Rhizomes are natural adaptations that allow ferns to reproduce effectively. As rhizomes grow horizontally, usually just above or below the soil surface, the tips of the structures develop new fronds that grow into adult ferns. The rhizomes, which have small roots that extend into the soil, may be soft and pliable or nearly as hard as wood. In addition to aiding reproduction, rhizomes transport water and nutrients to the plant.
Before dividing fern rhizomes, cultivate the soil in the planting spot to a depth of 18 to 24 inches and then dig at least 4 inches of organic matter or compost into the top 6 inches of soil. Organic matter promotes drainage and helps maintain the proper pH level. Most ferns perform best in acidic soil with a pH of 5.5 to 6.5. To determine your soil pH, use a soil test kit available at most garden centers. If your soil is alkaline, acidify the soil by adding a product containing sulfur or ammonium sulfate at the rate indicated by the soil test results and product's manufacturer.
Ferns are best divided as soon as new growth emerges in early spring. To divide fern rhizomes, first determine the growing style of the rhizomes, which varies depending on the type of fern. Division is easiest if the rhizome has multiple branches. For this type of fern, cut off a segment with at least one small frond, because this is the point where frond clusters develop. If the rhizomes grow in a straight line and don't branch, division is slightly more involved. Use a sharp knife to slice partway through the rhizome. Injuring the rhizome stimulates new growth, and a small frond will appear at the cut spot later in the season. When the frond is apparent, cut the remainder of the way through the rhizome and remove the severed rhizome.
To plant fern rhizomes, dig a shallow hole with a trowel and plant the rhizomes with the crown -- the point where the frond meets the rhizome -- just above the surface of the soil. Water as needed to keep the soil evenly moist throughout the first year, but don't overwater or allow the soil to become waterlogged or soggy. A 2- to 3- inch layer of mulch such as pine needles or chopped leaves spread around the plant keeps the soil cool and moist.
Divide only domestic ferns grown in the home garden. Ferns growing in nature are important parts of the ecosystem. Don't disturb ferns growing wild, as colonies of the plants are often destroyed by excessive and inappropriate division. Certain types of ferns are on threatened and endangered lists in some states. Removing these ferns is illegal.