The success of any weed infestation is the result of at least one of several ecological processes such as establishment, competence, dispersion, or herbicide resistance. Any weed species that becomes troublesome in a particular location is because it possesses characteristics that ensure the reproductive success through one of those ecological processes. In the case of Russian-thistle (Salsola tragus L.) that success is mainly due to its dispersion ability and its high seed production.
The dispersal mode of this species is by plant tumbling. After the plant dies, the main stem breaks near the soil surface leaving the plant free to roll and tumble with the wind (Image 1). The bouncing movement causes Russian-thistle seeds to scatter in a range of about 0.04 miles to 2.5 miles from the original position. A single plant can produce more than 50,000 seeds that are spread during the dispersion process. This allows the weed to colonize new areas and spread its progeny, which will reduce the intraspecific competition and increase the probability of potential high biomass and fecundity of future plants. Big plants are more competitive than smaller plants and can produce higher yield losses in agricultural areas.
Dispersal success, measured as the distance achieved from the original spot, is highly dependent on extrinsic factors (i.e., wind speed) and intrinsic factors (i.e., shape and weight of plants). Russian-thistle plants have a globose-elliptical shape that allows them to roll easily with the wind. In this species, the plant itself acts like a sail “catching” the wind to travel farther distances. Therefore, the bigger the sail, the higher the probability for farther dispersion.
Conversely, the remaining stubble from the previous crop could function as a barrier that reduces wind speed. A tall stubble could reduce the wind speed impacting the tumble weed and consequently reduce Russian-thistle plant dispersion. Developing weed management practices that reduce Russian-thistle dispersion would help to minimize the colonization of new areas/fields and prevent future agricultural yield losses. Since Russian-thistle seeds have a short viability in soil (no more than 2 or 3 years), avoiding new seed deposition is a beneficial strategy to diminish the infestation quickly, if adequate management is applied in the following two years.
To confirm the ideas mentioned above on how plant size and stubble height can impact Russian-thistle dispersion, an experiment was conducted in two consecutive years (2020 and 2021) at the Columbia Basin Agricultural Research Center (CBARC) (Adams, OR) (Image 2) and in a grower’s field in Ione, Oregon in 2020. At CBARC, the different stubble heights were 15 and 5 inches, and in the grower’s field, we compared standing stubble with stubble trampled by the combine wheels (Image 3). Plant dispersion was evaluated by counting plants right before they were killed by freezing temperatures (end of October) and multiple times during winter and the following spring.