Turf Notes: Poa trivialis

Above is a look at roughstalk bluegrass (Poa trivialis) which makes up a large portion of our fairways and tees at Sugar Creek.  It is considered a weed and was not planted intentionally.  The "trivialis" in the plant's Latin name indicates that it is very common, not trivial.  This member of the bluegrass genus is an invasive species from Europe that can be found growing wild in most areas of North America.  It spreads aggressively by seed and stolons (stems that run along the ground) during the spring and fall.  It does not, however, like our summers.

In many previous posts, I have discussed the difficulties of dealing with Poa annua, or annual bluegrass, on golf course turf.  I occasionally mentioned Poa trivialis (or roughstalk bluegrass) along with its annual, bunch-type cousin.  Because Poa annua affects putting greens it often gets more press than P. trivialis.  You may hear superintendents just say "Poa" as shorthand for both weeds, but there are major differences in the their life cycle and growth habit.  After working with both for quite some time, I am finding that P. trivialis is a far greater problem for our fairways and tees than annual bluegrass.

The problem with Poa

 Poa trivialis is commonly called roughstalk bluegrass -- but not due to any roughness of its shoot tissues.  The rough "stalks" are the stolons that creep along the ground sending tender shoots upward and tiny roots downward.  Roughstalk bluegrass actually has soft, fine blades that are tightly spaced when mowed regularly.  It can easily be mistaken for creeping bentgrass without close inspection.  Below is the 4th tee looking good in June.  It is mostly Poa trivialis.

4th tee - Mostly Poa trivialis

Poa trivialis looks good and plays well most of the year.  It would make a decent turfgrass -- except for one BIG problem.  It completely shuts down during hot weather.  The grounds crew successfully kept it going through very dry conditions by handwatering and syringing through the day, but the heat wave in early July with 3 consecutive 100 degree days caused large areas of Poa trivialis to decline from heat stress.

July 5th and 6th -- the 2nd and 3rd 100 degree days -- were brutal on cool-season turfgrass.  Simply put, grass species in our area are not adapted for optimal growth during the summer.  They are adapted for the cooler seasons of spring and fall.  Kentucky bluegrass, creeping bentgrass, and ryegrass can often take one extremely hot day under the right conditions, but several in a row is a different story.  Turf managers attempt to coax an often unwilling plant through the rough patch of summer when stress and play are at their highest.  It doesn't always work. 

In the photo below taken the morning of July 6th you can see patches of turf in the process of going dormant due to heat stress.  You can also see some cart tire tracks turning purplish-brown.  That afternoon was the breaking point for a lot of Poa trivialis and Poa annua:

7th fairway morning of July 6th - Patches going dormant

Why is it here? 

Poa trivialis is everywhere.  Small amounts can come with regular Kentucky bluegrass and ryegrass seed -- especially in the past.  It is possible that it was in the original seed used on the course, but it is just as likely that seed and stolons were already here before the golf course.  These areas have been seeded and sodded many times over the years just to be over-taken by Poa species.

We have the perfect conditions for Poa.  Poa trivialis thrives during spring and fall in compacted and poorly drained soils.  Many of our fairways are made up of clay mixed with rock, asphalt, brick, and concrete from the old rock dump on the site.  Here is a soil profile from a random spot on this fairway:

Clay mixed with bits of asphalt, brick, and gravel

This picture was taken 12 hours after a heavy rain.  The soil problems are compounded by poor drainage.  Unfortunately, the design of the course routes a large amount of water through the middle of the fairway.  Some drains have been added over the years in these wet spots but they are not adequate to move lots of water quickly.  The result is that puddles stay in the fairway for days after a thunderstorm.  The 7th fairway is especially bad.

7 fairway after heavy rain

The puddles perfectly line up with the areas of dormant Poa trivialis because the poor drainage encouraged this grass in the first place and discouraged desirable species of bentgrass or bluegrass.  Under the right conditions these puddles also heat up and scald the turf.

The turf in these areas is not technically dead.  It is just dormant.  When it dries out, you can peal up the turf and see new shoots forming on the stolons and some seedling germinating.

These seedlings may be Poa annua or trivialis, but they are seedlings and not connected to stolons:

So far, we have used slicing tines on the fairways and tees to encourage some gas exchange.  We will also be slit seeding and aerifying during August to speed recovery and try to establish hardier species of grass.

Can you get rid of it? 

Poa trivialis is almost diabolically difficult to control because it can lay dormant for long periods.  In 2010 and 2011, we slit seeded these areas with bentgrass.  The seed germinated in August and began establishing only to be overwhelmed by Poa annua and Poa trivialis in September and October.  Without removing the existing turf and starting fresh, it is difficult to impossible to permanently establish a desirable turfgrass. 

There are two ways to get rid of it: Multiple applications of non-selective herbicide (Round Up) or physical removal (total renovation).  There is research on a selective herbicide (Velocity) that can weaken it in a mixed stand but would not be effective in this situation.  The underlying cause of Poa also needs to be dealt with: poor drainage and poor soil.  

Because it is not possible to eradicate without a total renovation, we try to keep as much alive as possible during the summer months.  In years like 2009, it was possible to keep it all alive.  The past two summers it has not been possible due to extremes in heat and moisture. 

How can we speed up recovery?

We do several things to speed up recovery and limit further damage:

• Maintain adequate moisture when it does not rain
• Limit cart traffic to affected areas
• Use lighter mowers or walk mowers on affected areas
• Aerify multiple times
• Topdress affected areas
• Slit seed
• Sod worst areas if necessary
• Maintain proper fertility

In 2011, August was fairly mild with only 3 days over 90.  An August like that would be ideal for recovery.  Here is some slit seeding from 2011:

Slit seeding on #1 fairway in 2011

Most of these seedlings were overwhelmed by Poa the following spring, but the slit seeding did significantly speed up recovery last fall.  We are going to keep experimenting with techniques to establish more desirable turfgrasses in the middle of Poa trivialis this year.  Stay tuned . . .