Pansies (Viola spp.) provide a wide array of flower colours for annual beds during the cool months of the year. In the southern U.S., where winters are chilly but mild, gardeners plant pansies in October and November. They persist and bloom across winter and reach a peak in March and April. Farther north, the much colder, subfreezing winters makes pansies seasonal flowers for planting in fall and again in early spring. Light trimming pansies, reducing size by no more than 50 per cent, helps them grow dense and again produce flowers.
Cool temperatures and full sunlight exposures keep pansy plants healthy and growing with a compact, bushy habit. The ideal temperature range for pansies falls in between 4.44 and 15.6 degrees Celsius. Extended subfreezing temperatures can dry out or kill leaves and flower buds, but the plant survives. Once daytime temperatures consistently reach higher than 80 degrees, pansies stop flowering and the stems tend to elongate, creating spindly, floppy plants. Insufficient sunlight also diminishes flowering and makes plants slender and sparse.
One method to trim pansies is called pinching, since it's most easily done by pinching your finger tips across the plant stems. To reinvigorate leggy plants, or overall tidy the appearance of the flower bed, pinch back stems to a lower leaf node. The leaf node is the point of attachment of a leaf to the main plant stem. Make the pinch cut 1/4 inch above a leaf node. The dormant buds in the node then sprout and yield more leaves and flowers later. Apply a mild solution of liquid fertiliser after pinching to encourage a quick flush of new leaves and flower buds.
Similar to pinching is a process known as deadheading, or removal of spent flowers. In massive flower beds with hundreds of pansies, deadheading isn't practical. However, in small clusters of plants or in containers, occasionally deadheading pansies improves the look of the planting and encourages further production of flowers. Once plants begin to set seed, hormones conducive to flower production wane. Pinch off old, faded flowers or seed heads by removing the long flower stem as low as possible to the main plant stem.
Pansies continually produce new leaves and flowers during the cool season, so mass trimming with a hedge trimmer or large shearing implement isn't a good idea. Arbitrarily cutting back pansies this way creates lots of debris that yellows and dries and needs removal to keep the flower bed attractive. Rather than pinching or trimming back pansies that get long and floppy in shady or warm weather conditions, it makes more sense to pull up the waning plants. Replant the area with more shade-tolerant flowers or annual flowers that prosper in the warmer weather.