When horticultural perlite is added to heavy clay soils, water ponding and surface crusting may be eliminated. In addition, plant roots may more easily penetrate the perlite/soil growing media and develop more fully. Horticultural perlite can be particularly advantageous in modifying the structure of general garden soil, patio plantings, and roofscapes.

What is Perlite?

Perlite is a unique volcanic mineral which expands to about 13 times its original volume when it is heated to a temperature of approximately 1600 F (871C). During the heating process, the mineral particles pop like popcorn and form a granular, snow-white material that is so light in weight it weighs only about 5 to 8 pounds per cubic foot (80-128 kg/cubic meter).

Each particle of perlite is comprised of tiny closed air cells or bubbles. The surface of each particle is covered with tiny cavities which provide an extremely large surface area. These surface cavities trap moisture and make it available to plant roots. In addition, because of the physical shape of each particle or perlite, air passages are formed in the growing media thereby providing excellent aeration. Fertilizer appropriate to the plants being grown should be added. Horticultural perlite is available in several different grades. The coarse sand size has been used for many general gardening and horticultural applications, but the finer grades work best when used outdoors.

Advantages of Horticultural Perite

• Improves aeration and drainage
• Makes moisture and nutrients readily available to plants
• Is inorganic and does not deteriorate.
• Has an essentially neutral pH of 6.5 to 7.5.
• Serves as an insulator to reduce extreme soil temperature fluctuations.
• Is sterile and free of weeds and disease.
• Is clean, odorless, lightweight, and safe to handle.

This garden had approximately 2 inches of fine grade perlite tilled into the soil early in the growing season. The results after an extremely dry summer with almost no watering!

General Garden Applications

Soil in vegetable and flower gardens can be conditioned by cultivating, with a fork or rototiller, to a depth of 6-12 inches (150-300 mm). A 4 inch (100mm) layer of 100 fine horticultural perlite, or a mixture of one half horticultural perlite and one half peat moss, then is worked into the cultivated soil. (If the soil is rich in organic material, peat moss really does not have to be used.) The same procedure may be followed in preparing lawn areas for sodding or seeding. Established lawns also may be renovated with horticultural perlite. The lawn area should be spiked or plugged and horticultural perlite spread on the surface and raked into the spike or plug holes. The perlite will work its way into the root zone to provide aeration and drainage.

If the garden area is very large, the home gardener may condition only the rows or areas where planting is to take place. Thus, if the procedure is continued for 2-3 years, the entire garden area will eventually be conditioned. Because horticultural perlite is inorganic, it will not deteriorate in the soil but will continue to function for many years.

When shrubs or trees are to be planted, a planting pocket 6 inches (150mm) larger than the plant roots should be dug. A mixture of one-third soil, one-third horticultural perlite, and one third peat moss or compost may be used to fill the hole and compete the planting operation. The shrub or tree should be thoroughly watered. The roots of the plant can easily penetrate and develop in the friable growing media.

Patio Planting

Horticultural perlite is an ideal material for patio plantings in container boxes, tubs, and ornamental containers. Because a perlite soil mix is so light in weight, handling headaches are reduced as planters filled with a perlite soil mix may be easily moved to rearrange a patio planting display or to protect plants against adverse weather. A suggested soil mix for patio planters is one third horticultural perlite, one third peat moss and one third composted wood product such as bark or shavings, although 100% fine perlite also can be used! Patio plantings in either type of media will require more frequent watering than garden plantings since they tend to dry out faster.

Roofscape Gardening

For the apartment dweller with a limited outdoor roof garden, perlite can be of enormous benefit. The weight of planting mixes is very important when the strength of the supporting structure must be considered. Wet, sandy loam weighs from about 120-140 lbs./cubic foot (1920-2240 kg/cubic meter) while a soil mix consisting of equal parts of perlite and peat moss weighs only about 35 lbs./cubic foot (560 kg/cubic meter), and even less if 100% fine perlite is used. As a result, a perlite growing mix can be several times as deep as a conventional soil mix without increasing weight. This enables the rooftop gardener to grow larger trees. shrubs, and plants and to have more extensive gardens.