During the last month or so, I’ve had several people tell me how aggressive marestail (horseweed, aka Conyza canadensis) is, and how this is a particularly bad year for it. One person suggested marestail should be added to Nebraska’s noxious weed list. This week, Olivia and I drove from our Platte River Prairies to the Niobrara Valley Preserve – right through the center of our state – and I tried to document what is certainly a summer of abundance for marestail.
Here are a few things you should know about marestail right off the bat. First, it is native to Nebraska and most of North America. It acts as an annual plant in states to the east of us, but acts as a biennial here, usually germinating in the fall and blooming the following summer. In its native habitats (including grasslands), marestail is a colonizer of bare ground, filling spaces between plants left open by disturbances like grazing, trampling, animal burrowing, drought, or fire. Because marestail loves open soil conditions, it isn’t surprising that it has become a weed in crop fields. It has garnered special attention lately because it has a strong ability to become resistant to herbicides, including glyphosate, which it started showing resistance to way back in 2006.
In other words, marestail is a tough native plant that has always scraped out a living when and where it can. However, it’s not a plant that can push other plants around. Instead, it sits in the soil (as a seed) and waits for a time when surrounding plants are weakened and abundant light is hitting the soil. Then it pops out of the ground and tries to grow, bloom, and produce as many seeds as it can during its short window of opportunity. In any particular year, marestail can be found here and there in most Nebraska prairies, especially those in the western 2/3 of the state. However, it also seems periodically to respond to certain weather patterns and exhibit a flush of abundance across a larger region – as it is doing this year. Many short-lived plants do the same thing, each with its own individual preferences for weather patterns. Many Nebraskans might remember the huge sunflower party across the Sandhills back in 2013, for example, following the big drought of 2012.
Whether it’s sunflowers or marestail, huge regional flushes in abundance don’t last long. By 2014, annual sunflower numbers in the Sandhills had returned to normal – patches of yellow flowers here and there, around livestock tanks and fence corners, and wherever else there was open soil to grow in. Marestail will do the same thing in 2019. That pattern of boom and bust is not evidence of an invasive plant. Instead it characterizes a plant that is too weak to compete most of the time and has to take ultimate advantage of the few windows of opportunity it gets. When it is abundant, marestail isn’t stealing resources from other plants, it is taking resources that weren’t being used. I don’t know for sure what weather patterns led to rampant marestail germination last fall, but I’m sure this year’s abundant rains have played a big role in the survival of a large percentage of those seedlings.
When short-lived plants like marestail and sunflower (along with ragweed, gumweed, and many more) are in the middle of a short-term explosion in your prairie, you could choose to fight them. You could, for example, mow them off, trying to prevent them from making seed. However, that’s a lot of work, and the plants will do everything they can to regrow and still produce seed – it’s what they do, and they only get one year to do it. Even if you do keep them from going to seed, there are many thousands of seed already in the soil, ready to spawn the next generation of plants whenever they get the chance. You could also spray short-lived opportunistic plants with herbicide, but I wouldn’t recommend it. First, you’ll likely kill the surrounding plants (the ones that normally out-compete marestail and sunflower) and just trigger another explosion of opportunistic plants the f0llowing year. Second, with most short-lived plants, by the time they’re big enough that you notice them (especially by the time they’re flowering) herbicide treatments just make them produce seed more quickly, so are counterproductive.
The smartest choice is to just sit back and marvel at these periodic phenemona, knowing you’re watching a short-term and harmless event. Marestail, of course, doesn’t have the wide aesthetic appeal of sunflowers (though not everyone likes sunflowers either), but it has its own distinctive charm. I think it adds an attractive texture to the landscape, but I’ll admit I’m a little odd. Regardless of whether you find it attractive or not, it’s here, and it’ll be here whether you like it or not.
Fighting back against these periodic flushes of marestail and other opportunists is expensive and futile, and usually results in weakening the plant community that normally keeps them in check. Most importantly, remember that, at least in grasslands, marestail doesn’t steal resources from the plants you like, it just takes what they can’t use. What’s to dislike about that?