How to Overwinter Boston Ferns: 9 Pro Tips
A big beast of a Boston fern is too handsome to lose to winter frosts. Whatever its size, the lushly fronded tropicals make good houseplants in cold months. Explore ways to overwinter the favorite ferns with gardening expert Katherine Rowe.
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Boston ferns carried over from year to year become old friends as they create a presence in their hanging baskets, pots, and pedestals. Their rotund crowns expand with each season’s nurturing.
The ornamental fountains of fronds have arching stems and a lush, fresh look. They soften garden corners and brighten the interiorscape. In the right growing situations, they make handsome houseplants. They thrive outdoors in warm conditions and round out the houseplant jungle in cold months.
Boston ferns withstand cool temperatures but won’t endure hard frosts. Hardy in USDA zones 9-11, leave them outside in the ground or pots in warm, frost-free climates. Welcome them into a sheltered space in colder zones.
About Boston Ferns
Nephrolepis exaltata is a naturally occurring selection of tropical sword fern. Sword ferns grow natively in humid forests from northern South America to Mexico, Florida, and the West Indies. In the wild, N. exaltata can reach a stately seven feet tall. ‘Bostoniensis’ is a more graceful, compact variety that makes growing it in temperate climes easier. There are many similar cultivars in circulation, with dwarf habits and frilly or ruffled leaves.
The beloved tropicals tolerate cool temperatures, but when they drop into the low 40s (~4°C), it’s time to consider winterizing. Ideal nighttime temperatures are near 65°F (18°C), with warm daytime temps below 95°F (35°C).
Trim Selectively
A few minor maintenance tasks prepare Boston ferns to overwinter. The first is to do a light clean-up. Cut out yellow, brown, or dead stems. Trim long, wayward fronds to five inches. Clip lower fronds by several inches, but leave the central crown intact. Take care not to remove more than one-third of the vegetation to avoid stress. This is a light trim rather than severe pruning.
Give the fronds a good shake to displace any shedding leaves, a characteristic of the favorite fronded tropicals. Hose them off with a good soaking to rid them of dust and any pests before they move to shelter.
Bring Them Indoors
The most successful way to overwinter Boston ferns in cool climates is to protect them with shelter. They thrive as houseplants in the right situations.
Tropicals that move from outside to cozy conditions, though, make a big transition in the seasonal shift as growth slows. Meeting their proper light, temperature, humidity, and watering requirements are the key factors.
Let Them Rest in an Unheated Space
An enclosed, unheated space is another excellent option for protecting sensitive specimens. Bring ferns in hanging baskets or pots into a garage, basement, shed, barn, or cold frame. Keep the indoor temperature range around 55°F (13°C) or higher for best overwintering in a dormant state.
If you didn’t catch the fern before frost, its roots may still be alive. Cut off frost-damaged fronds (even if it’s all of them) and move the container to a protected area. You may see new growth emerge.
Moderate Temperatures
Drastic temperature fluctuations are common as we move plants this time of year, but the resulting stress causes leaf drop. Ferns benefit from cool indoor temperatures between 55°F (13°C) and 75°F (24°C). Place pots in a cool room or near a window for lower temperatures.
To avoid sudden chill or heat and decreasing humidity, keep them away from drafts. Heating and air vents, open windows, space heaters, and fireplaces cause sweeping fluctuations. Take care not to have them touch a chilly windowpane or sit too close to a fireplace or heater.
The Right Light
These tropicals require medium levels of indirect light to flourish. Near a bright window and out of direct sunlight is a good situation. Direct sun, especially afternoon rays, can scorch stems and leaves.
East-facing windows work well, or slightly away from south or west-facing positions to miss intense rays. On short winter days, west and south-facing windows offer the most light. Year-round, these spots may be too intense or require a filtered curtain. Tropical ferns don’t do well in low light, and northerly spots may not offer ample exposure.
Reduce Watering
The overwintering Boston ferns need much less water this time of year to retain even moisture. As growth naturally slows, roots aren’t absorbing moisture or nutrients as readily. Indoors and in unheated spaces, water only when the soil feels dry. It can be hard to gauge moisture when fronds offer wall-to-wall coverage, but the pot will feel lighter in addition to dry surface soil.
Watering frequency lessens now to about every four to six weeks. Check soil moisture every few weeks so roots don’t dry out completely. Water fluctuations cause stress. Overwatering during inactive growth results in roots sitting for prolonged periods in soggy situations. Fungal problems like root rot can develop.
Use tepid or lukewarm water at each session to promote humidity and avoid shocking the roots. Water deeply until it flows from drainage holes. Don’t let pots sit in wet trays for extended periods.
Stop Fertilizing
In the wild, tropical ferns receive nitrogen through raindrops and other nutrients from decaying material like dropped leaves from the tree canopy. They don’t often need fertilizer, but if you are in the habit, hold off as temperatures drop. Stop applications in August or September to avoid stimulating new, leafy growth as plants enter the less active growing season.
Pick up fertilizing again in early spring as new growth emerges. Opt for an organic fertilizer like fish emulsion. Or, use a houseplant formula at ½ strength every four to six weeks in the warmer months.
Overwintering Boston ferns may show paler green foliage than their normal lush, rich look. This is part of its inactive resting state.
Increase Humidity
Bostons thrive in the natural humidity of their forest habitats. Indoors, they’re prone to dropping leaves as they transition from outside. If you notice continued excessive drying and leaf drop, low humidity may be the cause.
Our cozy heated homes become drying as moisture decreases. To raise the humidity level around the fern, place its pot on a dish of gravel or pebbles and water. The pebbles serve to keep the container above the water (we don’t want to water from the base here, just to add immediate surrounding moisture).
Some gardeners lightly mist the fronds once or twice a week. They don’t need to drip (which can cause foliar diseases); just a light spritz adds humidity to the vicinity. Humidifiers can help, too. Still, the best moisture control is efficient irrigation.
Ready for Spring
As the fronds spend the cold season indoors, they may accumulate dust or pests (possible scale and mealybugs) and need a good rinse. Run them under lukewarm water in a sink, or dip them upside down in a bath.
To move Bostons out in the spring, do so gradually to get them acclimated to life back outside. Move them out to a partially shaded spot for several hours a day, returning them inside in the evenings for up to a week. Once outside for the season, be mindful of lingering chilly nights that accompany warming days. Bring them in, or use a frost cloth or thermal blanket to cover them on cool (not freezing) nights.
