Lack of Sunlight
Most flowering annuals, perennials and vegetable bedding plants require a minimum of six to eight hours of direct sunlight each day. When plants receive less than their basic requirement they will development increased top growth in an attempt to try and catch more sun.
This growth spurt occurs when the plant is already stressed and surviving on low nutrition stores and weakens the plant further still. This type of growth results in limp, paler, leggy growth with few leaves and blooms.
Aging & Seasonality
Annual bedding plants will become leggy and weak looking late in their regular growth season. If temperatures remain warm for longer than usual or if you live in a temperate or semi-tropical clime that does not have killing frosts, bedding plants will survive long beyond their season. Though they persist, they will look spindly and past their prime until they are shocked and renewed with fertilizer and pruning or their seasonal growth pattern resumes.
Need of Pruning & Fertilizer
Leggy, pale and weak bedding plants can often be revived with pruning and feeding. Cut back spindly plants to just one to three leaf nodes on each stem. Feed with a high nitrogen or balanced complete fertilizer formulation such as a 12-8-8 or a 10-10-10 and water in the fertilizer until the soil is drenched.