Growing tropical ferns indoors
Nephrolepis exaltata 'Bostoniensis'
Most ferns thrive in filtered light or shady sites outdoors, but the tropical ferns we use as houseplants are poor candidates for low-light locations. Golden pothos (Epipremnum aureum), heart-leaf philodendron (Philodendron scandens oxycardium), snake plant (Sansevieria trifasciata), and Chinese evergreen (Aglaonema commutatum) are much better candidates for locations near north windows or other poorly lit places in your home.
Tropical ferns actually grow best indoors in "medium" light such as what you'd find in an east-facing window or a few feet from a west or south-facing window.
During our short days from November through February, ferns can be placed directly in a west or south window with no ill effects. But as days lengthen and sunlight becomes more intense, you run the risk of burning delicate foliage in such bright places, unless the light in those windows is filtered by large trees nearby, or by sheer shades or curtains, inside.
Without adequate light, no fern will prosper indoors, but there's more to growing ferns than providing enough light.
You'll find that new ferns come with care tags suggesting they need to be kept evenly moist. That means the soil should never be allowed to get very dry; it doesn't mean it should always stay wet.
It's important to water ferns thoroughly whenever you water. Don't just dribble a little water on top of the soil from time to time. Use room temperature or lukewarm water; cold water can damage tropical roots. If at all possible, avoid using softened water. Its repeated use will result in an accumulation of salts in the soil, which eventually injures the plant's roots.
Water until excess moisture begins to drip through the container's drain holes, then spill out whatever remains in the tray or saucer after a few minutes. Wait to water again until the soil surface begins to feel dry to the touch. It's a mistake to water before that; soggy soil encourages root rots.
Ferns are usually potted in highly organic soil that's porous, yet moisture-retentive. When it's time to transplant them into larger containers, choose potting soil that contains a large percentage of peat moss.
Place ferns far enough from walls and other plants to insure good air circulation. Ferns look particularly handsome displayed on pedestals or in wicker ferneries. They're also commonly grown in hanging baskets, but you must be careful not to display them too near the ceiling. The higher they are, the hotter and drier the air, especially during the heating season. This may cause the tips of your fern's fronds to turn brown and die.
Ferns are known for their high humidity needs. Some people still mist their ferns to increase humidity, but it's not very effective. Unfortunately, misting also increases the likelihood of foliar leaf spot diseases; it's better to rely on room humidifiers.
Because it's difficult to improve humidity significantly indoors, it's usually better to concentrate on proper watering to eliminate moisture stress.
Typically ferns have modest fertilizer needs; they can be damaged more easily than most houseplants if you overdo it. Over-fertilizing will result in browning and drying that begins at the tips, then works its way back into the rest of the fronds. A lack of nutrients results in foliage that pales and loses its vibrant green color.
Fish emulsion seems to work well, though other fertilizers meant specially for houseplants are fine, too, provided they're mixed ½-strength and applied sparingly. Fertilize only when the plants are actively putting on new growth, or if the foliage appears a paler green than normal.
Keep them clean
It's important to keep fern foliage clean. Dusty leaves may provide a haven for mites or insects. The dust also filters sunlight so less reaches the foliage. You can wash ferns with a gentle spray of lukewarm water from the sink, or swish them upside-down in a sink or laundry tub of lukewarm water to which you've added a few drops of mild dishwashing liquid.
- Boston fern, Nephrolepis exaltata 'Bostoniensis' is an old-fashioned fern with long, arching fronds. A dozen or more named "sports" or mutations of the original Boston fern have all but taken its place by now. Many boast foliage that's more ruffly or finely divided than the original. Dallas ferns are particularly notable for their compact stature and their ability to survive at lower light levels than Boston ferns and most of their descendants.
- Rabbit's foot fern, sometimes called Squirrel's foot fern, Davallia fejeensis, is known for its tan "furry" rhizomes that grow down over the pot like little legs. Short, lacy green fronds sprout from these rhizomes, creating wonderful contrasts in color and texture. Because of its growth habit, this fern must be suspending in some type of hanging apparatus.
- Mother fern, Asplenium bulbiferum, is aptly named for the little plantlets that form on its large, feathery fronds. To propagate Mother ferns, all you need do is pluck off the little "babies," plant them in moist, peaty potting soil, then enclose the container in a plastic bag. Keep them in bright, indirect light until they root. (Direct sunlight would overheat them while they're still bagged.)
- It might be difficult to find a Holly fern, Cyrtomium falcatum, but it's well worth the added effort. Unlike more delicate-appearing ferns, the stiff fronds on this plant are rather coarse and leathery, with an attractive, deep glossy green surface. And unlike most indoor ferns, this plant does best when temperatures are on the cool side and you allow the soil to dry between waterings.
- Asparagus ferns such as the Sprenger fern (sometimes sold in spring as "sprengeri"), Myers fern, and Plumosa fern are not really ferns at all. Named for their fern-like, feathery foliage, they're actually close relatives of the asparagus we eat, and as such, grow best in full sunlight. Commonly kept outdoors in summer, they often suffer from inadequate light in winter, elongating and dropping many of their tiny leaf-like structures.
- Two to avoid unless you're looking for a real challenge: Maidenhair fern, Adiantum species, and Staghorn fern, Platycerium bifurcatum. Each has specialized growing requirements, including high humidity levels more typical of greenhouses.