Fertilizing Boston Ferns

A single frond of Bostern Fern.

Boston fern, Nephrolepis exaltata bostoniensis, is a perennial tropical fern, commonly grown as a houseplant throughout most of the year in all but temperate and tropical climes above U.S. Department of Agriculture zone 8b. You most often see the plants potted or used in hanging baskets where soil space and nutrients are somewhat limited. While not heavy feeders, Boston ferns thrive in a rich and easy draining humus soil and according to North Dakota State University, benefit from gentle fertilizing, high ambient humidity and regular watering.

Type of Fertilizer to Use

Boston ferns, like most ferns, are sensitive to over-fertilization and chemical burns to their roots and lower fronds according to Minnesota State University. To protect the plants from these damaging and possibly deadly side effects the University of North Dakota recommends the use a slow release granular fertilizer with a low level guaranteed analysis ratio. Look for either an organic or chemical fertilizer with a guaranteed analysis of 8-8-8 or 10-10-10, but not any higher in nutrient concentration. Alternatively, source a fertilizer formulated and labeled precisely for use on ferns.

When to Fertilize

Boston ferns should be fertilized during the main growing seasons during the year and allowed a rest interval during the winter when growth is naturally slower, advises the University of Florida IFAS program. Feed your ferns from roughly April through to November and then resume fertilizing again in April the following year. Some fertilizer brands will recommend monthly applications while others will recommend every couple of months. Heed the label directions of your specific fertilizer within the feeding window and know that a break from fertilizing is actually a good thing for your Boston fern.

Amount of Fertilizer to Use

Slow-release fertilizers typically come in a granular form designed to break down into the soil gently over time. Recommended amounts and frequency of application can vary widely by brand, depending upon the fertilizer chemistry. Always follow the label recommended application guidelines for your size of Boston fern and the surrounding soil volume. Always err on the side of less fertilizer, not more. With ferns and most other plants, over-fertilizing is worse than no fertilizer when it comes to plant health and vitality, according to the University of Illinois.

How to Apply the Fertilizer

When planting or re-potting your Boston fern mix in the label recommended amount for one dose of slow release fertilizer evenly into the planting soil. When fertilizing after planting, scatter the fertilizer grains over the surface of the soil being careful to keep the fertilizer from collecting around the base of the plant. For ground-planted ferns, keep the fertilizer around the outer drip-line of the plant, and for potted ferns, from the mid-way point to the outer edge of the pot. Do not dig the fertilizer into the surface of the soil, as this can disturb and sever healthy surface roots.

Water Liberally and Consistently

Ferns thrive in moist humus soil and high ambient humidity year-round. When using fertilizers, water is even more critical as it's the water that distributes the nutrients throughout the root zone and protects the plant from excess buildup of fertilizer salts in the soil.

Water your fern immediately after each application of fertilizer with a generous amount of water, applied until the planting soil is entirely saturated and excess water runs out of the drainage holes of potted ferns. Watering should be generous and consistent to keep the soil very moist-to-wet all year long, even when you suspend fertilizing. Frequent misting with distilled water will also help to keep the plant hydrated and green in low humidity conditions, according to Horticulturalist Ron Smith at the University of North Dakota.

References

• Cornell University: New York State Horticultural Study Guide for Youth: Boston Fern (Nephrolepis bostoniensis); October 2004
• University of North Dakota Extension Service: Hortiscope: Questions on Ferns; Ron Smith; July 2001
• University of Illinois Extension: Watering and Fertilizing Container Plants; February 2008
• University of Minnesota Extension: Growing Tropical Ferns Indoors; Deborah Brown; July 2001
• University of Florida IFAS Extension: Nephrolepis exaltata Boston Fern, Sword Fern; Edward F. Gilman; June 2007