During the winter the bermuda grass goes dormant/turns brown so if you want a green lawn during the winter, you have to plant it every fall. We decided to try it. Here's what you do:
1. mow your bermuda grass really short
2. put down winter grass seed (and hope it doesn't get eaten by birds)
3. cover it with dirt
It has taken some time, a lot of 'tending,' getting rid of some pesky gophers (knock on wood) and a really good week of rain, but we finally have a nice green lawn in our backyard (nevermind that the yard is totally devoid of other trees or shrubs...those will come when we own a house). There are still some bald spots and yellow patches, but I'm pretty proud of our work.
