Roughly 12,000 species of ferns exist in the world, and fern varieties range from common houseplants such as the Boston fern (Nephrolepis exaltata 'Bostoniensis') to native shade species such as the northern maidenhair fern (Adiantum pedatum). Ferns vary greatly in their appearance, hardiness and care needs, although all can be propagated in the same simple way.
Climate and Hardiness
Hardiness varies wildly among the different kinds of ferns. Boston fern, a hot-house specimen, does best with a nighttime temperature of 65 degrees Fahrenheit and a daytime temperature not above 95 F; it survives outdoors all year in only U.S. Department of Agriculture plant hardiness zones 9 through 11. Northern maidenhair fern grows as a perennial plant outdoors in USDA zones 3 through 8 while the Japanese shield fern (Dryopteris erythrosora) is perennial in USDA zones 5 through 8. The large, showy ostrich fern (Matteuccia struthiopteris) is perennial in USDA zones 3 through 7, making it a common choice for cold-climate woodland gardens.
Ferns reproduce naturally from spores, but spore culture is not a practical propagation method for most home gardeners. Division -- cutting a large plant into several smaller plants -- provides the best and most effective means of multiplying established ferns at home. Start divisions in spring just after new growth begins to emerge at the ferns' bases but before the fiddleheads -- the new fronds, or leaves -- unfurl.
Dividing ferns takes very few pieces of equipment and little planning, although certain key elements must be prepared ahead of time. A sharp, sanitary cutting tool is vital so you don't transfer disease to the fern. Soak the blade of a knife in a solution that is one-half rubbing alcohol and one-half water for five minutes; then rinse it with water and wipe it dry. Do the same with plastic pots -- one pot for each new fern, or fern division, you want to grow; ensure each pot has plenty of drainage holes in its base, or else your fern divisions may rot.
The right growing medium is crucial to growing healthy, lush ferns. A basic soil mixture composed of equal portions of milled peat, coarse sand and potting soil works well. One teaspoon of garden lime per each 1 quart of soil mixture will help create a soil mixture with the right pH, or acidity, level. Species such as the northern maidenhair fern grow best in a soil mixture that is one-half peat, one-quarter potting soil and one-quarter that is a combination of sand, rotted manure and charcoal chips, with the addition of 1 tablespoon of lime for each 1 gallon of the soil mixture.
Despite their delicate appearance, ferns are incredibly resilient when it comes to division. The main threat to their well-being is drying out. Preparing your tools and watering a fern before it is divided will help prevent serious trauma to the plant, as will dividing the fern on a cool, cloudy day.
Water the fern the day before you plan to divide it; water softens the soil and hydrates the plant's roots. The next morning, fill the bottom one-half of each plastic pot with the prepared soil mixture. If the fern you want to divide is in a pot, then gently remove it from the pot. If the fern grows in the ground outdoors, then measure 2 to 4 inches outward all the way around the fern's outer edges, and dig roughly 2 inches deep at that point all around the plant, using a sharp garden spade. Pry the fern's root ball loose from the soil, and lift the fern from the ground.
Successful divisions have an equal share of roots and shoots, or fronds. So gather the fern's fronds into two to four equal portions, and tie each portion gently with string or twist ties to hold it in place. The size of each division is largely a matter of personal preference. Large divisions, however, may need to be divided again in just a few years.
Cut straight down through the root ball with your sanitized knife, removing a section of roots in equal proportion to the corresponding section of fronds. If the roots are compacted or very bound together, separate them gently with your fingers, and spread them over the surface of the soil mixture in the pot. Cover the pot's roots with additional soil mixture, using only enough so the roots are at the same soil depth at which they grew previously, and water the soil well. Repeat the procedure for each division you want to take from the fern plant, using one division per pot.
The key to rooting fern divisions is to provide the right conditions. Moisture, humidity and shelter from strong winds all play a role. Position potted divisions of hardy ferns outdoors in a wind-sheltered location with bright, indirect sunlight, and put potted divisions tender, hot-house ferns indoors near a north-facing window or a lightly shaded east-facing window. Keep their soil mixture evenly moist at all times, but don't let it become saturated.
Increase humidity by misting the divisions or by placing each pot on a tray containing pebbles and water nearly to the pebble tops. Outdoor fern divisions also benefit from being positioned near a spigot or other continually moist location. Roots typically grow in just a few weeks, at which point the ferns will perk up.