The easiest giveaway is the color. Annua is a lighter green - stands out like a sore thumb. Further, you'll notice it in limited areas (unless you have an infestation). If you have a KBG lawn, expect it to seed relatively simultaneously. If it's Annua, it will come in patches AND have the light green color (hideous color). Annua typically tends to go to seed before KBG, though the weather can change this.

Annua also has looser clusters of seedheads. KBG are somewhat tight and may have a purplish tint.

The leaves are another giveaway, which you can easily see here:

http://pastureinfo.massey.ac.nz/grasspages/ugpoacompleaves.html

Then, there's the growing habit. Annua doesn't spread by rhizomes. It grows in bunches and will seed no matter how low you mow it (I guarantee you don't have a mower that can keep up with PA).

So to answer the question, there are a lot of factors that need to be accounted for. However, you're seeing these posts at peak Annua season, which is probably why you're asking...

Those answering are either seeing or addressing the same thing.

Easy test? Spot spray with Tenacity - if it bleaches it's Annua. You may be surprised what else turns white too! :D

Edit: Here's the post I was thinking of when posting. It really helps in distinguishing them: http://goo.gl/f7vjow

This post was edited by gsweater on Tue, May 27, 14 at 22:27

I've been crawling around comparing the leaves and peering thru a magnifier. But there seems to be some characteristic difference in the seedheads that people recognize instantly.

Poa annua is the grass that is producing seeds in most places now. Even in southern Ohio I am not seeing other grass species producing seed heads.

That does help

When does perennial bluegrass set seed?