Contributed by Fernando Oreja and Judit Barroso, Oregon State University
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.
Image 1. Mature Russian-thistle plant ready to be dispersed.
Photo credit: Nicholas Genna (previous OSU post-doc working on this research)
Image 2. Russian-thistle plants growing on tall and short stubble at CBARC, Adams, OR in 2020.
Photo credit: Judit Barroso, OSU Associate Professor
Image 3. Russian-thistle plants growing in trampled and short-to-medium stubble in Ione, OR in 2020.
Photo credit: Judit Barroso, OSU Associate Professor
Figure 1. Russian-thistle dispersion (%) in Ione, OR for different plant sizes (small, medium and big) and stubble heights (standing and trampled). Values are the means and vertical bars are the standard error of the mean (SEM).
At both sites, the dispersion rate increased with time. At CBARC, a higher plant dispersion was observed on plants growing in short stubble than in tall stubble. In 2020, dispersion in short stubble was 66% compared to 14% in tall stubble, and in 2021, the values were 53% in short stubble compared to 20% in tall stubble. In the grower’s field, dispersion in trampled stubble was 88% compared to 43% in standing stubble and big plants were more dispersed than small plants (86% vs. 48% respectively) (Figure 1).
For growers that struggle to control Russian-thistle post-harvest, leaving the stubble tall at harvest, could reduce Russian-thistle dispersion in their fields and neighboring fields. Preventing plants from becoming big (e.g. mowing) as part of an integrated weed management program could also reduce dispersion.
We would like to thank the USDA-NIFA program for funding this research and the grower in Ione for letting us have Russian-thistle in his field for this study.