Expanded Vermiculite as a Rooting Medium

Our work at Glenn Dale demonstrated that expanded vermiculite is usable as a rooting medium for cuttings and has some exceptional properties that probably place it close to the ideal in rooting media. Experiments were conducted with a species difficult to root, Chionanthus retusus; by far the best results were obtained with vermiculite. We got similar results for tests with many other kinds of plants. The rooting of cuttings is usually somewhat more rapid in vermiculite than in other materials, and the root systems are noticeably heavier.

Vermiculite is an insulating material sold by dealers in building supplies. It is a mica-like mineral composed of hydrated silicates and has the property of expanding into a light fluffy material when heated. Many of the properties of the expanded material are valuable. It is inert, sterile when fresh, and has many adsorbent pore spaces and inner surfaces that retain water for long periods and hold large amounts of air. The medium or coarser grades combine in a unique way the ability to provide excellent aeration and to retain large amounts of water.

With vermiculite one need not water so often; on the other hand, if drainage is adequate, large amounts of water may be applied frequently without harming the cuttings. This material may be used alone or mixed with sand. Mixtures of vermiculite and peat have given good results with plants that need acid soil.

Recent work at Glenn Dale in the use of opaque propagating houses or miniature propagating cases represents a new and radically different approach to the ideal of propagating structures that are nearly or completely automatic and therefore require little labor or skill. The opaque structure, combined with the use of modern types of fluorescent lighting, has given results as good as those obtained with more effort in conventional greenhouses.

The construction cost can be low. The structure can be partly or, preferably, completely underground. Temperature thus can be controlled in winter and summer. If the insulation is good, the reduced heating cost offsets much of the cost of the current for illumination. In the absence of sunlight, the humidity can be kept constant at a high level, without mechanical apparatus. Because artificial light is used, the space required and consequently the construction and heating costs can be cut sharply by a vertically tiered arrangement of the cutting beds.

Various types of small propagating cases with fluorescent lamps and thermostatically controlled units for providing bottom heat have also been used at Glenn Dale. The results equal, we believe, those obtainable in any glass-covered propagating house or frame. Small units of this type should be helpful to the amateur who wishes to start plants for the garden in a basement or spare room. In certain types of units, many kinds of cuttings will root before watering again becomes necessary.

The possibilities of controlling the quality of radiation in opaque structures are most interesting. The modern fluorescent lamps, with their high efficiency and low heat production, made the use of opaque structures practical. They are available in a variety of colors, some of them definitely superior to natural daylight in promoting the rooting of cuttings. Results of many trials have all shown that rooting is usually more rapid and heavier in light that has a relatively high proportion of the orange-red end of the spectrum. Rooting is usually inferior under blue light.

The color range available in high-voltage cold cathode tubes is much greater. The rooting obtained under the fluorescent tubes (orange, rose) invariably has been exceptionally heavy. Under these colors, cuttings of a somewhat tender unripened growth become firm and turgid long before those under other colors, including white or daylight. After a few weeks under the fluorescent tubes, the leaves of the cuttings often have a firm, leathery quality that make them distinguishable from those under argon-mercury cold cathode tubes of various colors.