Sweet Clover: Ugly but Harmless? Or Dangerous Invasive Species?

Why is sweet clover the target of aggressive control by some prairie managers and largely ignored by others?  After talking to a number of people across the Midwest and Great Plains, I think there are a couple of things happening.  First, the usually biennial sweet clover can be very abundant and showy in the years it blooms, but is harder to find in other years.  I think some prairie managers see those big flushes and mistake abundance for aggressiveness.  However, I also think that some soil/precipitation/latitude(?) conditions may lead to real negative impacts from sweet clover on plant diversity.

One of the lessons that’s been strongly reinforced for me this summer is that it can be difficult to extrapolate successful prairie management/restoration strategies from one region to another.  Just during the last several months, I’ve visited prairie managers in Nebraska, Indiana, Missouri, and South Dakota and I’ve seen tremendous variation between (and even within) those states in terms of which species are invasive and which are not.  It’s dangerous to assume that just because a species like sweet clover isn’t causing problems in one prairie, it won’t cause problems in another.  I hope we’ll eventually learn enough to accurately predict when to worry and when not to, but in the meantime, it behooves prairie managers to carefully evaluate species at their own sites.

Yellow sweet clover. This exotic species is still planted in some wildlife and ground cover grassland plantings because of its purported wildlife value and cheap seed. However, it appears to be invasive in some places and/or situations.

I’ve been working with prairies along Nebraska’s Platte River for nearly 20 years now, and my observations have led me to conclude that sweet clover is more of a big ugly plant than a true invasive species in those prairies.  Years of data collection on my plant communities support those observations.  That annual monitoring work entails listing the plant species I find in each of about 100 1m2 plots across a prairie.  Those plots are stratified across the prairie so the site is evenly sampled.  Once I have those plotwise species lists, I calculate the floristic quality  (FQI) inside each plot, a calculation that takes into account both the number of species present and the average “conservatism” value of those species.  I can then look at changes in mean floristic quality over time to help me see how the plant community changes over time.  I monitor a few prairies annually, and others on a periodic basis.

Those data show the same thing I’ve seen observationally – sweet clover changes in abundance from year to year (though not as much as it appears visually), but the species doesn’t increase in abundance over the long term and doesn’t appear to negatively impact floristic quality.  Below are graphs from three sites that show both sweet clover frequency (% of plots occupied by sweet clover) and mean floristic quality.  Two of those sites were annually grazed during the data collection period, and the other was only grazed once – toward the end of the sampling period.  Cattle grazing almost certainly helps control sweet clover because it is one of their favorite plants to eat, but I don’t think sweet clover is causing me problems where I don’t graze either.

What my data don’t show is the flush of tall blooming plants that happens every other year or so.  I’m just counting whether at least once sweet clover plant is present in each of my small plots – not how big it is, or whether or not it’s blooming.  Nevertheless, sweet clover frequency changes from year to year but doesn’t appear to correlate at all with changes in mean floristic quality.

Nine of years of annual data from a 1995 prairie restoration seeding. The site has been under patch-burn grazing during each of the nine years of data collection. Sweet clover is never abundant at this site, but has also not increased over time, even through years of drought and heavy grazing. Error bars for floristic quality indicate 95% confidence intervals.

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Sweet clover was present in between 22% and 42% of 1m plots in this restored crop field, planted in 2002. Though the sweet clover frequency varied from year to year, the mean floristic quality of the plant community increased between 2004 and 2008 before leveling off after that - apparently independent of sweet clover. Sweet clover data from 2008 was eliminated from this graph because I later questioned whether I'd confused black medick and sweet clover in some plots. This site was not grazed except in 2009.

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These data are from a remnant mesic prairie under patch-burn grazing. Instead of being stratified across the entire prairie, these data are from a single patch that was burned in 2008, and that same patch was sampled again in 2009 and 2010. It was grazed very hard in 2008. In 2009, no new patch was burned, so it was grazed hard again, but then fenced out in early June. In 2010, the site was not fenced out, but received only light grazing while cattle focused on another portion of the site that was burned. These data are from only about 30 plots per year.

I feel pretty good about ignoring sweet clover and focusing on more invasive species on our prairies.  Both my observations and data support that strategy.  However, as I said earlier, just because the species doesn’t appear to be problematic for me doesn’t mean it isn’t an invasive species in other prairies.  It’d be great if we could compare data similar to what I’m presenting here from a number of sites to see if sweet clover is acting differently in different places.  Without data, it’s hard to know whether or not people are just interpreting the “invasiveness” of sweet clover in different ways.  For now, my answer to the question,  “Is sweet clover really invasive?” is still the same…

Maybe.

Grandpa’s “Accidental” Prairie Restoration Project

In the late 1950’s, my grandfather bought a quarter section of farmland just southwest of Stockham, Nebraska.  At the time, all but about 26 acres of that 160 acre land parcel was in row crops.  The unfarmed areas (the steepest slopes and wettest draws) were a combination of native prairie and other “waste” ground.  From what I understand, Grandpa’s intent was to buy the land and put much of the cropland back to native grass pasture, keeping only the flattest areas and most productive soils to farm.  Regardless of whether or not that was his initial intent, he did indeed do that restoration project in 1962 – using the Soil Conservation Service’s Great Plains Program to provide cost-share for the seeding and construction of a livestock dam.  Approximately 87 acres of cropland was planted with a mixture of native grasses, including primarily big bluestem, indiangrass, switchgrass, little bluestem, side oats grama, and western wheatgrass – with a little blue grama thrown in too.  (We still have the receipts from the seed company.)

