Forest Preserve District of Cook County, Illinois
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Weed Seeds
Nature Bulletin No. 469-A   November 4, 1972
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

The study of weed seeds can be fascinating, educational, and an inspiration for artistic designs -- especially if you use a good magnifying glass or hand lens. People wonder why weeds are so aggressive; why they succeed where desirable plants fail; why they thrive in spite of droughts, grass fires, mowing and cultivation. One answer, in the case of most weeds, can be found in the seeds that they produce.

Some weed seeds have such a hard durable outer coat that they are unharmed by an ordinary grass fire. If eaten by a bird or a grazing animal, they are not digested in its stomach and are widely distributed. Some kinds have such vitality that they have been known to sprout and grow when brought to the surface after being buried deeply for many years. Many weed seeds have interesting appendages which cause them to be carried long distances by winds or which enable them to cling to passing animals.

The seeds of some weeds are enclosed in small "nutlets". Beggar-lice, nutlets of one of the Stickweeds, are covered with clusters of from 3 to 5 fine barbs resembling the claws on the foot of a housefly. On another kind the clusters are star-shaped like the end of a dentist's drill. A Puncture Vine nutlet has spines arranged so that, no matter how it lies, one will be pointing upward -- exactly like the French caltrop and the Roman tribulus used to stop an enemy's cavalry. Similarly, a Sandbur will cause painful injury to a barefoot boy, the paws of dogs and cats, or even puncture a bicycle tire.

The Burdock and the Cocklebur produce quantities of burs with hooked spines arranged in ornate but very effective patterns. The coats of dogs or other animals, and the tails of horses and cows, frequently become matted with these pesky burs which, when they are pulled off, spill out large numbers of small seeds. The cocklebur is an example of plants that produce more than one type of seed; some of its burs germinate the following spring, others in two years, and so on.

There are weed seeds armed with slender pointed bristles, called "awns", that expand and contract according to the amount of moisture encountered. Thus, the Wild Oat seed has two awns which enable it to creep along the ground until it can enter some crevice. Porcupine or Needle Grass seed has two which twist like an auger and bore into the soil.

Some weed seeds have odd shapes which are responsible for the common name of the parent plants. Those of the Moonseed vine are crescent-shaped like a new moon. The Spanish Needle has a long slender seed armed at the broader end with three prongs and each of these has a sharp curved hook. The Bootjacks or Beggar-ticks, with two such awns, resemble that old-time implement used on farms and ranches. Docks, Smartweeds and Sorrels have wedge-shaped seeds which help them to easily work into soil openings.

Some weeds, such as Dandelion, Wild Lettuce and the Thistles, have tufts or "parasols" of fine hairs attached to their seeds, and these serve like parachutes which are carried long distances by winds. Those parachutes are interesting. The Seed of the Goatsbeard, now a roadside pest, has one with a lacy arrangement of silky threads which travels daily until it passes over a moist area. Then the parachute, called a "pappus", closes and the seed drops to the ground.

A race of plants depends upon its seeds -- each one a masterpiece of arrangement and construction.

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