 site years in Kristin’s research. This is because of interactions between temperature, precipitation, day-length and dandelion biotype. I remember being surprised to learn from Dr. Linda Hall that there are at least 60 different biotypes of dandelions in Canada. In general cooler temperatures lengthen dandelion flowering period. Moist conditions after seed dispersal will favour seedling emergence, especially if there are frequent rainfall events. These seedlings will not compete significantly with the annual crop in which they are growing, because they are about two months behind. 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. Kristin’s research looked at controlling the dandelions with glyphosate alone, glyphosate + florasulam and glyphosate + tribenuron. The treatments were applied either in the spring prior to seeding the wheat, or in the fall post harvest at various rates. In addition 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 were very definite in 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) sometimes gave reasonable control when applied in the fall. Higher rates and those combined with florasulam or tribenuron gave better control. By applying herbicide post harvest one is able to control established dandelion plants, emerged seedlings, and plants that emerged from rootstock earlier in the year. 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!