To get to root of potting, try variety of materials

A few weeks ago, I met with the St. Albans Garden Club, a group of pleasant and gracious women, who sent me off after an evening of gardening talk with several related gifts. One of the items was an interesting brick of coconut fiber pressed into a 4-by-8-by-2-inch block. The fiber was produced in Sri Lanka for the Tropical Gold Co.

“Growing medium for indoor and outdoor use,” the wrapping touted. “Expands 8-9 times its volume.”

The coconut fiber brick is just one of many rooting materials available for starting plants indoors. My daughters and I seeded some winter greens into the medium, so time will tell how coconut fiber compares to peat, the substance that constitutes the basis of most commercial growing medium material.

There are many different types of peat, with sphagnum and hypnum mosses the most widely used in horticulture today. Peat looks similar to coconut fiber, being light brown in color, with a fine texture that promotes water and air retention in the root zone of potted or garden plants. Peat is slow to decompose and tends not to tie up soil nitrogen in the composting process.

Sphagnum is the most widely used moss, holding up to 60 percent of its volume in water. It is a fairly acidic material and requires the addition of lime to elevate its natural pH of 3-4 to a level acceptable for plant propagation. Hypnum moss has a pH of 5.5 or so and, when mixed into a formulation with vermiculite, does not require the addition of lime to influence the pH.

What is vermiculite? If you’ve purchased commercial potting soil from your local nursery or garden center, you’ve probably noticed that the mix is mostly peat, with flecks of white material. Careful examination may reveal that there are actually two types of white material: One is a shiny, micalike material that may appear layered like a miniature accordion; the other is a rocklike material that crunches into a powder between your fingers. The former is vermiculite, the latter, perlite.

Vermiculite in its natural state is compressed. To prepare it for use in potting medium, it is heated, which causes its many layers to expand. This expansion transforms vermiculite into a lightweight material that increases the water-holding capacity of the mix. A cubic foot of vermiculite weighs about 8 pounds. The material also enhances the movement of gases (air) within the root zone. Vermiculite is a valuable component of root medium in that it has a high ability to hold onto plant nutrients, making them available for plant use rather than allowing them to wash through the soil.

Perlite is volcanic rock that is crushed and heated to about 1,800 degrees F. to cause expansion among the particles. It is an even lighter weight than vermiculite, weighing about 6 pounds per cubic foot. This siliceous material is an important component of potting material because it can dramatically increase the water-holding capacity of root medium. It is also used in hydroponic production to provide stability and structure for the developing roots of plants.

Growing mediums that contain peat, vermiculite and perlite are known as soilless mixtures, since they provide some of the same horticultural qualities as soil, yet aren’t technically soil. A soilless mixture might contain 50 percent peat, 40 percent perlite and 10 percent vermiculite. While commercial formulations are available, it is possible to concoct your own potting medium by purchasing these three components (or adding others such as compost or composted manure, for example).

While one potting mix may be appropriate for many of your garden plants, you may find that special formulations – one, say, with an abundance of vermiculite (or a higher vermiculite-to-peat ratio) – may be appropriate for the smallest seeds. As you start seedlings in the coming months, be aware of the formulation of rooting material you use in the different phases of production.

Diana George Chapin is the NEWS garden columnist. Send horticulture questions to Gardening Questions, RR1, Box 2120, Montville 04941, or e-mail them to dianagc@prexar.com. Selected questions will be answered in future columns. Include name, address and telephone number.