Annual Blue Grass
I have made some editorial changes to the text below which are printed in brown text. (additions in February 2019)
Poa annua or annual blue grass is a very common problem in most places in the US and also in rest of the world. It is very prevalent in the Piedmont North Carolina where we are and is difficult to eradicate. The reason for that is that the seeds normally germinate in the fall, and if you are planting or reseeding a fescue lawn a pre-emergent weed killer will make it difficult to re-seed your fescue. Most pre-emergent weed killlers have a residual that last for 90 days or longer. Using most pre-emergent weed killlers will harm if not kill fescue. If you have a healthy strong lawn going into fall, and you keep your grass mowed about 4 inches during the late summer and early fall, you usually can keep the Poa annua from being a problem.
Another reason the Poa is hard to control is that it seed heads never get very high and in general grass mowers don’t cut low enough to cut off the seed head. Seeds remain dormant for many years, so if the soil is bare, and especially compacted, you will have Poa annua.
Most pre-emergents will keep Poa annua from germinating as said above, but there is one that appears to be pretty effective, and does not destroy fescue grass. That is ProGrass. Prograss will kill the very young Poa annua, but once the grass matures Prograss does not do much to the mature plant. Prograss is applied in the early fall when Poa annua is starting to germinate, and then again once more mid winter. Another interesting thing is that you can sow or spot seed fescue immediately after treatment with Prograss. That is what the chemical label says, but that doesn’t make sense to me.
Besides Prograss, many lawn care companies use a material now with the tradename of Poa Constrictor. A season like 2019 which has been excessively wet with some heavy rains has washed away seed and fertilizer making it very difficult to get a good stand of grass for the fall and winter.
Poa annua thrives in moist cool weather especially in compacted soil that is near the edge of a sidewalk or driveway. Sidewalks and driveways, especially brick or pavers tend to heat up and retain that heat so the plants germinate earlier. (added feb 2019)
Below are a couple of photos.
The bright green grass and has taken over this planting strip. There are several bare spots and generally poor soil.
At this point one could use a chemical that would kill all the grass, thus destroying the seed producing poa. Then start over and reseed. I am not a fan of Roundup, but it is translocated through the plant and kills the grass roots pretty effectively.
I have seen Poa annua start to sprout in late August as I recall, and that was pretty surprising to me since I think the soil temperature needs to be less than 70 degrees.
By the time I was sure it was poa annua that I was seeing, it was pretty late to treat it with Prograss. Having said this, I think I will start a detailed page on Prograss and also Poa Constrictor.
The next photo is of a close up of Poa annua and shows the seed heads