Petunias are among the most popular flowering annuals, known for their long blooming period and wide variety of colors and forms. Petunias are excellent candidates for garden beds and borders, providing season-long color from spring until the first hard frost. They are a favored choice for gardeners looking to fill large spaces with vibrant, continuous blooms, serving as color masses, borders, or seasonal groundcover depending on the variety selected.
Site Selection and Soil Preparation
Successful growth begins with selecting the right location, as petunias require abundant sunlight to thrive. Choose a site that receives a minimum of six hours of direct sun exposure daily; insufficient light will cause the plants to become spindly and produce significantly fewer flowers.
The soil structure is equally important for root health. Petunias require well-draining soil to prevent waterlogging, which can quickly lead to root rot. Ideal conditions involve a light, fertile soil structure with a slightly acidic to neutral pH, generally ranging between 6.0 and 7.0.
If your garden soil is heavy clay, it needs substantial amendment to ensure proper drainage. Incorporate generous amounts of organic matter, such as compost or well-rotted manure, into the top 8 to 10 inches of the bed. This addition improves soil aeration and porosity, allowing the root system to establish itself without sitting in excess moisture.
Transplanting Petunias Into the Ground
The best time to transplant petunias into the garden is in the spring, once the danger of the last frost has passed and the soil has warmed to at least 60°F. Nursery-raised plants must be “hardened off” first by gradually exposing them to outdoor conditions over about a week to minimize transplant shock. This involves placing them in a sheltered outdoor area during the day and bringing them back inside at night.
When planting, dig a hole slightly wider than the plant’s root ball, ensuring the top of the root ball sits level with the surrounding soil surface. Spacing varies by type: mounding varieties like Multiflora and Grandiflora need about 12 inches apart, while aggressive spreading varieties should be given 18 inches or more to accommodate their extensive growth habit.
Carefully remove the petunia from its container and gently loosen any tightly coiled roots before placing it in the hole. Backfill the hole with soil, firming it gently around the base to eliminate air pockets. Immediately after planting, give the petunias a deep and thorough watering to settle the soil around the roots and aid establishment.
Ongoing Care for Ground-Planted Petunias
Once established in the ground, petunias require consistent maintenance to ensure continuous blooming throughout the summer months. Watering frequency depends on the weather, but a thorough, deep watering is needed when the top inch of soil feels dry to the touch, especially during periods of high heat or drought. Avoid frequent, shallow watering, as this encourages the development of a weak, shallow root system that is less tolerant of stress.
Petunias are heavy feeders because they produce flowers constantly, requiring a steady supply of nutrients. Supplementing the initial slow-release fertilizer mixed into the soil at planting is beneficial. A monthly application of a balanced, water-soluble fertilizer formulated for blooming plants will support robust growth and flower production. Spreading varieties may even benefit from a bi-weekly feeding regimen due to their rapid development.
Deadheading and Pruning
Removing spent blooms, known as deadheading, is necessary for older petunia varieties to encourage new flowers and prevent the plant from diverting energy into seed production. Pinch off the entire seed pod at the base of the faded flower. While many modern hybrid petunias are self-cleaning, meaning they drop their spent flowers naturally, pruning back leggy stems by about one-third in mid-summer will also promote bushier growth and a fresh flush of blooms.