Even now, they’re lingering along the roadside waiting for you to pick them up and take them wherever you’re going. Some will ride inside your car, others on the chassis and a few lucky ones will find their way into your clothing. Yes, weeds that spread by people, or hitchhiking, have certainly taken advantage of you this year. In fact, the average car carries two to four seeds for hitchhiker plants at any given time!
There are at least 600 weed species that travel by hitchhiking with humans or on machines, 248 of which are considered noxious or invasive plants in North America. They come from every kind of plant, from herbaceous annuals to woody shrubs, and occupy every corner of the world. A few plants you might be familiar with include the following:
What are Hitchhiker Weeds?
Once those weeds emerge, digging them out is the only cure. Make sure to get three to four inches (7.5 to 10 cm.) of root when the plant is young, or else it’ll grow back from root fragments. If your problem plant is already flowering or going to seed, you can clip it at the ground and carefully bag it for disposal – composting will not destroy many of these types of weeds.
Although it might sound like all fun and games, the weeds spread by people are not only difficult to contain, they’re costly for everyone. Farmers lose an estimated $7.4 billion each year in productivity to eradicate these pest plants. Humans are spreading these seeds at a rate of 500 million to one billion seeds a year in cars alone!
You can help slow the spread of these hitchhikers by carefully inspecting your clothing and pets before emerging from a wild area full of seeding plants, making sure to leave those unwanted weeds behind. Also, reseeding disturbed areas like your garden plot with a cover crop can ensure that there’s too much competition for hitchhikers to thrive.
O ne of the most interesting cases of hitchhiking involves seabirds and a tropical pisonia trees of the four o’clock family (Nyctaginaceae), including Pisonia umbellifera and P. grandis . On some atolls of the South Pacific, pisonia is the only dominant tree on otherwise barren islets. Marine birds use the large, edible leaves to construct nests in the trees. Pisonia trees produce tiny white flowers in terminal, many-flowered clusters. Individual apetalous flowers have a tubular, petaloid calyx that resembles a sympetalous corolla. The lower portion of the calyx tightly enwraps the ovary and is persistent around the fruit as an anthocarp. The calyx base plus the enclosed seed-bearing fruit is the unit of dispersal. In some members of the Nyctaginaceae, the persistent calyx base bears sticky glandular projections that aid in dispersal by adhering to the bodies of animals. This is especially true in pisonia trees in which the numerous glutinous anthocarps stick to the feathers of seabirds. This is an effective method of dispersal to distant atolls and islands of the South Pacific region. Sometimes a hapless seabird is completely covered by clusters of the sticky anthocarps, to the point where flight is difficult or impossible. Unable to remove the water-resistant, glue-like anthocarps from its feathers, the seabird drowns in the surf and is consumed by ravenous beach crabs.
Note: Ranking & SRDUs are purely hypothetical without any quantitative data to support them.
S tick-tights or beggar’s-ticks (Desmodium cuspidatum) produces slender legume fruits that break into small, one-seeded joints covered with tiny barbed hairs. Technically, this special kind of legume fruit is called a loment. The individual joints are so flat that they are exceedingly difficult to remove from your socks. Like little flat ticks, you must individually pull off each one. This can be exasperating when your socks are covered with them. Several species of this remarkable hitchhiking herb are native to the midwestern and eastern United States.
N aturalized hitchhiking plants often grow very well in disturbed sites, and this is how some of California’s most noxious weeds were introduced. The earliest reported collection of puncture vine in California was made at Port Los Angeles in 1903, presumably the result of a ballast dump. Within a few decades it had spread throughout the state, reaching epidemic proportions in some cultivated valleys. The burs were carried in the wool of sheep, in hay, straw, other feed, manure, melons, alfalfa, cotton, potatoes, picking sacks and boxes, tents, sand, gravel, farm and industrial machinery, and in rubber tires of automobiles and airplanes. Puncture vine is the scourge of every bicyclist and is a major factor in the popularity of inner tube repair kits. Although it only has a SRDF of 3 in the above table, this may be a gross underrating for this naturalized hitchhiker.
The Undisputed Most Painful Hitchhiker
A Tenacious Hitchhiker Of The Colorado Desert
T he seeds of some parasitic mistletoes are also coated with a very sticky substance that aids in dispersal by birds. Desert mistletoe ( Phoradendron californicum ), a common parasite on cat-claw acacia ( Acacia greggii ), produces juicy, bright red berries that provide numerous birds with food and water during the winter months. If you walk along a desert wash in the Colorado Desert region of the southwestern United States, you can often spot a black bird called the phainopepla ( Phainopepla nitens ) near clumps of mistletoe. This bird is easy to identify because of the conspicuous crest on its head. Mistletoe seeds are covered with a glue-like substance that sticks to the bills of birds. When birds try to clean their bills, the seeds adhere to the limbs of other trees and shrubs. The seeds also pass through the bird’s digestive tract and are transported from one bush to another in the bird’s droppings. In fact, this probably explains the derivation of the word mistletoe: from two Germanic words: mista (dung) and tan (twig); referring to bird droppings on a branch or stem. Apparently when the word mistletoe was first used in Europe, people were already aware of the dispersal of mistletoe seeds by birds.