Using RoundUp is a bad idea but so is leaving the Bermuda grass. It is the worst weed you can have in a bed. Plastic, both solid and the so-called weed-blocking kind, is also a bad idea because it doesn't work well and fouls up the most important part of the soil - the surface just under the mulch. That's where the temperature is ideal, minerals are available, life is transitioning back and forth between the mulch and the soil, etc. The best method is to remove the top 1 and 1/2 to 2 inches of soil and toss it in the compost pile. Then apply the compost and other organic amendments. Do not till the area first - that drives pieces of the stems (rhizomes and stolons) down into the ground allowing it to come back forever as a horrible weed.
QUESTION: I have lots of weeds in my buffalo grass lawn. What kind of weed killer should I use? Should I water less during the winter? V.S., Colleyville
ANSWER: Cool-season weeds can be killed with a vinegar-based herbicide while the grass is dormant. Fatty-acid herbicides such as Scythe also can be used. In the summer, spot-spraying, molasses applications and hand weeding are the best weed-control methods. Buffalo grass does not need extra water during the winter.