A heavy nutrient boost not always wise for landscaping plants

I received this query last week: “I bought a large number of ostrich ferns late last summer and planted them in a semi-sunny patch at my home on Gouldsboro Point. The soil has been treated heavily with seafood compost. Moss tends to flourish in this area. What can I do to give the ferns a nutritional boost this season?”

This is a common question from gardeners, coming in as many versions as there are varieties of garden plants. The question is based on an assumption that garden plants need a nutrient boost every year, a misconception created by manufacturers and purveyors of fertilizers as they urge us to push fast growth with high levels of nitrogen. In fact, numerous studies show that plants producing lush growth will be plagued by more insect and disease problems than plants growing more slowly under low to moderate rates of nitrogen. In keeping with our goal of growing healthy plants, we should trade off slower growth for greater insect and disease resistance. (Of course, those same purveyors of packaged nitrogen will be happy to sell you pesticides, too!)

My short answer to this question is always the same: Show me your soil test results. The ostrich ferns may not need a nutrient boost this season, and only by examining a soil analysis, interpreting it in the context of the specific cultural requirements of ostrich ferns, could I say more.

Plant growth problems, which are too quickly attributed to lack of nutrients, are often due to other limitations that would be revealed by soil analysis. For example, soil pH that is too low or too high can interfere with root uptake of nutrients, making them unavailable to plants. In this specific case, the seafood compost may contain enough shell residues to excessively raise the soil pH above the optimum 5.5 to 6.0 required by ostrich ferns. Only a soil test will tell.

Plant species differ in optimum nutrient conditions for best growth, while recommendations from the soils lab typically address only broad categories of garden plants, such as lawn grasses, herbaceous perennials and annuals. Not being a fern grower myself, I checked several references, discovering that ostrich ferns are sensitive to high levels of nitrogen, easily burned by over-fertilizing. Several sources recommended limiting nutrient applications to top dressing with compost. Thus any fertilizer recommendations based on a soil analysis should be reduced for ostrich ferns. I would suggest annual top dressings of composted litter from a nearby horse stable as the best approach to maintaining a stable nutrient base for healthy growth.

This will be the establishment year for the ferns in question, a year that should be devoted to development of an extensive root system rather than pushing shoot growth. For all of our garden plants, any nitrogen boost that would be appropriate for established plants should be reduced by at least half during the establishment period.

A soil test should be conducted before planting a new section of the garden and whenever existing plants show symptoms of decline such as yellowing leaves, branch dieback or lack of any new growth. The University of Maine Cooperative Extension office in your county will provide instructions on how to sample the soil and forward your samples to the soils lab in Orono for analysis. It will take about two weeks to receive the results.

Send queries to Gardening Questions, 116 N. Main St., Orono 04473, or e-mail manley@adelphia.net. Include name, address and telephone number.