Growing Media

Peat and Peat-Like Materials

Peat moss is formed by the accumulation of plant materials in poorly drained areas. The type of plant material and degree of decomposition largely determine its value for use in a growing medium. Although the composition of different peat deposits vary widely, four distinct categories may be identified:

Hypnaceous moss – this type of peat consists of the partially decomposed remains of hyprum, polytrichum and other mosses of the Hypanaceae family. Although it decomposes more rapidly than some other peat types, it is suitable for media use. Many of the peat deposits in the Northern United States are Hypnaceous.

Reed and Sedge – are peats derived from the moderately decomposed remains of rushes, coarse grasses, sedges, reeds and similar plants. These fine textured materials are generally less acid and contain relatively few fibrous particles. The rapid rate of decomposition, fine particle size and insufficient fiber content make reed and sedge peats unsatisfactory for media use.

Humus or Muck – consists of the decomposed debris of finely divided plant materials of unknown origin. Humus often contains large quantities of silt and clay particles, and when mixed with soil does not improve drainage or aeration. Due to its rapid rate of decomposition and particle size, humus is considered to be undesirable for growing media use.

Sphagnum moss – is the dehydrated remains of acid-bog plants from the genus Sphagnum (i.e. Spapillosum). It is light in weight and has the ability to absorb 10 to 20 times its weight in water. This is attributed to the large groups of water holding cells, characteristic of the genus. Do not rely on sphagnum or any single ingredient to prevent damping-off. Use clean materials and containers, manage moisture, and provide suitable growing conditions.

Sphagnum moss is perhaps the most desirable form of organic matter for the preparation of growing media. Drainage and aeration are improved in heavier soils while moisture and nutrient retention are increased in lighter soils. Germany, Canada and Ireland are the principle regions of Sphagnum moss production.

Wood Residues

Wood residues constitute a significant source of soilless growing media. These materials are generally bi-products of the lumber industry and are readily available in large quantities. Nitrogen depletion by soil microorganisms, during the decomposition process, is one of the primary problems associated with these materials. However, supplemental applications of N to the growing media can make most wood residues valuable amendments.

Leaf Mold – maple, oak, and sycamore are among the principle leaf types suitable for the preparation of leaf mold. Layers of leaves and soil are composted together with small amounts of nitrogenous compounds for approximately 12 to 18 months. The use of leaf mold can effectively improve the aeration, drainage and water holding properties of a growing media. Although these materials are readily available at low cost, leaf mold is not extensively used in container production.

Sawdust – the species of tree from which sawdust is derived largely determines its quality and value for use in a growing media. Several sawdusts, such as walnut and non-composted redwood, are known to have direct phytotoxic effects. However, the C:N of sawdust is such that it is not readily decomposed. The high cellulose and lignin content along with insufficient N supplies creates depletion problems which can severely restrict plant growth. However supplemental appli-cations of nitrogen can reduce this problem.

Barks – are primarily a bi-product of the pulp, paper and plywood industries. Suitable particle size is obtained by hammer milling and screening. This produces a material which is suitable for use in container media. Physical properties obtained from tree barks are similar to those of Sphagnum moss.

Bagasse

Bagasse is a waste bi-product of the sugar industry. It may be shredded and/or composted to produce a material which can increase the aeration and drainage properties of container media. Because of its high sugar content, rapid microbial activity results after the incorporation of bagasse into a media. This decreases the durability and longevity of bagasse and influences N levels. Although bagasse is readily available at low cost, (usually transportation), its use is limited.

Rice Hulls

Rice hulls are a biproduct of the rice milling industry. Although they are extremely light in weight, rice hulls are very effective at improving drainage. The particle size and resistance to decomposition of rice hulls and sawdust are very similar. However N depletion is not as serious of a problem in media amended with rice hulls.

Several other organic materials are suitable for use with container media. Included are: manures; corn cobs; straw; peanut and pecan shells. However these do not constitute major commercial sources of organic amendments.

Sand

Sand, a basic component of soil, ranges in particle size from 0.05mm to 2.0mm in diameter. Fine sands (0.05mm – 0.25mm) do little to improve the physical properties of a growing media and may result in reduced drainage and aeration. Medium and coarse sand particles are those which provide optimum adjustments in media texture. Although sand is generally the least expensive of all inorganic amendments it is also the heaviest. This may result in prohibitive transportation costs. Sand is a valuable amendment for both potting and propagation media.

Perlite

Horticultural perlite is a lightweight mineral ingredient used to increase air space in potting mixes. Its effectiveness depends on particle size and the other ingredients in the mix.

Fluoride associated with perlite can be a concern for fluoride-sensitive plants, such as spider plants and dracaenas. This does not establish that horticultural perlite is generally toxic to plumeria. Follow the product’s handling instructions and avoid breathing dry dust.

Vermiculite

Vermiculite retains substantial moisture; it is not interchangeable with perlite when adjusting a mix’s water-holding capacity.

Calcined Clays

Calcined clays are formed by heating montmorrillonitic clay minerals to aproximately 690^oC. The pottery-like particles formed are six times as heavy as perlite. Calcined clays have a relatively high cation exchange as well as water holding capacity. This material is a very durable and useful amendment.

These inorganic soil amendments are generally utilized to increase the number of large pores, decrease water holding capacity and improve drainage and aeration. Other materials such as: pumice; cinders; and pea-gravel are also suitable for this use.

Use ingredients intended for horticultural containers

For home-grown plumeria, choose established potting-mix ingredients with known properties rather than substituting packaging foam, construction foam, or unidentified industrial by-products. Check product information for intended use and suitability.

Preparing Soilless Growing Media

Although amendment combinations may vary, basic objectives in the preparation of a growing media are alike. An effective program should produce a growing media that is:

1. porus and well drained, yet retentive of sufficient moisture to meet the water requirements of plants between irrigations;
2. relatively low in soluble salts, but with an adequate exchange capacity to retain and supply the elements necessary for plant growth;
3. standardized and uniform with each batch to permit the use of standardized fertilization and irrigation programs for each successive crop;
4. free from harmful soil pests; pathogenic organisms, soil insects, nematodes and weed seeds
5. biologically and chemically stable following pasteurization; primarily free from organic matter that releases ammonia when it is subjected to heat or chemical treatments.

Since innumerable amendment combinations can produce a growing medium with these characteristics, it is important to consider both the economic as well as cultural optimums. Factors that determine the cost of a growing medium include: transportation, labor, equipment, materials and handling. In many cases the cost of mixing a “custom” growing medium exceeds that of the commercially prepared materials. These factors should be studied carefully before making a decision.

Example mixtures to evaluate

The ratios below describe example ingredient combinations, measured by volume. They are starting points for evaluation, not proven recommendations for every plumeria.

Ingredient grade and particle size affect performance, so two mixes with the same proportions may behave differently. Test a small batch in your intended containers before using it throughout your collection. Check drainage, moisture retention, and drying between waterings.