Weeds with feathery wind blown seeds
Iowa State University
Wind-Blown Weed Seed
by Bob Hartzler
May 26, 2004 – The 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 pappus 1 , 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|
|13.2 -26.4 ft|
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.
1 Pappus is the botanical term for the feathery ‘stuff’ that allows the seed to float in the wind.
Weeds with feathery wind blown seeds
Annual herb with spreading taproot; stems bushy, much branched, 1/2 to 4 feet tall, 1 to 5 feet in diameter, rigid, spiny, spherical, often reddish-purple in age, young stems and leaves green and succulent.
Leaves: Leaves alternate, the first-formed leaves fleshy, cylindrical or awl-shaped, 7/16 to 2-1/2 inches long and less than 1/16 inch wide, with a pointed tip, the latter-formed leaves shorter, stiff, dilated and thickened at the base, ending in a hard sharp spine.
Flowers: Flowers June to October; flowers small, greenish, mostly solitary in the axils; petals none; sepals 5, papery and persistent; 2 bracts at the base of each flower are rigid, spine-tipped; fruit surrounded by the 5 enlarged sepals, each developing a fan-shaped, strongly veined wing on its back, 1/8 to 3/8 inch in diameter.
Fruit: Seeds numerous (to 250,000 per plant), top-shaped, 1/16 inch in diameter, with a yellowish coiled embryo, visible through the thin gray wall.
Cultivated and disturbed or degraded sites in grassland and woodland communities, and roadsides in any type of well-drained, uncompacted soil, however, it is most frequent in alkaline or saline soils within elevations that generally range below 8,500 feet.
Reproduces by seed; Russian thistle is a highly effective reproducer, after seeds mature in late fall, the plant stem separates from the root, the plant is then blown by wind; seeds, held in the leaf axils, fall to the ground as the plant tumbles.
Native to the Mediterranean region; Russian thistle aids in spreading fire; burns easily because stems are spaced in an arrangement that allows for maximum air circulation; dead plants contribute
to fuel load by retaining their original shape for some time before decomposing. In general appearance, this species could be confused with kochia. This species generally occurs as a weed in wildland areas of the Southwestern Region rather than as an invasive plant.
When You Wish upon a Fluff
You know those cute yellow flowers you see in the grass? Or have you seen round, white poofs of fluff that you can blow into the air to make a wish? Those two flowers are the same flower. They’re called “dandelions,” which comes from the French words for “lion’s tooth.” They’re bright and friendly-looking, but grown-ups can’t stand them. Dandelions are weeds, meaning they suck up more than their share of water and vitamins from the dirt. That makes it harder for grass to grow. But until the 1800s, people used to grow dandelions on purpose. We can eat any part of the flower — the leaves taste pretty good in a salad! Luckily for us, each flower holds up to 400 seeds, which can sail as far as 5 miles. Dandelions are here to stay.
Wee ones: If you have 7 yellow dandelions and 4 white fluffy dandelions, of which kind do you have more?
Little kids: If 1 dandelion seed sails 5 miles in one direction, and another seed flies 5 miles in the opposite direction, what’s the farthest apart they can land? Bonus: If each time you blow on a dandelion you blow 5 seeds free, how many wishes does it take to blow 20 seeds away? Count up by 5s!
Big kids: Dandelions can have up to 400 seeds, but they usually have around 180. How many more seeds does a mega-fluffy 400-seed dandelion have than the usual? Bonus: If you blow off 300 seeds, and each of those makes a new flower that sends off 300 seeds, how many seeds sail off in that 2nd round? (Hint if needed: what if each of the 300 sent off just 3 seeds…then what if each sent off 30 seeds instead…then how about 300?)
Wee ones: More yellow dandelions.
Little kids: 10 miles apart. Bonus: 4 wishes: 5, 10, 15, 20.