When Is it Too Cold for Boston Ferns Outside?

Boston ferns (Nephrolepis exaltata "Bostoniensis") are effective ground-cover plants for shady areas in warm climates. Healthy Boston ferns can survive temperatures as low as 15 degrees Fahrenheit. They must be cared for properly, though, in order to keep them healthy and able to withstand such cold weather year after year.

Ferns
credit: DebraMcGuire/iStock/Getty Images
Boston ferns vary in height from 6 inches to 4 feet.

Winter Hardiness

Boston ferns are hardy in U.S. Department of Agriculture plant hardiness zones 8b through 11. Their fronds, the leafy stems above ground, are killed by the first hard frost of fall or winter, but the underground roots survive. When temperatures warm in spring, the fronds grow right back.

Water and Fertilizer

Water Boston ferns when the top of their soil begins to dry. Provide the water in morning so that the fronds will dry quickly, which helps prevent fungal infections. Morning watering also ensures the moisture will be present for the ferns during the heat of the day, when they need it most. If the Boston ferns begin to turn gray and grow more slowly than usual, then they are not getting enough water. Tip burn or dry, brown leaflets occur when the irrigation water is too salty. Rain can be collected in barrels for watering if the area's water contains too much salt.

Sprinkle slow-release fertilizer on the soil around the Boston ferns early in spring, as soon as they begin to grow. Use a granular, 12-4-8 fertilizer with a six- to nine-month duration. Apply ¼ pound of the fertilizer per 50 square feet of soil surface. Slow-release fertilizer gives plants small doses of nutrients throughout the growing season.The plants should use up the fertilizer by the first hard frost. Boston ferns should not receive fertilizer after that time.

Fungal Diseases

Pythium root rot can infect Boston ferns, especially if their soil is kept too wet. The soil should remain uniformly moist, not muddy or dripping wet. Boston ferns with Pythium root rot wilt, become stunted and turn gray or yellow. Their roots turn gray and mushy or wiry. Rhizoctonia aerial blight also may attack Boston ferns. Brown lesions develop all over the leaflets and at the center of the affected ferns. The center or crown of the fern is commonly wet with the disease. Lesion nematodes sometimes infect Boston ferns. Signs of a nematode infestation are the same as those of Pythium root rot.

These fungal diseases and lesion nematode infestations can be treated with a fungicide containing Thiophanate methyl. A soil drench and foliar spray should be used. Many commercial fungicides contain Thiophanate methyl. Read your fungicide's label carefully to ensure the product is safe for use on ferns. A common dilution rate for a 40 percent wettable powder soil drench is 1 ½ to 3 ounces of fungicide to 25 gallons of water to treat 100 square feet of soil. Pour the solution over the soil, and apply plain water to the area to get the solution into the soil. Read and follow the fungicide manufacturer's instructions carefully, and wear protective eye-wear and clothing that covers your skin while handling the fungicide.

Sap-Sucking Pests

Mealybugs, spider mites, thrips and scale insects feed on Boston ferns and weaken them. Mealybugs, spider mites and thrips usually can be controlled by simply spraying them off the Boston ferns with water from a garden hose.

Mealybugs are small, oval, slow-moving or immobile bugs that are usually white or off-white. Spider mites are difficult to see without a magnifying glass. The first sign of a spider mite infestation is silvery stippling on leaflets. The mites eventually create a fine web between fronds. Thrips cause Boston fern leaflets to look dusty, and they leave tiny, black, fecal deposits on leaflets. All of those pests pierce leaflets and suck plant juices. Scale insects are flat, oval, immobile insects that are usually tan but can be white or other colors. They feed on plant juices, too.

Mealybugs and thrips are usually killed by a strong spray of water, and those that survive have a hard time getting back on plants. Spider mites and thrips shy away from moist conditions. Spray the ferns every other day until a spider mite or thrips infestation subsides. Spray both the tops and bottoms of the fronds as well as the stems in morning so they will dry quickly. A light infestation of scale insects can be controlled by scraping the scales off with a thumbnail or old toothbrush.

All of these pests also can be killed with insecticidal soap, which is available as a concentrate or ready-to-use spray. The concentrate is mixed into water at the rate of 5 tablespoons per 1 gallon of water. Pour the diluted mixture into a spray bottle or hand-pump pressure sprayer. Spray the tops and bottoms of the ferns' fronds as well as the stems in morning or evening when temperatures are below 85 degrees Fahrenheit. Use a strong spray from the garden hose to wash the insecticidal soap off the ferns after one to two hours. The soap kills only the insects it is sprayed on, and, if it is left on the ferns, it could damage leaflets. Repeat the treatment every four to seven days until the pests are gone.

Chewing Pests

Snails and slugs eat Boston fern leaflets, leaving holes in them. If the pests become a problem, sink empty tuna or cat food cans into the soil around the Boston ferns so the top lip of each can is level with the surrounding soil. Fill the cans with beer, which attracts snails and slugs. They crawl into the cans and drown. Check the cans each morning. If snails and slugs are in the cans, then empty them, put them back in the soil and refill them with beer.