Mexican petunias (Ruellia brittoniana), also known as Mexican bluebell or ruellia, are herbaceous flowers valued for their colorful blossoms, dark green foliage and ease of cultivation in the landscape. The plants bloom during the summer and fall, producing multiple blooms in shades of blue, white and pink, depending on the variety. The flowers reach 1 to 2 inches in diameter and attract beneficial insects such as butterflies to the garden. Native to Mexico, Mexican petunias require relatively warm temperatures to thrive and perform as perennials in USDA hardiness zones 7 through 10. Elsewhere, gardeners treat the flowers as annuals.
Choose a planting site for Mexican petunias that consists of fertile, well-drained, moist soil and receives six to eight hours of bright sunlight throughout the day. Plant Mexican petunias during mid-spring, after the danger of frost has passed in your area.
Sow seeds a half-inch deep in the soil so the plant can readily emerge after germination. Space each seed 24 to 36 inches apart to allow room for the plant's mature spread. Water lightly after sowing to initiate new growth. Germination will occur in three to four days.
Water the plants once per week during the first month of growth to help establish the root system. Reduce watering frequency thereafter to once every 10 days, or whenever the top 2 inches of soil are dry to the touch. Soak the soil to a depth of at least 2 inches at each application to provide adequate moisture for the roots.
Feed Mexican petunias once per month with a balanced 10-10-10 NPK fertilizer. Check the instructions provided by the manufacturer for proper application and dosage information. Water lightly after fertilizing to dissolve the nutrients for easier absorption by the roots.
Cut the plants back to the ground during early winter, and then cover with a 2- to 3-inch layer of mulch to protect the roots from freezing. Remove the mulch during late winter, just before the plants begin actively growing again in early spring.