When it comes to vegetables, steaming is better than boiling, say nutritionists (both canine and human). While some can be eaten raw, dogs get maximum benefit if their veggies are cooked and cut down to size. Vegetables should make up about 25 to 30 percent of your dog's diet.
Boiling is a moist cooking method in which food is added to a large amount of water heated to 212 degrees Fahrenheit. This cooks the food, but leaches out many valuable nutrients from it and kills some of them deader than Julius Caesar. Vitamin C is destroyed by heat (and light, and air), but healthy dogs make their own internally, so that's not often a problem. Some of the nutrients can be recovered by saving the water vegetables were boiled in and using it as a base for soup or stock -- this is the origin of the South's famous "pot likker" -- or for cooking rice to be included in the dog's dinner.
Steaming is a moist cooking method in which food is placed in a container over, but not touching, boiling water or in a pressurized vessel with a small amount of water (a pressure cooker). Steam is actually hotter than boiling water and cooks the food more quickly, but does not dissolve the nutrients. It also tenderizes the vegetables so that a dog can digest them more easily.
The smaller and more tender the pieces of vegetable, the more benefit a dog derives from them. Chop or shred the veggies before or after cooking. You can also puree them in a blender or food processor for maximum effect. If you're not a cook, you can give your dog baby or junior vegetables from a jar -- just be sure to read the label to see if these have been seasoned with onion powder, which can be toxic to dogs.
The Good, The Not-So-Good and The Dangerous
Most dogs can eat and profit from squash, pumpkin, zucchini, carrots, peas, beets, yams, green beans and potatoes (both white and sweet) when boiled or steamed and chopped or pureed. Be weary of the cruciferous vegetables, such as broccoli, cauliflower and cabbage, along with bell peppers, as they may cause your dog to perfume the general area with homemade methane from his digestive tract. Leafy greens, such as spinach, really need to be pureed for dogs. Dogs can eat cooked or raw ripe tomatoes, but never green ones, and should be kept away from tomato plants, as the leaves, stems and unripe fruit can poison their nervous systems. All members of the onion family (leeks, shallots, chives and garlic) in any form (boiled, steamed, fried or powdered) can be toxic to dogs, so omit them and anything that contains them.