One cup of cooked cauliflower contains about 2.6 grams of sugar, while the same amount of broccoli has about 2.2 grams. Leafy green vegetables generally contain less than 0.5 grams of sugar per cup. The amount of sugar in vegetables varies but is usually quite small.
Vegetables such as mushrooms, tomatoes and radishes, as well as cruciferous vegetables such as cauliflower and broccoli, all contain less than 5 grams of sugar per cup. The vegetables that are lowest in sugar are leafy greens such as lettuce, spinach, Swiss chard and kale. Cucumber, green onions and potatoes are almost as low. These vegetables each contain roughly 1 gram of sugar per serving. Asparagus, squash and green snap beans all contain around 2 grams of sugar per serving.
Starchier vegetables are often sweeter. One 85-gram serving of peeled baby carrots contains 4 grams of sugar. A large ear of corn on the cob contains about 5 grams of sugar. Some of the sweetest vegetables are sweet potatoes, with 7 grams of sugar per serving, and onions with 9 grams of sugar.
These amounts are for raw and plain cooked vegetables. Certain preparation methods such as pickling can significantly increase the amount of sugar in a vegetable-based dish.