What is annual rye grass?

Annual ryegrass is also known as lolium multiflorum. It is a very aggressive type of grass that seems to adapt to many environments very quickly. It is an awesome grass (glossy look) for lawn for a quick fill (only lasts for one season) and looks great in the winter, but is a terrible weed in cereal crops such as winter wheat.

Ryegrass should not be confused with rye. They are completely different species.