New Guinea impatiens (Impatiens hawkeri group) that are shy to bloom may be spending too much time in the sun. Hardy in U.S. Department of Agriculture plant hardiness zones 10 through 12, New Guinea impatiens flower prolifically when given the right light levels, fertilizer and growing conditions. Plants grow in clumps 1 1/2 to 4 feet tall and 1 1/2 to 3 feet wide, and feature blooms in shades of purple, violet, red, orange pink or white. Non-flowering plants may need shade or a change of season before blooming resumes.
A partially shady spot encourages flowering in New Guinea impatiens. Morning sun and afternoon shade provide the best light levels for these plants and promote prolific blooming. In areas of the garden that receive more than eight hours of sunlight a day, blooming is reduced. The few flowers that do appear are smaller than usual, and the plants' leaves may be scorched and damaged.
Heat and light also affect blooming in New Guinea impatiens in warm zones of the United States, where the plants grow year 'round. In USDA zones 10 through 12, these plants may stop blooming over summer, even when they're growing in the shade. In cooler USDA zones, New Guinea impatiens should begin blooming soon after planting outdoors after the final local average frost date.
New Guinea impatiens flower best with moderate fertilization. Overfertilizing is a common cause of nonflowering in plants. Excessive fertilizer produces plenty of lush, green growth but discourages blooms. In container plants, too much fertilizer can also harm plant roots.
Fertilize newly planted New Guinea impatiens with a ready-to-use, slow-release, granular fertilizer. Sprinkle 12-4-8 fertilizer granules over the ground at a rate of 4 tablespoons per 4 square feet, and water the granules to begin the fertilization process. To fertilize New Guinea impatiens in containers, dissolve 1/2 teaspoon of a soluble 24-8-16 fertilizer in 1 gallon of water, and apply the fertilizer solution every two weeks. Containers for New Guinea impatiens must have drainage holes.
Evenly moist, organically rich soil provides good growing conditions for New Guinea impatiens and boosts blooming. Water New Guinea impatiens when the soil surface is dry, applying enough water to moisten the soil to the depth of the plants' root balls. Water plants in containers until water flows through the drainage holes.
Increase the organic content of the soil by spreading organic mulches, which also help prevent water from evaporating from the soil. Over time, worms and other soil organisms break the mulch down and mix it into the soil. Spread a 3-inch layer of garden compost, leaf mold, shredded bark or other organic mulch around in-ground New Guinea impatiens plants.
When New Guinea impatiens plants grow tall and leggy, pruning creates denser, more compact shapes and more blooms. New Guinea impatiens produce blooms toward the tips of their stems. By mid-summer, plants can develop just few long stems and few flowers. Pruning these long stems encourages more stems and more blooms.
Sterilize pruning shears by wiping the blades with a cloth soaked in rubbing alcohol, and prune all New Guinea impatiens stems at one-quarter of their length, above a leaf or leaf node. Leaf nodes are regularly spaced, raised areas of stem. Sterilize the pruning shears again when you've finished.