Match the marigold varieties with the size of vegetable garden. If the majority of the plants are small, such as carrots and radishes, then plant French marigolds (Tagetes patula), which reach 6 to 24 inches tall. If the vegetables include tall plants including sweet peas and tomatoes, then plant tall American marigolds (Tagetes erecta), which grow 3 to 5 feet tall. Hot dry sites benefit from signet marigolds (Tagetes tenuifolia), which tolerate these conditions.