Dispersion of Weeds
How do weeds manage to spread into new areas? There are several methods of dispersion:
Animal Agency. Animals help to spread weeds around for various reasons. Some weeds produce seeds that can pass through the digestive tract of birds and mammalian herbivores. They remain viable because of a hard coating that resists digestive juices. As these animals move around and defecate, the seeds are deposited in new areas where they can germinate. For example, Solanum nigrum (Black Nightshade) produces small juicy fruits that contain yellow seeds. Birds and small mammals eat these fruits as a source of carbohydrates, but the seeds often pass through their gullets unharmed. Other weedy plants have edible foliage, which is eaten by mammalian herbivores as a source of protein, carbohydrates, and other nutrients. Some of these weeds have already bloomed and have produced seeds, which are unharmed by their passage through the digestive tracts of such animals. In this manner, livestock, deer, rabbits, and other herbivores spread the seeds of the weeds that they have eaten far and wide. Examples of such weeds include Ambrosia trifida (Giant Ragweed), Daucus carota (Wild Carrot), and Rumex crispus (Curly Dock).
Ants disperse seeds with elaiosomes. An elaiosome is a gel-like coating or appendage of the seed coat that contains fatty acids or carbohydrates. Seeds with elaiosomes often release a chemical scent that attracts ants. The ants carry such seeds back to their nests, where the elaiosome becomes a source of food for the adult ants or their larvae. The seeds are eventually discarded with other refuse from the ant nest, where they have a better chance to germinate. Ant dispersion is typical of many wildflowers in woodlands, possibly because of lack of wind and other factors. Weedy species that rely on ant dispersion include Chelidonium majus (Celadine) and Croton capitatus (Hogwort).
Many weeds have seeds or seedpods that cling to the fur of animals and the clothing of humans as a result of sticky hairs, hooked spines, or barbed awns. Examples of weeds using this method of dispersion include Lappa minor (Lesser Burdock), Torilis arvensis (Common Hedge Parsley), and Bidens frondosa (Common Beggar's Ticks). Other weeds have seeds that become sticky while wet, which enables them to cling to the bottom of hooves, webbed feet, or shoes. Species using this method of seed dispersal include Plantago lanceolata (English Plantain) and several Chamaesyce spp. (Prostrate Spurges).
People have deliberately introduced many species of plants into new areas as food crops, as sources of forage, or as ornamental plants for gardens and landscape projects. Some of these introduced plants become aggressive because they lack natural enemies and are later regarded as weeds. For example, Alliaria petiolata (Garlic Mustard) was originally introduced as a potherb. It has since spread rampantly through deciduous woodlands in many areas of the eastern United States. Another species, Melilotus officinalis (Yellow Sweet Clover), was introduced as a source of forage for livestock and a source of nectar for honeybees. It now occurs throughout the United States in such places as railroads, roadsides, pastures, meadows, and waste areas. It has become a problem for ecologists who manage prairies. Another species that is rapidly spreading along the edges of woodlands, thickets, fence rows, and other areas is Hesperis matronalis (Dame's Rocket). This species was originally introduced into the United States as an ornamental plant for flower gardens, and it is still cultivated for this purpose, even though it is considered an invasive weed of both natural and disturbed areas.
Machine Agency. People spread many weeds around through their machines. For example, automobiles and trucks raise gusts of wind as they travel up and down highways. As a result, tiny seeds or seeds with tufts of hair are easily carried aloft in the wind and blown along the highway with each passing motor vehicle. In this manner, many weeds are distributed along roadsides. Similarly, locomotives and their freight cars generate even stronger gusts of winds as they travel up and down railroad tracks. As a result, weeds having tiny seeds or seeds with tufts of hair are distributed up and down railroads. For example, Chaenorrhynum minus (Dwarf Snapdragon) is common along railroad tracks because it is well-adapted to dry gravelly areas and its tiny seeds are blown back and forth by passing trains.
Earth-moving equipment and dump trucks often carry the seeds and rhizomes of weedy plants to new areas, including construction sites and landfills. Other weeds can become snagged on the undercarriage of automobiles and pick-up trucks as they travel along dirt paths, fields, or an open range. These are typically tall-growing weeds with wiry stems and sizable flowerheads, including Centaurea biebersteinii (Spotted Knapweed) and Daucus carota (Wild Carrot). Still other weeds become dispersed by motor vehicles because their seeds or seedheads become stuck in the tread of tires; sometimes they become trapped with the mud. Even motorboats help to spread some aquatic weeds from one lake to another because they become snagged on the undercarriage that is used to lift the boat in or out of the water. When the motorboat shows up at another lake, the aquatic plant rehydrates and establishes itself at its new home.
Lawn maintenance companies help to spread weeds from one lawn to another in urban areas. This is because weed seeds can become trapped within the undercarriage of power mowers amid the grass clippings. Eventually, some of the old grass clippings and the weed seeds of other lawns are blown out from the power mower onto a new lawn. Farmers help to spread weeds to other agricultural areas through shipments of contaminated grains and bales of hay. The transportation of livestock by trucks and freight train spreads weeds along roadsides and railroads through the droppings of such animals while they are being transported from one area to another.
Wind and Water. Gusts of wind resulting from local weather conditions helps to spread those weeds with seeds that are tiny or have tufts of hair. Even large seeds can be blown across the ground or carried aloft by the strong winds of thunderstorms and hurricanes. Examples of wind-dispersed weeds include Asclepias syriaca (Common Milkweed), Taraxacum officinale (Dandelion), and Conyza canadensis (Horseweed). Each of these species have seeds with tufts of hair. Weeds with tiny seeds that are easily blown about by the wind include Artemisia vulgaris (Mugwort) and Consolida ajacis (Rocket Larkspur). Some plants produce seeds or seedpods that are flat, light-weight, and surrounded by a thin papery membrane (i.e., they are winged). This enables even larger seeds to be blown about by the wind. Weeds adopting this method of wind-dispersion include Linaria vulgaris (Butter-and-Eggs) and Thalaspi arvense (Field Pennycress). Another group of plants break off at the base of the stem (or the base of the inflorescence) and act like a "tumbleweed" in an open terrain, scattering their seeds along the way. Weeds adopting this method of wind-dispersion include Sisymbrium altissimum (Tumble Mustard) and Kochia scoparia (Kochia). Such weeds often assume a more or less spheroid shape, which enables them to roll across the ground in response to strong winds.
The seeds of some plants can float on water and be carried by currents downstream (some of these are also blown about by gusts of wind). This method of dispersion is especially common among wetland species. Examples of weeds that rely on water dispersion to some extent include Lythrum salicaria (Purple Loosestrife) and several Rumex spp. (Docks). More conservative species of wetlands, such as Iris shrevei (Blue Flag Iris), depend on this method of dispersion as well.
Mechanical Ejection. Some plants fling their seeds several inches or several feet by mechanical means. This typically involves the release mechanism of their seedpods as they mature. Weedy plants that rely on the mechanical ejection of their seeds, in whole or in part, include Erodium cicutarium (Stork's Bill), Oxalis stricta (Yellow Wood Sorrel), and some Euphorbia spp. (Spurges).
Conclusion. As can be seen from the preceding discussion, weeds have many ways of getting around. Perhaps the leading cause of their dispersion across the planet are people and the activities that they engage in. Not only do people and their machines spread weeds from one area to another, but they also create highly disturbed habitats in which weedy plants can thrive.