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Wind-Blown Weed Seed   
by
Bob Hartzler

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May 26, 2004The towering purple flowers of musk thistle are a sign of the onset of summer in southern Iowa.  In addition to being a harbinger of summer, these plants often serve as a source of contention between neighbors.  Musk thistle, and other thistles in the Carduus and Cirsium genus, are classified as noxious weeds in Iowa (see Iowa Noxious Weed Law).  The intent of the Weed Law is to protect landowners from having their property invaded by weeds present on adjoining land.  A primary reason for including  thistles on this list of notorious weeds is their wind blown seed.  This trait provides the thistles an effective dispersal mechanism, allowing easy movement across property lines.  

Although thistle seeds have a prominent pappus1, the relatively large seed size limits the distance seeds are carried in the wind.  During peak flowering of musk thistle large 'clouds' of pappus can be seen floating gently in the breeze.  While giving the impression that the seed are being carried long distances, in most instances these high flying remnants of the musk thistle are detached pappus with no seed attached.  While some seed may be carried long distances from the source, the majority of seed falls relatively close to the mother plant.  The distance seed is carried obviously is strongly influenced by wind speed.  Research at Virginia Tech found that under calm conditions 95% of the seed fell within 30 ft of the mother plant (Figure 1).  With wind speeds of 12 MPH, 87% of the seed fell within 75 ft of the mother plant.  Approximately 4% of the seed was carried more than 200 ft.

Researchers in Argentina planted plumeless thistle in fields previously not infested with the species, and then determined the emergence of thistle seedlings in relation to distance  from the mother plant.  More than 95% of the seedlings emerged within 2 ft of the mother plant, indicating that most seed fell directly to the ground.

Table 1.  Emergence of plumeless thistle seedlings in relation to mother plant.

Distance from mother plant % of seedlings
0- 0.8 ft 66
0.8 - 1.6 ft 30
1.6 - 3.3 ft 3
3.3 - 6.6 ft 1
6.6 - 13.2 ft <1
13.2 -26.4 ft <1

Feldman and Lewis.  1990.  Weed Research 30:161-169.

Many important weed species have adaptations for efficient dispersal of their seed.  Although thistle seed are adapted to wind dispersal, their relatively large seed size limits long distance movement under most conditions.  Musk thistle plants directly across a fence line can serve as a source of infestation, but as the distance increases to more that 100-200 feet the likelihood of seed traversing the distance is greatly diminished.  Dandelion and horseweed, two additional weeds with wind-blown seed,  have much smaller seed than thistles, and therefore are more likely to be carried long distances by wind currents.

1Pappus is the botanical term for the feathery 'stuff' that allows the seed to float in the wind.

 

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
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