Bluegrass, Annual (Poa annua) Small cool season grass is a particularly serious weed problem in closely mowed areas. It begins to emerge in late summer and early fall when night temperatures are in the 60’s and moisture is present. Annual bluegrass seeds continue to germinate through the fall, winter and spring making chemical control more difficult. Favored by moist soil conditions and cool temperatures. It has a strong competitive advantage over warm season grasses from fall through spring. Annual bluegrass is greatly reduced by taller mowing heights and limited use of water. Seed heads form in late fall and winter, but seed head development is greatest in the spring and early summer. Control with pre and post emergence herbicides like corn gluten meal.