Dealing With a Pervasive Invasive – Kentucky Bluegrass in Prairies

Many of the prairies we manage have pretty degraded plant communities, characterized by low plant diversity and dominance by a few grass species – including the invasive Kentucky bluegrass (Poa pratensis).  Our primary objective for these prairies is to increase plant diversity, which, in turn, bolsters ecological resilience and improves habitat quality for a wide range of prairie species.  Because bluegrass is so pervasive in our prairies, we’ve had to modify our objectives and strategies from those we use to address most other invasive species.

Kentucky bluegrass can stifle plant diversity by crowding out other plants. In many prairies degraded by years of overgrazing and/or broadcast herbicide use, bluegrass is now the dominant plant species and few other plants can compete with it.

When attacking invasive plant species, a common strategy is to contain, and (hopefully) shrink, patches of invasive plants in order to protect plant diversity in non-invaded areas.  In the case of Kentucky bluegrass, however, we have to take a different approach because the species already spans the entire prairie.  Kentucky bluegrass acts like a thick blanket of interwoven stems, roots, and rhizomes – smothering most other plant species beneath it.  Because our goal is to increase plant diversity, we want to make that blanket thin and porous enough that a wide variety of other plant species can grow up through it.   

An illustration of a floristically diverse prairie (left) with several large patches of an invasive plant.  When dealing with many invasive species, we can focus on reducing the size of infestations in order to restore a more diverse plant community.

   

Kentucky bluegrass acts as a thick blanket that smothers most other plant species.  In a case like this, we need to manage the prairie in a way that increases the mesh size of that blanket and allows a more diverse plant community to poke through.

Our primary strategy for suppressing Kentucky bluegrass is the periodic application of prescribed fire and grazing.  We can weaken bluegrass by burning prairies when bluegrass is just starting to flower and/or by grazing prairies harder in the spring than in the summer.  We mix those treatments with rest periods within a patch-burn grazing regime.  The result has been a steady increase in plant diversity in most of our degraded prairies.

If we were fighting a different invasive species, we might expect that if plant diversity was increasing, the amount of territory occupied by the invasive species would be decreasing.  With Kentucky bluegrass, however, our bluegrass blanket is getting thinner, but still covers the whole prairie – something that shows up clearly in data I’ve been collecting over the last decade.  Through the use of nested sampling plots of 1m2, 1/10m2, and 1/100m2, I’ve been tracking plant diversity and floristic quality through time, along with changes in the frequency of various plant species (the percentage of plots in which they occur).  Over the last 8-10 years, as plant diversity within 1m2 plots has increased, the frequency of Kentucky bluegrass has stayed about the same.   Even at smaller plot sizes, which are more sensitive to changes in the frequency of very abundant species, bluegrass is still in nearly every plot.

We also have a number of restored (reconstructed) prairies in and around our remnant prairies.  Within restored prairies, plant diversity is in pretty good shape, but Kentucky bluegrass is rapidly invading.  In one particular prairie, Kentucky bluegrass is now in almost 90% of 1m2 plots and more than 50% of 1/100m2 plots.  That sounds bad, but as bluegrass becomes more abundant, plant diversity – and the frequency of other prairie plant species – is actually holding steady.  There are a few small areas in which bluegrass appears to be forming near monocultures, but for the most part, it looks like bluegrass is just filling in around the other plants instead of actually displacing them.  My guess is that some soil conditions provide such ideal growing conditions for bluegrass, it’s going to be king of those areas no matter what we do.  Elsewhere, however, I think our management is preventing it from becoming dominant.

Here’s the take home lesson for me:  When trying to manage for plant diversity in the face of a pervasive species such as Kentucky bluegrass, it’s more important to track plant diversity than to worry about how much territory bluegrass occupies. 

Just consider the data from our prairies… if I concentrated only on how much Kentucky bluegrass is in our prairies, it would look like we’re failing miserably in our management attempts.  We’ve got just as much bluegrass as we ever did in our degraded remnant (unplowed) prairies, and we’re quickly losing ground in some of our restored prairies.  However, my data also shows that plant diversity is increasing in remnant prairies and holding steady in restored prairies.  Since the ultimate goal is to have a diverse plant community, that’s success! 

Click here to see a PDF showing some of the data I mentioned in this post.

Photo of the Week – April 21, 2011

Spring is the time for early wildflowers, cool rain showers, and northward-migrating birds.  It’s also the time during which smooth brome and other invasive cool-season grasses get their annual jump on the competition.

Suppressing invasive grasses and allowing native grasses and forbs to remain competitive in the plant community is one of the most important jobs for prairie managers in our area.  In some cases, well-timed herbicide treatments can be successful at knocking back those invasives, but it is usually very difficult to find a window of time when the temperature is warm enough for the herbicide to be effective but when most native plants are safely in dormancy.  More commonly, the best option is to suppress the growth of invasive grasses and decrease their competitive ability for that season.

 
Cow grazing smooth brome in a recently-burned restored prairie. Repeated defoliation throughout the spring season will greatly weaken this brome, allowing other plants to encroach upon its "territory".

We’ve found that grazing is particularly effective at suppressing cool-season grasses, because the grasses are repeatedly defoliated during their growth period, greatly reducing their vigor.  The strategy works particularly well for smooth brome because, at this time of the year, brome is by far the favorite food of cattle in our prairies.  They’ll eat other invasive grasses too (I watched them working on Kentucky bluegrass this morning, in fact) but if smooth brome is available, they’ll hit that first.

Sometimes we use short-term intensive grazing so that the cattle graze the brome hard, but then are removed from the pasture at about the time warm-season grasses are starting to emerge.  That works well to suppress cool-season grasses, but often we end up simply trading dominant cool-season grasses for dominant warm-season grasses – without helping forbs much.  This is because the warm-season grasses tend to take advantage of the weakened invasive grasses better than forbs do.

In the case of the above photo, we’re using a variation on patch-burn grazing, in which only a portion of the prairie is burned so that cattle will concentrate their grazing is focused within that patch.  In this version of patch-burn grazing, cattle will be present all season, but we will reduce the stocking rate in late May.  The idea is to get high grazing pressure on smooth brome during the spring, but also to leave enough cattle in the pasture during the summer so they will selectively graze the warm-season native grasses.  With a light summer stocking rate, the cattle eat almost exclusively grasses, leaving wildflowers to grow with less competition from both invasive cool-season grasses and native warm-season grasses.  Outside the burned patch, very little grazing takes place at all, especially after the stocking rate is reduced.  Next year, a different patch will be burned so we don’t repeatedly stress the same plants each year.

This photo is a good demonstration of the attraction of both recently-burned patches and smooth brome to cattle – even when the brome is only about 2 inches tall!  This cow is working over a patch of brome in the restored prairie, even though it also has access to the unburned parts of the same pasture (seen in the far distant background of the photo), which includes some degraded native prairie with lots of smooth brome and other grasses in the 5-8″ height range.