The night and day differences of smooth scouringrush with Marija Savic

Smooth Scouringrush sample emerging from ground.

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2022 Weed Control Reports

Weed science research at WSU

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Episode transcription:


Drew Lyon: Hello, welcome to the WSU Wheat Beat podcast. I’m your host, Drew Lyon, and I want to thank you for joining me as we explore the world of small grains production and research at Washington State University. In each episode, I speak with researchers from WSU and the USDA-ARS to provide you with insights into the latest research on wheat and barley production.

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My guest today is Marija Savic. Marija is a graduate student at Washington State University. Marija is from Serbia. She is a graduate research assistant in the Weed Science Program, working on smooth scouringrush management with me and Dr. Mark Thorne. Prior to coming to WSU, she got her bachelor’s degree at the University of Belgrade in plant protection sciences, and she completed an internship at the University of Nebraska-Lincoln.

She’ll be graduating in May, and I’ve asked her to join us today to tell us more about her research. Hello, Marija.

Marija Savic: Hello, Drew.

Drew Lyon: So smooth scouring rush–why should people be interested in this? Why are weed scientists interested in it? And why should Washington wheat growers be interested in it?

Marija Savic: First of all, thank you for having me today.

So smooth scouringrush is a creeping, non-flowering perennial, and it has a high silica content in stems. And up until recently or relatively recently, it wasn’t known as a troublesome weed. It started entering and spreading on production fields only after we introduced no till or reduced tillage systems, which here in [the] Pacific Northwest started somewhere in [the] late seventies, in [the] past century.

And this is one old but new weeds–how I like to sometimes say–because this plant group or plant genus is one of the oldest living vascular plants on Earth. And one fun fact is that even dinosaurs back in time used it as a very nutritious and primary food source.

Anyway, smooth scouringrush, as I said, became a weed in row crops relatively recently. And to answer finally your question, why we are interested in this plant is because no till growers in eastern Washington have expressed their concerns regarding this weed and its presence in their fields or in their row crops, and it is definitely spreading and invading our production fields. Usually you will see it in dense patches present in fields. And another part that makes it problematic is that it’s really hard to control it with herbicides, which we know that our main tool for no till growers for weed control, and we don’t have many options. So that is why we are doing research on this weed.

Drew Lyon: Okay. Yes, I don’t think it was a weed on my radar when I first showed up here in 2012, until a grower showed me some fields north of Reardon where it was just spreading clear across fields, which really surprised me. So been trying to work on it for a while and brought you in to do some research on it. I wonder if you can explain a little bit about what your research is and what some of the findings from your research have been.

Marija Savic: So to give a little bit of background, we know before that one herbicide, chlorsulfuron, controls smooth scouringrush. It is very effective, but we have some problems with introducing it into crop rotations here in eastern Washington, especially in higher rainfall zones. So we started, in my program before I came here, started exploring glyphosate and looking into higher rates.

So what I found or what we found in my research is that high rates of glyphosate can be really effective for smooth scouringrush control, but only when it’s used with a surfactant, which is pretty interesting. And when I say high rates, I just want to mention that those are upper label rates for single application–I’m talking here about three quarts of herbicide per acre. We used RT3 in this case, so three quarts of RT3.

And in my research we tested several surfactants and organosilicone ones were the most consistently effective on smooth scouringrush, and especially when we applied these treatments during the day. This is because we hypothesize or we had implications that organosilicones can help in increasing herbicide efficacy by increasing the stomatal flooding or herbicide uptake through stomata. So that is why the applications worked really well.

And a second thing that I would like to point out is that organosilicone surfactants do increase droplet spreading on the surface. And we found that if weather is not optimal or if it’s hot and dry, this herbicide efficacy can be compromised because we have that higher evaporation from the plant surface. So that is one secondary point from my research that I want to point out is that, yes, organosilicones work during the day and increase glyphosate efficacy. But that is not always the case, it can happen to fail and that we don’t see results from the herbicide.

Drew Lyon: Okay. You mentioned “work during the day.” Your study looked at both day and night application. Can you explain why you looked at applications during the day versus at night?

Marija Savic: So I mentioned that yes, the applications work, but we had some implications from the literature that when organosilicones are included in tank mix, they increase the stomatal flooding and stomata are actually open during the day in most of the vascular plants because stomata to be open they require light or energy to be produced so the certain protein pumps can work and actually open stomata.

Drew Lyon: And at night they’re closed, right? So, what was the thinking, during the day if they’re open and stomatal flooding is occurring, we would get greater uptake; whereas at night maybe we wouldn’t because we thought the stomata might be closed. Is that what you found?

Marija Savic: Yes, exactly. But as I said at some cases that was not the case. The hypothesis didn’t hold and we suspect that [the] environment played a role and also the droughts that occurred in one of the years that we applied these treatments.

Drew Lyon: Okay. So if the droplets evaporating quickly under hot, dry conditions, then probably shouldn’t be applying it under hot, dry conditions. So later in the season, maybe earlier in the season would be better application time than later?

