Poa constrictor #729393 - Ask Extension


Poa constrictor #729393

Asked October 07, 2020, 6:15 PM EDT

If I use Poa constrictor on my yard to prevent annual bluegrass or Poa grasses from growing And taking over my yard what is the best time of year and how many times can I use it without damaging my perennial ryegrass?

Multnomah County Oregon

Expert Response


PoaConstrictor (ethofumesate) can only legally be applied to lawns by a professional turfgrass applicator.   

The herbicide label gives applicators all the information they need to make the necessary applications including rates, spray intervals, and maximum number of applications (or total amount allowed per year).

To be effective, 3 applications in the fall are needed.  And it will need to be applied every year, without stopping.  However, there are resistant biotypes of Poa annua that will not be killed.  Those biotypes will flower and produce seed which will germinate.  Over time, these biotypes will invade more and more of your lawn.  In other words, by using PoaConstrictor, you are only delaying the inevitable.

One example that stands out is this:  In western Oregon and Washington, all golf courses older than 20 years have converted to almost all annual bluegrass (Poa annua), in spite of million dollar budgets to keep it out.

Another option is to apply a pre-emergent herbicide in the spring and fall.  Example products are Barricade, Dimension, or Pendulum.  There are granular formulations available for these products which allow you to use a drop spreader to apply.  Unfortunately, these products only prevent the Poa from germinating and will not kill mature plants.

If you still want to apply PoaConstrictor, hire a licensed pesticide applicator.

Brian McDonald Replied October 08, 2020, 1:52 PM EDT
Thank you for your response! 
The Question Asker Replied October 08, 2020, 9:15 PM EDT
Can you please tell me what height we should be cutting are perennial rye grass at? we live in Gresham Or.
and does the height vary from spring to fall?
The Question Asker Replied October 08, 2020, 9:32 PM EDT

There is a range of acceptable heights depending on your goals, your mowing frequency, and the smoothness of the soil surface.  The range for perennial ryegrass is from 1.5" to 2.5".  If you mow it lower, annual bluegrass (Poa) and other grasses will encroach and take over the lawn much faster.  If you mow it higher, the lawn with lose density (won't be as thick) and the leaves will thicken.  Higher mowing heights also create an environment for turf diseases in our wet Oregon climate.  The only exception to this is if your lawn is in shade, in which case, you should mow your lawn at 2.5" to maximize the leaf area to catch the diminished light.

In general, the lower and more frequent you mow your lawn within its tolerance range, the finer texture the leaves become and the denser your lawn.  So mowing your lawn at 1.5" will produce the best looking lawn but this requires a smooth surface and frequent mowing - 2x per week during the growing months assuming you are watering it regularly.  If the grass is too long and you try to mow at 1.5", your lawn will not look good and this scalping is not good for the lawn as you are removing too much leaf tissue at one time.  If you can't be consistent with your mowing, then you are better mowing your lawn at a higher height.  1.5" is for people who want the best lawn possible and are willing to spend more time mowing it.  Remember, your decision does not  have to be permanent.  If you start at 1.5" and you get tired of mowing it that much, raise the height to 2" and adjust your mowing frequency.

If you mow it at 2", you can mow it 1x per week for most of the growing season and 2x per week when it is growing fastest (April - May and possibly a couple times in late September into early October).  You may also have to mow it more frequently for a short time after fertilizing.  If your soils get soft in the winter, it's o.k. to raise your mowing height to 2.5" because your wheels are sinking in the ground a bit.  Use your judgement based on how the lawn looks after mowing.  Try a 2" height in a small area (1 mower pass about 5' long) out of sight and see how that looks.  If it looks like it is too low (removes too much grass and the grass is more yellowish), then raise the height to 2.5". 

Keep in mind that one of the biggest keys to having a nice perennial ryegrass lawn is to make 2 fertilizer applications in the fall.  Perennial ryegrass is susceptible to quite a few winter fungal diseases and fertilizer can make a huge difference in severity.  Apply 1 lb. of nitrogen per 1,000 sq. ft. using a slow release fertilizer.  Now is a good time to make the first application.  Make the 2nd application a month later. 

To calculate how much fertilizer to apply to get 1 lb. of nitrogen per 1,000 sq. ft. divide 1 by the nitrogen percentage in decimal form on the bag (the first number).  For example, if your fertilizer is a 25-3-10, divide 1 by 0.25, which equals 4.  So apply 4 lbs. of this fertilizer to 1,000 sq. ft. to get 1 lb. of nitrogen  per 1,000 sq. ft.  The reason this is important is because fertilizers come with a wide range of nitrogen percentages.  For example, organic fertilizers often have 6% nitrogen or less.  So to apply 1 lb. of a 6% nitrogen fertilizer, it would require 16.7 lbs. of fertilizer (1/.06) vs. only 4 lbs. in the first example. 

I hope this helps.  Good luck.
Brian McDonald Replied October 09, 2020, 12:28 PM EDT

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