To solarize to kill grass and weeds, you need to mow it short, water the area, and apply the plastic. DO NOT MULCH ... expose the plastic to strong, hot sunlight to build up enough heat under it to kill the plants.

Compost will prevent the heat from penetrating, so it is not a good idea.

Black or clear plastic for this? Nancy

They say clear works better, passes more heat thru. The watering before as mentioned is important as is sealing the edges to hold in the heat, plus something to keep the plastic from blowing away.

A digital thermometer may be helpful if you want to compare black to clear plastic. The idea is not to block the sun but to get the soil hot enuff to kill all seeds in that area of the soil. Smaller sizes of plastic are much easier to handle and manage.