Slow cookers operate unattended at temperatures from 170 to 280 degrees Fahrenheit. Most recipes call for cooking pot roast, or any food, for eight to nine hours on low, or five to six hours on high. Although Michigan State University Extension Service recommends cutting pot roast into 1-inch pieces, the University of Vermont Extension says it is safe to cook a pot roast whole in a slow cooker. Actually, most recipes call for using a whole roast.
Defrost the pot roast before adding it to the crockpot; never put frozen meat in a slow cooker. In addition, when using a slow cooker, lifting the lid causes fluctuations in the temperature, so avoid taking the lid off while the food is cooking, says Michigan State University.
Some pot roast recipes call for using vegetables and others do not. Whether you cut the pot roast in large pieces or roast it whole, put vegetables on the bottom and along the sides as they cook faster than the roast. When using a roast whole, choose a cut that fits the pot, or trim it to fit. Put the seasonings on top of the meat and then pour the water, wine or other liquid over the top of the meat.