Starting in 1964, Grandpa started grazing the new pasture – including the unfarmed portions – and it’s been grazed annually ever since.  The pasture grazing was designed to be both profitable and sustainable, and as far as I can tell it has been.  There is no evidence of chronic overgrazing, and a number of “grazing sensitive” plant species are still abundant, especially in the old remnant areas.  My grandpa died in 1990, and Grandma died earlier this year.  After Grandma’s death, most of the other family farm land was sold, but after some long discussions, we kept the ¼ section in the family.  My wife and I now own a 2/3 interest and my aunt and uncle own the other 1/3.  Although I was helping Grandma manage the pasture for more than a decade before her death, actually owning the land makes me see the pasture in a much different way.  Before we shelled out our hard-earned money to buy it, the pasture was an interesting place to go cut cedar trees and walk around, and I tried to help Grandma set up the grazing leases to ensure that the land stayed in good condition while bringing in a reasonable income.  Now, as an owner, that lease income means a whole lot more, as does the current and future condition of the grassland.  Decisions about how to balance stocking rate and income with plant community impacts are a little more real!

I bring all this up as background discussion for the real subject of this post.  Though I’m sure it wasn’t Grandpa’s intent when he seeded the site back to grassland in 1962, he actually did some pretty great prairie conservation work.  He took a series of very small isolated remnant prairie islands and filled the space between them with grassland habitat.  It’s hard to know how many plant and insect species populations have larger and more viable populations now because of his work.  Certainly the site has improved habitat conditions for the grasshopper sparrows, western meadowlarks, and other grassland birds that are nesting there.

A portion of remnant (unplowed) prairie on our farm. It has retained a good mixed-grass plant community, with fairly abundant leadplant, stiff sunflower, prairie clovers, and other characteristic prairie plants. Shown here are Missouri goldenrod, white sage, stiff goldenrod, and many others.

While any grassland is better than no grassland, the re-seeded areas of the prairie are still distinct in appearance and composition from the unplowed areas.  Many of the prairie remnants contain fairly abundant populations of conservative forb species, including leadplant, stiff sunflower, prairie violet, prairie clovers, and many others.  A few of those species have moved into the re-seeded areas, but mostly at low abundances.  The re-seeded areas are dominated by grasses, but also have an abundance of many common forb species such as goldenrods, white sage, ironweed, hoary vervain, yellow prairie coneflower, dotted gayfeather, yarrow, and others – along with strong populations of sweet clover.

This photo shows some of the re-seeded prairie. Many native forbs have moved into the stand of grass that Grandpa planted, but others are still rare or missing.

I only recently found some hand-drawn maps showing the exact locations of the unplowed areas, and was able to cross-check those with old aerial photos from before and after the 1962 seeding.  Now that I know those locations more exactly, I’ll be able to start making even better comparisons between the remnant and re-seeded areas.  I started that process this last weekend, taking an inventory of plant species in the re-seeded areas (I found 65 species).  I’m sure I’ll add to that list over time, but that’s not too bad, considering only 6-7 grasses were planted there initially.  I don’t know yet how many plant species are in the remnant portions – I’m still working on that.

This aerial photo from 1956 shows our 1/4 section just prior to Grandpa buying it. The darkest areas are the unfarmed portions. You can see how small and isolated many of them were. Only the steepest and wettest areas avoided the plow.

In some ways, it’s amazing to see the diversity of plants in those previously farmed areas.  If I took a botanist to the site without divulging its history, I’m pretty they’d have no idea it had once been farmed.  At the same time, while there is good plant diversity at the site, it’s interesting to see how few conservative plant species have made their way into the previously farmed areas.  I’ve seen a few individuals of leadplant, a few patches of purple and white prairie clover, some areas of purple coneflower, and a few stiff sunflower colonies.  Prairie violets have begun creeping from the remnants into the re-seeded areas too – but in 50 years, they’ve only made it about 20 or 30 yards.

All of this points out the importance of protecting and managing remnant prairies to avoid losing those conservative plant species.  Once they’re gone, it’s not realistic to expect them to just come waltzing back in from nearby sites.  During the last 10 years or so, I’ve been overseeding portions of the re-seeded prairie with locally harvested seed.  As is typical, the results of that have been fairly muted, but I’m hoping my work gets those plants to establish a little faster than they otherwise would have…

This false sunflower is one of a handful of species that is showing up here and there across the old farmed portions of the prairie as a result of our overseeding efforts. It's a slow process, but one that will, I hope, pay long-term dividends.

Of course, if Grandpa’s restoration project was being done today, and the main goal was really to ecologically reconnect those small prairie islands, the cropland around the prairie remnants would be seeded with a high-diversity mixture of prairie plant species.  That would help ensure that the seeded area facilitated a number of ecological needs, including the availability of host plants for a variety of insects and genetic flow between plant species.  In 1962, no one in Nebraska was even thinking about anything like that, and Grandpa’s goal was (I think) simply to take a piece of land that was being overused and make it into productive agricultural land.  I’m pretty sure he’d never heard of a grasshopper sparrow.  Regardless of his initial goal, there is now a 108 acre prairie in southern Hamilton County, Nebraska – and that’s a rare and valuable commodity.  The nearest prairie to ours is at least several miles away, across many acres of cropland.

I don’t know exactly know how to measure the ecological value of our prairie, but I’m sure proud to own it.