Marija Savic: Yes, our earlier applications were really holding up and following what we assumed, what we expected. But our later applications just turned out a little bit different. So, we would not recommend those late applications or applying herbicide when it’s really hot and dry.

Drew Lyon: Okay. You mentioned chlorsulfuron has been found to be very effective, but the growers, particularly in the high rainfall area, were a little concerned with it–I assume, because of the carryover issues. It pretty much limits you to small grain crops for several years after its use. There’s also a concern with the ALS-inhibiting herbicides, which chlorsulfuron is, with herbicide resistance.

So in a species like smooth scouringrush that primarily spreads by asexual reproduction, there’s the rhizome spreading. Should we be worried about herbicide resistance development in smooth scouringrush?

Marija Savic: So as I said, smooth scouringrush is a creeping and non-flowering perennial. When I say creeping, I mean it reproduces vegetatively via rhizomes. And when I say nonflowering, it means that it doesn’t reproduce from seed, but it reproduces from spores. That is asexual reproduction and that is different from most of the living plants that we know today.

However, sexual reproduction is quite rare to happen in environments such as eastern Washington because it’s pretty dry, and the conditions that smooth scouringrush requires for sexual reproduction are really specific. It’s, as I said, very rare to happen. So herbicide resistance, we don’t have some major concerns about herbicide resistant development because it’s primarily reproduces by rhizomes, so all the plants and all the offspring is actually just clonal. And the genetic variation or the exchange of genetic material is not happening because we don’t have sexual reproduction.

Drew Lyon: Okay. So if that’s a growers concern, they should maybe think about it, but it’s not a major concern with this particular species, but it might be for other species in the field.

Marija Savic: Yes, that is another thing why we started looking into other herbicides, because it’s not the only–smooth scouringrush, for example—[weed] there in fields. And it is known that we have a widespread ALS-resistance in some weeds here.

Drew Lyon: Okay. I found smooth scouringrush to be fascinating. It seems like for every question we answer, we come up with two or three more questions. So what are some of the unanswered questions that remain after your research?

Marija Savic: So as I said in the beginning, it is very interesting that this plant exists on Earth for such a long time, but on the other side, we don’t know about it so much, especially in a weed science world and from the weed management or its management in row crops.

So from my research, we for example, hypothesize that stomata are open during the day, but there was just assumption that we made. So this is something that we need to look into and to look into environmental factors that will affect this cycle of stomata opening or closing, and to see if our assumptions were completely correct, or that in some instances, for example, this cycle can switch. Or if we were looking into this hot and dry weather and the drought conditions, we can see that we can maybe find out something else that we didn’t know so far.

And also, as I said before, there are only few herbicides known to control this plant and this plant group is naturally tolerant to many herbicides–that is pretty interesting. And stems of a smooth scouringrush are rich in silica, and we know that the silica can increase over time. And this is something that can also affect our herbicide efficacy and can be potentially something that we should look into more.

Drew Lyon: Okay. Yeah. I think, like you said, at two of your sites the day applications worked quite well and kind of followed the thoughts we had on stomatal flooding. But that third site where the daytime didn’t work as well–actually the nighttime worked better–and that might suggest that under certain conditions the stomata are open at night, which would be a bit unusual, I think. But you had some indications perhaps from some greenhouse work, didn’t you, that perhaps the stomata in drought stressed plants don’t close fully at night? Is that what you found?

Marija Savic: Stomatal conductance definitely decreases with droughts and higher temperatures, but it seems that this difference between day and night disappears, let’s say, and the difference in stomatal conductance between day and night, it’s non-existent sometimes. So yes, this cycle can definitely change. This is something that we just preliminarily looked into. We didn’t design a full experiment. It was more just for our knowledge. But as you said, we usually come up with two or three extra questions after answering one.

Drew Lyon: Yes, very good. Yes. Your research helped to answer a few questions. I think we have a little better understanding when and with what Roundup or glyphosate should be applied to this plant. But then we created some other questions in the process. So, if our listeners want to learn a little bit more about smooth scouringrush and its control, is there a place they can go to find that information.

Marija Savic: Yes, they can go and check Washington State University Wheat and Small Grains webpage. Annually there are reports published from all the research that is ongoing and from the previous research, and they can find out more about smooth scouringrush. We have all of these [studies] that I’ve been talking [about] published there.

Drew Lyon: Alright, Marija, I’ve enjoyed working with these last two years. I’m wishing you luck in whatever you do next. And thanks for sharing your research information with our listeners today.

Marija Savic: Thank you. It was a pleasure.

Drew Lyon:


Thanks for joining us and listening to the WSU Wheat Beat podcast. If you like what you hear don’t forget to subscribe and leave a review on iTunes or your favorite podcast app. If you have questions or topics you’d like to hear on future episodes, please email me at drew.lyon — that’s — ( You can find us online at and on Facebook and Twitter @WSUSmallGrains. The WSU Wheat Beat podcast is a production of CAHNRS Communications and the College of Agricultural, Human, and Natural Resource Sciences at Washington State University.

I’m Drew Lyon, we’ll see you next time.


The views, thoughts, and opinions expressed by guests of this podcast are their own and does not imply Washington State University’s endorsement.