Weed Wisdom Sept. 2020
Bluebur [Lappula squarrosa (Retz.) Dumort.]
By: Stephanie Gross and Charles Geddes
Bluebur [Lappula squarrosa (Retz.) Dumort.] is a summer or winter annual weed native to Eurasia. Although every province and territory in Canada has traces of bluebur, it is more commonly found in the prairie provinces. The species is invasive and was first recorded in Alberta in 1899.
Bluebur is from the Boraginaceae family and its Latin name means little bur. Bluebur can grow up to two feet tall and is covered in short, thick white hairs. The lower leaves are oblong and stalked while the upper leaves are not. Each blue flower has five petals and produces four nuts. These flowers closely resemble forget-me-nots. The nuts have two rows of hooked prickles. This weed also gives off an unpleasant mouse odour.
Bluebur reproduces sexually using seeds. Seeds can persist in the soil for approximately four years. Plants can persist over winter as a vegetative rosette. Temperatures near freezing will slow seed germination but encourage reproduction.
This weed is often found in waste areas and along roadsides. The burs spread by attaching onto sheep wool or tails of cows and horses. Animals can transport these seeds short or long distances. Humans can also spread bluebur when the seeds attach to their clothing and get dispersed in new areas. Abiotic factors such as wind and water could also spread bluebur. It can thrive in a diverse range of moisture, showing growth in dry gravel beds and roadsides as well as on the perimeter of ponds. Bluebur can be problematic in cropped fields.
There are several ways to limit the spread of bluebur. One method is to carefully remove the burs from clothing. This will prevent bluebur from being introduced to new areas by human activity. These weeds can easily be pulled by hand if found in small areas or numbers. Planting crops with a high seeding rate will help them out-compete bluebur. Tillage can be an effective method to manage bluebur mechanically. Fall or spring tillage can decrease bluebur abundance in the field. In annual crops in Saskatchewan, bluebur mortality rate is very high among winter annuals and early emerging summer annuals due to spring cultivation.
Herbicides, like the phenoxys 2,4-D or MCPA, can control bluebur in some crops like wheat. Other effective herbicides fall into the acetolactate synthase inhibitors, synthetic auxins, or photosystem II inhibitors, or groups 2, 4, and 6, respectively (see your provincial crop protection guide). Mowing or swathing should not be done to remove bluebur. By removing only the apex of the plant, it can grow back stronger and more-competitive. Seed production is likely to increase if sufficient stem is left behind.
Alberta Biodiversity Monitoring Institute (ABMI) Bluebur information page
Frick, B. (1984). The biology of Canadian weeds. 62. Lappula squarrosa (Retz.) Dumort. Canadian Journal of Plant Science, 64(2), 375-386. doi:10.4141/cjps84-053
Profile of Invasive Plant Species Within the Peace River Regional District. (2017). Ecology of Weeds and Invasive Plants, 349-383. doi:10.1002/9780470168943.ch9
Best, K. (1965). Bluebur-a weedy post. Blue Jay Journal, 43. Retrieved from: https://www.bluejayjournal.ca/index.php/bluejay/article/view/3183/3172
Alberta Biodiversity Monitoring Institute. 2019. Bluebur (Lappula squarrosa). ABMI. Retrieved from: abmi.ca/home/data-analytics/biobrowser-home/species-profile?tsn=99002709.