In the semi-arid region of inland Pacific Northwest (PNW), Russian thistle (Salsola tragus), kochia (Bassia scoparia), and prickly lettuce (Lactuca serriola) are three important weed species that can regrow after harvest and use water. However, the way in which these species compete for soil water, the most limited resource, has not been explored adequately. Below ground competition for soil water may occur in two ways. First, deeper rooting architecture with a tap root and a faster rate of root elongation may allow weeds to access water that spring or winter wheat cannot reach. Any weed that is capable of rooting deeper than wheat will have a considerable advantage during the growing season, especially during dry years. Further, post-harvest regrowth will be fueled by access to a stable source of water when upper soil layers are too dry to support growth.
A second reason why Russian thistle, kochia, and prickly lettuce are an issue in wheat may relate to root physiology. The historic consensus was that transpiration stopped in soil that was drier than -1.5 MPa. (1.5 mega pascals, which is equal to 15 bars or 15 atmospheres of suction, or -220 psi. This is about 8.3% water content in a Walla Walla soil). This lower limit is called the permanent wilting point and was thought to be typical across plants. However, nowadays a term called the biological wilting point has replaced the permanent wilting point concept allowing for species dependence. Therefore, it is possible that Russian thistle, kochia, and prickly lettuce may possess a biological wilting point that is lower than wheat, giving these species a physiological advantage during drier years or during post-harvest regrowth.
Last year, we, in collaboration with Dr. Nicholas Genna, started research to quantify the competitive ability of Russian thistle, kochia, and prickly lettuce compared to winter and spring wheat below ground. In other words, we wanted to know how each species interacts with soil water. An understanding of a potential below ground advantage for the weed species, and quantification of the water extracted if weeds are left untreated after harvest, could provide scientific information to farmers seeking a return on investment for controlling weeds post-harvest and guide researchers and breeders to develop more competitive varieties.
A preliminary analysis of the data indicates that several of the studied weeds extract more water from the soil and also deeper in the soil profile than spring wheat. Which means that Russian thistle and certain other weeds can cause significant deficits in soil water storage even when the crop has used “all” available water. In Figure 1, we see that all plots lost water, including the unplanted control plot, from late March to early October of 2021. However, the soil under a prickly lettuce plant, kochia plant or Russian thistle plant had significantly less water than the soil with no plant (control) or the soil under a spring wheat plant growing in fallow conditions. The fourth foot of prickly lettuce had similar soil water than the control and the spring wheat, which might indicate that this species does not root as deep as kochia or Russian thistle. Kochia and Russian thistle had similar water extraction patterns, although it seems that Russian thistle was drying the fourth foot of soil more than kochia.
(Select photo to enlarge)
Figure 1. Difference of soil water content between the March and October soil sampling for the first four feet of soil (0 to 1, 1 to 2, 2 to 3, and 3 to 4) in a fallow trial for each of the studied species. The soil water was calculated based on a dry mass basis (g of water/g of dry soil). The boxes in the graph indicate the range of 50% of the data, with the central line marking the median value. Lines that extend from each box are the remaining data, and dots indicate extreme points.
Image 1. View of the fallow experiment in 2021 at CBARC before the last soil sampling in October. The plot at the front is a control plot without any plant. Plots were separated by 4m (≈13ft) center-to-center.
Image 2. a) Photo of the biggest Russian thistle plant in the study. Its fresh weight was 29 lb on October 4th, and b) Photo of the biggest kochia plant in the study a few days before taking the last soil sample close to the plant main stem (in a one-foot radius).
We plan to repeat the studies this coming season to be able to project our findings on soil water extraction by these late season weeds into an economic yield loss when weeds are left untreated post-harvest. Do not hesitate to contact us if you want to know more about the research and keep tuned for more complete information by the end of 2022.
This blog post was co-authored with Stewart Wuest, USDA-ARS.