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Linda Naeve, Reiman Gardens, (515) 294-8946,
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,

Reiman's Pick for the week of Feb. 23, 2004

Perlite - A Valuable Potting Soil Ingredient

By Linda Naeve
ISU Extension Coordinator,
Reiman Gardens

Society and government today have made us very conscious of the ingredients in everything from spaghetti sauce to shampoo. In addition to product labels, we find ourselves reading the list of ingredients before purchasing many products.

This week's Reiman's Pick, perlite, is an essential ingredient found in many potting soil mixes. You may not have heard of it before, but it is likely your houseplants are growing in perlite. Check the list of ingredients on your bag of potting soil or look for white particles that resemble Styrofoam beads in your soil.

Perlite is a natural occurring volcanic glass that, when heated to 1600 degrees Fahrenheit, pops like popcorn and expands to about 13 times its original volume. The end product is a lightweight, snow-white, granular material that weighs only five to eight pounds per cubic foot.

According to the Perlite Institute, each particle of perlite is comprised of tiny, closed air cells that do not absorb moisture into the particle, but holds water around the outside. When closely examined, the surface of each perlite particle is covered with tiny cavities that trap moisture, making water available to the plants roots.

Perlite is used in potting soil mixes because it modifies the structure of soil and improves aeration by keeping it loose and preventing compaction. A potting mix containing one part garden loam, one part peat moss and one part perlite is excellent for containers because it holds just the right amount of water and oxygen.

Perlite is also an excellent rooting medium for cuttings. Roots formed in perlite are much stronger than those formed in water because perlite allows oxygen to reach the roots as they develop, enabling the roots to adapt more readily to a soil environment.

A simple and carefree way to propagate easy-to-root plants such as coleus, English ivy, German ivy, Swedish ivy and other houseplants only requires a clear quart- or gallon-sized, zippered plastic bag, perlite and water. First, fill the bag about 1/3 full of perlite. Remember to avoid breathing the dust from dry perlite because it can be an irritant and may cause severe coughing.

Add enough water to moisten the perlite and leave a small amount of excess water at the bottom of the bag. Take 3- to 4-inch cuttings from the new growth of your plant and remove the lower tier of leaves from the cuttings. Insert the stems into the perlite so that the node - the point where the lowest leaves were attached - is slightly buried. Place your cuttings in the bag, but be careful they don't touch each other or the side of the bag.

Next, puff a little air into the bag and seal it. Place the propagation bag where it will receive indirect light. After two or three weeks, check the bottom of the cutting for root development by tipping the bag to the side. If the roots are less than half an inch long, cover them and wait a week or two more. Cuttings are ready to be planted into potting soil when they have an ample supply of roots measuring 1/2 to 1 inch long.

Besides horticultural uses, perlite has many other applications. It is used as loose-fill insulation in masonry construction and as an aggregate in Portland cement and gypsum plasters. Perlite also is used as a filter media for pharmaceuticals and in water filtration for municipal systems and swimming pools. Additionally, perlite is an abrasive in soaps, cleaners and polishes.

Perlite can be purchased at most lawn and garden centers. Look under the plants when you visit the Reiman Gardens Conservatory and Christina Reiman Butterfly Wing and you will see that perlite is a major component that keeps the plants growing and blooming beautifully.


Editors: A color photo, suitable for publication, is available at right. Click on the thumbnail photo to go to the fullsized photo. The picture's fullsize photo is 288K.

Caption: Cuttings from many plants root successfully and quickly in moist perlite.

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