Ravine near Moab. Photo by D. Sillman
(Click on the following link to listen to an audio version of this blog … Another Look at Tumbleweeds
Last summer, when we were in Moab, Utah, we spent one afternoon at a reservoir south of town. There was a hiking trail that climbed up a nearby ridge with a waterfall at its apex. Along the trail there were a number of deep gullies that were stuffed to their rims with tumbleweeds! The weeds must have rolled off of the surrounding dry steppes all through the previous fall and winter and maybe even years before that and gathered in these sheltered ravines. There were thousands and thousands of tumbleweeds in the piles. They were compressed down and tangled up into solid masses and were so dry that they looked like they could burst into flames with just a single match! Our rental house in Moab had a patio full of tumbleweeds, too. We watched them roll in with the steady wind and then grab onto each other around the patio furniture. My grandson, Ari, picked a small, very round one out of the pile and kept it as a “pet.” Sadly, a gust of wind one afternoon carried his tame-tumbler away!
Photo by J. Choate, Flickr
Tumbleweeds, as I have written before (Signs of Spring 6, April 8, 2021) , are an iconic symbol of the American West (as depicted in movies and on television, anyway), but they are really relatively recent invaders into our North American landscape. A ”tumble weed” is an annual plant that, as part of its seasonal senescence, detaches either all or part of its above ground biomass from its anchoring roots. This then allows the stems and branches and dried, seed-filled flowers to roll across a sometimes considerable stretch of land. The purpose of this detachment and “tumbling” is, of course, the dispersal of seeds. Ecosystems like arid grasslands, prairies and steppes that have regular winds and few biological or physical obstructions are most likely to have tumbleweeds.
Seed dispersal via plant tumbling is a fairly unusual plant strategy. Most plants spread their seeds either via gravity or via modifications in seeds to allow wind dispersal (like dandelion seeds or maple tree samaras). Plants also use flotation systems and water dispersal (like we have seen recently in the blogs about palm trees and mangroves). Plants also utilize a variety of animal assisted seed dispersal mechanisms. Some of these are “epizoochoric” (a sticky or spiny seed gets attached to the outside of an animal), while many others are “endozoochoric” (a seed inside of an edible fruit passes through an animal’s digestive track and is deposited (often along with a dab of fertilizer!) at some distant location).
Photo by D. Jandy, Flickr
Many different kinds of plants around the world make tumbleweeds. In North America, though, there are two major “tumble-weeding” plant species: Salsola tragus (common name “Russian thistle”) and Bassia scoparia (common name “kochia”).
Russian thistle is a plant from Ukraine and western Russia. Its seeds were brought to North America in a shipment of flax seed
Russian thistle. Photo by F and K Starr. Wikimedia Commons
back in the early 1870’s. The Russian thistle seeds germinated and matured in the flax fields of South Dakota and then rapidly spread across the continent. This spread was accomplished not only by the very efficient, long-distance tumbling of the dried plants (each Russian thistle plants contains approximately 250,000 seeds!) but also via transported agricultural equipment and train cars (especially livestock cars). By 1895, Russian thistle had reached California. It can now can be found in every state in the country with the exception of Alaska and Florida. Over 100 million acres of land all across the United States contain Russian thistle.
Kochia. Photo by Rameshng. Wikimedia Commons
Kochia was brought to North America in the late 1800’s as an ornamental plant for landscaping and for gardens. Its drought tolerance, deep taproots and C-4 energy biology, though, facilitated its escape into the wild. C-4 metabolism is a relatively uncommon plant adaptation (it is only seen in about 3 % of all terrestrial plant species) that enables a plant to take up carbon dioxide without losing excessive amounts of water via transpiration and especially without exposing some of its delicate, carbon fixing enzymes to toxic levels of oxygen. The bottom line for a C4 plant is that it must spend more energy than other types of plants (collectively called the “C3 plants”) to make sugars, but if the stress of drought or arid conditions threaten to dry out a normally transpiring plant, then the extra energy cost is a small payment to insure survival. C-4 plants grow most quickly during times of drought and high temperatures, conditions, of course, that are very common in the middle to late summer.
Kochia (in autumn). Photo by Kenpei. Wikimedia Commons
Kochia is now found in every state west of the Mississippi River and is a common weed of prairies, steppes, agricultural fields and roadsides. Kochia has a long list of common names including several (like “burning bush,” “firebush,” and “fireweed”) that are based on its bright red autumn foliage. A mature kochia plant is typically one to four feet tall but may be taller when growing in optimal conditions. A single plant can make 15,000 to 50,000 seeds.
Russian thistle seeds lack protective coverings and stores of nutritional energy. They cannot germinate until moistened by spring rains and warmed by rising spring temperatures. The initial shoots are grass-like with long, double, green leaves supported on red or purple stems. These initial leaves are quite palatable and are eaten by both wild and domesticated grazing animals.
Kochia seeds germinate in the late winter or early spring. Early shoots are also quite palatable and are readily eaten by cattle and many wild, grazing species. As the kochia grows, though, it accumulates high levels of nitrates and becomes potentially toxic to animals. Kochia also produces abundant, allelopathic chemicals from its roots that can poison other plant species. As it spreads in a soil system, fewer and fewer other plant (which would potentially be competitors for kochia) are also able to grow there. Crop plants, garden plants and grass plants are all negatively affected by the kochia toxins! Kochia has also developed broad tolerances to the major herbicides that have been used to control it.
Cattle in New Mexico. Public Domain
Cattle eating young, Russian thistle and kochia can keep the plants’ populations under control. When range land is left fallow and un-grazed, though, these two invasive plants can grow explosively and set up very dangerous fall tumbleweed dispersals. Many acres of rangeland, because of drought and declining ground water levels, are no longer being used to graze cattle. This allows excessive growth of Russian thistle and kochia and greatly exacerbates fall and winter tumbleweed problems.
The dry, above ground plant mass of Russian thistle and kochia are extremely flammable. As they accumulate in sheltered areas of a dry ecosystem (like the gullies south of Moab that I talked about at the beginning of this blog!) they can serve as a rapidly burning fuel for wildfires. These burned areas are often only poorly revegetated by native plants and will often grow back primarily in Russian thistle and kochia, establishing a dangerous cycle of more and more tumbleweeds and more and more wildfires.
Blowing tumbleweeds (which can be as large as small cars!) can interfere with car and truck traffic and can cause accidents via direct collisions. They can even build up in such great masses that they block highways. Crowley County in southeastern Colorado, for example, spent $100,000 (more than a third of the county’s annual budget) clearing its roads of tumbleweeds. El Paso County, Colorado in south central Colorado spent over $200,000 clearing its roads of tumbleweeds!
Photo by D. Updegrade, USAF
Tumbleweeds can also block irrigation ditches and have even been known to trap people in their homes! Most counties have laws that require property owners to keep their land free of nuisance vegetation like tumbleweeds. As the economic costs of fires, road clearing, irrigation ditch clearing continue to rise, stricter enforcement of these laws may be required even out here in the new “old west.”
To illustrate the steepness of the learning local curve concerning the seriousness of the tumbleweed invasion consider the response one local rancher made when she was asked how she dealt with tumbleweeds. She replied, “I pitchfork them over the fence onto my neighbor’s property and let him deal with them!”
Tumbleweeds, Old West icon, New West problem!