how to use perlite in potting mix

How to use perlite in potting mix


From an address by
George (Doc) and Kay Abraham
to The Perlite Institute
April 27, 1979

Before discussing various tests that have been conducted, we have to explain why perlite is such an important soil component in today’s container grown and potted plants.

Water is a nutrient. A plant is 90% or more water but it also needs oxygen. Roots supply water and oxygen to plants (perhaps 98% of the oxygen a plant uses comes through its roots. Since both water and air (oxygen) move in, through, and out of soil mixes, both water retention and drainage, and aeration are important considerations in any potting mix.

If the soil is poorly drained, or packed due to fine organic particles, oxygen is cut off from the plant. Also, whenever a plant is watered, it forces air out through the bottom of the pot or it bubbles up to the surface. In a loose, porous soil this is fine because it creates a suction effect which draws in fresh air from the top into the air spaces in the soil.

However, if soil is too moist, or lacks proper drainage, oxygen is excluded and roots gasp for air. Also, a lack of oxygen favors high concentrations of carbon dioxide (a waste product), cutting down on root growth, and certain soil organisms, which like high CO2 concentrations, grow. In the stagnant soil these organisms produce toxic chemicals, which have a disastrous effect on the health of plants. Many gardeners discover this first-hand when they set out container-grown plants and often note that the bottom one-inch is often stagnant and smelly.

This coupled with a low pH (that is, high acidity) and a toxic form of ammonia (NH3) helps to kill plants. The role of perlite particles in a soil mix allows air spaces to form where oxygen and water have free movement.

While most growers seem to prefer coarse grades of perlite, an educational effort is needed to share with them how medium and finer grades of perlite are just as effective in preventing oxygen starvation.

General Observations about Perlite

In general, after years of testing and experimentation, several observations can be made about perlite and its use in potted plants:

1-Plots and pots containing fine, medium, and coarse perlite have had exactly the same weight and size as those using traditional peat mixes. Many tests have proved there should be no hesitation in using finer grades, and that 100% perlite could be used and be just as successful as traditional peat mixes.

2-Tests on the effects of various grades of perlite on the rooting of cuttings and in the germination of seeds found that in all cases, regardless of size, rootings were the same in time, size, and quality. In fact, finer grades required less water to maintain healthy roots. In seed germination, however, coarse grades of perlite did not do as well as finer grades.

3-Perlite also was tested for use in drying out flowers. Flowers were placed in pans of fine perlite and covered lightly. After standing for 3-6 days (or long enough for drying to occur), the flowers were removed and dusted off. Flowers also can be dried using fine perlite and a microwave (approximately 3-5 minutes).

4-100% perlite has been used growing orchids. The fine and medium grades have been tried, but coarse perlite did the best. (For more information on growing orchids in perlite see Growing Orchids in Perlite)

    Perlite is one of nature’s best media for growing plants. It does not appear to make any difference which grade is used except with certain plants like orchids.

It is possible to grow most plants in perlite alone, although usually the finer grades and medium grades will work better and require less water.

Seeds can be started in any grade of perlite, but with smaller seeds, finer grades of perlite would be recommended.

Perlite is good for greenhouse benches. And as an added benefit, insects and snails do not like perlite!

Perlite (especially the fine grade) is excellent for drying flowers.

Perlite is ideal for outdoor containers. They can be moved around easily because perlite in the mix lightens it, besides improving drainage.

Hopefully the summation of this presentation has been helpful. If you have any questions, or need more information, please look at our other plant guides, or contact us:

The Schundler Company
150 Whitman Avenue
Edison, NJ 08817

Using Perlite in potted plants and container plants with a discussion of how fine, medium, and coarse grades can be used in most situations.

Perlite – How useful it can be in your Gardening Job!

Perlite is the name for a naturally occurring siliceous rock. When heated, it has the capacity to expand from four to twenty times its original volume. This is due to the rapid expansion of water within perlite, which creates the extremely light perlite that we use for gardening.

