Promoting Healthy Foliage
Provide good air circulation, adequate light and well-drained, moist soil. Don't crowd ferns, or place them too close to walls. Keep ferns evenly moist, which means not letting the soil get too dry or too wet. Water ferns when the soil surface feels dry, and water well. Give ferns an occasional shower, indoors or outdoors, because dusty fronds invite insect pests. Humid air also supports fern health. Use room humidifiers inside, or place individual potted ferns atop a layer of stones in a drain tray partially filled with water. Fertilize minimally in spring as new vegetation emerges; most people over-fertilize, which can cause fronds to turns brown, starting at the tips and working back. Use fish emulsion fertilizer at half strength, because ferns are particularly sensitive to the soluble salts in commercial fertilizers, or side-dress plants instead with well-rotted manure.
Removing Damaged Fronds
Fronds of ferns unroll in spring and then grow upward and outward as they mature. The entire frond is the general equivalent of a leaf; healthy fronds are necessary for photosynthesis, or the process of producing plant energy. Cut back seriously damaged fronds of herbaceous ferns when you notice them, removing them all the way to the plant base. Prune damaged fronds of evergreen ferns in late winter or early spring, as new growth begins to emerge. Heatons Nursery suggests cutting the entire fern off to the ground when battling a bad pest problem, allowing the plant to "reshoot."
Pruning Herbaceous Ferns
According to University of Missouri Extension, a general rule of pruning is plants that die back to the ground can be cut back in the fall. This is definitely true of deciduous or herbaceous ferns, those with tender fronds that die back when temperatures get cold. Cut deciduous ferns all the way back to the ground after a hard frost. Herbaceous ferns growing indoors as houseplants don't need to be cut back, but can benefit from an occasional haircut.
Pruning Evergreen Ferns
Never prune tree ferns, except to remove damaged vegetation. These slow-growing ferns are true trees, and will not regenerate fronds as other ferns do. Revitalize the foliage of evergreen or hardy ferns -- those that don't die back during winter -- in late winter at least every other year, suggests Collier's Nursery. Cut all fronds all the way back to the ground.