Photo of the Week – June 23, 2017

This is a good year for sensitive briar (Mimosa quadrivalvus) in the Platte River Prairies.  Sensitive briar is a spiny perennial legume that sprawls across the ground in dry prairies and has leaves that fold up when touched or blown about by the wind.  It’s an odd plant, and one that is hard to miss when it’s blooming because each plant has numerous pink flower balls scattered across an area about the size of a large bathtub.

A sensitive briar plant blooming on a sandy hill this year in the Platte River Prairies.

Sensitive briar is named for the sensitivity of its leaves to touch, but it must also be sensitive to moisture conditions or something else.  As I was preparing to write this, I scanned through my field notes because I remembered sensitive briar being extra abundant a few years ago as well.  I was right; I’d noted an extraordinary number of plants back in 2011.  In fact, I wrote a blog post about it!  I don’t have any better explanation this year than I did back in 2011 for why this perennial plant seems to ebb and flow so much in abundance.

This katydid nymph was one of many insects enjoying the abundance (and easily accessible pollen) of sensitive briar this year.

Maybe the ebb and flow is mainly about flowering, and many of our sensitive briar plants just don’t bloom every year.  The only thing giving me pause is an experience we once had with a large plot of sensitive briar plants in our seed production garden.  One year, we thought all the plants had died because they didn’t even come out of the ground that spring.  We wondered if they’d been accidentally sprayed or something the previous year.  Fortunately, we didn’t till the plot up and start over because the next year it was filled with mature sensitive briar plants again!  It’s not that I’m looking for more data collection projects to work on, but it would sure be interesting to mark some plants in our prairies and track them over 10 years or so to see what’s going on…

Just one more fun prairie mystery to solve!

Sensitive Briar Explosion

It’s been a cool and wet year spring and summer here.  Seed harvest is about 2 weeks behind schedule because plants are flowering later than in recent years, and we’re seeing unusual abundances (high and low) of some plant species.  I assume most of what we’re seeing is related to the weather, though it’s always hard to tell for sure.

One species I’ve been noticing over the last week or two is sensitive briar (aka cat’s claw or Mimosa quadrivalvus).  It actually doesn’t seem to be blooming too far off schedule, but what I’ve noticed is that it seems to be popping up everywhere!  In our Platte River Prairies, sensitive briar is a species that is found in scattered patches.  A small prairie might have one or two patches of it, usually in fairly specific habitat niches – on the slopes of alluvial sand ridges in our lowland tallgrass prairies, or on tops of hills in our upland sand prairie.  It also does well on some sandy roadsides.  It’s never widespread, but often forms pretty large patches (living room size or so) where it does grow.  On some of the steeper loess hills east of us I’ve seen much larger populations on steep hillsides.

The "koosh ball" flower of sensitive briar is pretty hard to miss - especially because it usually appears with many others on plants that can cover areas of 6 feet in diameter or more. It's a distinctive-looking plant. Besides the showy flowers, sensitive briar has abundant spines on its stems and compound leaves that fold up when touched.

I’m not really noticing more populations in our remnant prairies this year, though the existing populations are certainly full of flowers.  Our restored (reconstructed) prairies are where I’m really seeing a lot more plants.  I’ve seen sensitive briar for the first time ever in a couple of sites, and have seen many more plants than ever before in others.  These are obviously not first-year plants, because they’re large sprawling individuals with lots of flowers, so they’ve clearly been around for a while – unnoticed by me.  (To be fair to me, most of the prairies where I’m now seeing sensitive briar also have large populations of Illinois bundleflower, which looks very similar to sensitive briar when neither species is blooming.) 

Because the restored prairies are all of different ages, I’m pretty sure that I’m not seeing plants that are simply getting to the age where they begin flowering – I think something happened that made a lot of them flower for the first time (or made more of them flower simultaneously).  The question is whether this year’s weather is responsible, or whether something that happened last year or previously that set this up ahead of time!  Don’t you love prairie mysteries?

One thing is for sure, my kids will be happy to see the additional sensitive briar plants.  Whenever we’re hiking past a sensitive briar plant, we always have to stop and pet it so the kids can watch the leaves fold up.  It is a neat trick, but I’m beginning to wish I’d never shown it to them…  Besides having to stop at each sensitive briar plant, we also have to stop and check any plant that has any resemblance to sensitive briar (bundleflower, leadplant, Canada milkvetch, groundplum, etc.) to make sure its leaves don’t fold up too!

As an interesting aside, the spines on sensitive briar seem like they’d be well-designed to prevent aggressive grazing by large herbivores.  They’re aligned along the long sprawling branches so that they point back toward the center of the plant.  This means that if an animal picked up the stem and tried to strip the leaves by pulling away from the center of the plant, they’d be pulling right toward the teeth of those spines.  Under light stocking rates, sensitive briar is rarely grazed by cattle, but under heavier stocking rates they will eat it.  I’ve only gotten to watch the process once, and sure enough, the cow I watched picked up the stem and simply stripped all the leaves right off – against the spines – and didn’t seem to suffer any ill effects at all.  Those are tough lips!