The main function of perlite for the gardener is to aid water retention and aeration as well as improve compost mixtures.

Perlite is mined and expanded all over the world. The United States is estimated to be the largest consumer and producer of both crude and expanded perlite. However, other countries producing perlite include China, Greece, Japan, Mexico, Hungary, Armenia, Italy and Japan.


  1. Prevents compaction
  2. Improves aeration and drainage better than vermiculite
  3. Stimulates root initiation and vigorous growth
  4. Holds moisture but does not become soggy
  5. Almost neutral pH
  6. Does not decompose
  7. Free from disease, weeds and insects
  8. Insulates and minimises temperature fluctuations
  9. Inorganic, inert and sterile
  10. No known toxicity or fire hazard

Uses of Perlite
Seed Germination
Perlite speeds up germination and improves seedling growth.

For seeds, sow on a well-watered mixture of equal parts perlite and Sphagnum Moss Peat. Alternatively, add 1 part perlite to 2 parts ready-mixed potting compost.

For use pure, keep wet at all times by capillary irrigation or intermittent mist. After sowing, sprinkle seeds with a thin layer of fine peat and cover with glass or plastic to retain moisture until germination, then feed

Potting Mixes
Perlite is used in potting compost mixes to improves aeration, draining and insulation. Perlite can be used to open up the structure of ready-mixed loam or peat-based composts.

For soilless compost mixtures, use 3 or 4 parts of Sphagnum Moss Peat to 1 part of perlite (80/20).

For loam-based compost mixtures, use equal parts sterilised loam, peat and perlite (1:1:1) plus limestone and nutrients. Alternatively, a 1:2:1 mix may be used. Mix thoroughly, then water well after planting and feed as appropriate.

Rooting and Cuttings
Perlite speeds up rooting, reduces risk of damping off, provides an optimum balance of air and water, and makes water logging almost impossible. It also minimises damage to roots and growth disruption from transplanting.

For soft stem and leaf cuttings, use a mixture of equal parts perlite and Sphagnum Moss Peat (50/50). For harder cuttings and fragile plants, increase the proportion of perlite up to 4 parts perlite to 1 part peat (80/20).

For mist irrigation, perlite may be used 100% where sterility is essential. Keep well-watered but ensure free drainage. Feed plants as soon as roots develop.

Soil Conditioning
Perlite improves the texture of heavy silt or clay soils by increasing aeration and drainage. It also minimises the tendency to ‘cap’ over germinating seeds. These improvements will last for many years.

For difficult seed beds and flower beds, use up to 25% perlite worked into the top 5-10cm before sowing.

For trees, shrubs and roses, mix perlite with the soil when back-filling the planting hole to stimulate root growth.

Turf Dressing
Because Perlite improves aeration and drainage, it will assist the air-moisture balance and ensure better root development and turf growth. Golf course greens treated with perlite will show improved resilience and greater tolerance in use under extreme weather conditions – wet or dry.

For compacted and poorly drained areas on old turf and for establishment of new turf, use as follows: Spike over the affected area with a hollow tine, then spread a thin layer (2-5mm) of damp perlite pre-mixed with a suitable fertiliser. Rake or brush evenly, and water thoroughly. Note: a 100-litre bag of perlite will give on average a 2mm layer over 50sqm

Capillary Watering, NFT & Hydroponics
Perlite is an inert, sterile, neutral, ultra-lightweight aggregate with a very high air and water-holding capacity. To sterilise perlite for re-use, steam, flame-gun or treat with any proprietary chemical steriliser.

For capillary watering use, allow at least 25mm (1 inch) depth of perlite in place of sand or gravel in polythene-lined benches or suitable trays.

For ring culture and low cost commercial production of tomatoes, use perlite in a polythene lined trench or channel, and saturate with nutrient solution.

Special Note:
With electrical heating cables use a 50/50 mix of perlite and sand to prevent overheating.

Expert gardening advice from a team of professional gardeners and horticulturists.