Petunias are versatile annuals that do well in both warm and cooler weather, making them an excellent choice for spring, summer and fall gardens. Petunias tolerate most types of soil, as long as it is well drained and doesn't leave their feet soggy. Trimming back petunias is a fairly simple process that ensures a beautiful plant and a plethora of blooms, making for bright, colorful borders or adding a splash of color anywhere in the garden.
Trim back petunias when they are about six inches tall to ensure lush plants and an abundance of blooms. If purchased plants have already reached this height, they can be trimmed back at planting.
Trim back to about half of their height from mid to late summer to encourage stronger, more compact growth on leggy plants. ("Annuals and Perennials," p.106)
Cut back about half of the growth to four or five inches when bringing spindly petunias indoors. Indoors, provide necessary extra light by growing them under fluorescent lighting. When cut stems have developed new growth, cut back the other half in the same manner. This will help produce an abundance of blooms and eliminate long, leggy growth. (Dietz, p. 678)