Perlite and Vermiculite for Seed Sowing

Perlite and Vermiculite are both minerals found in nature

Both horticultural grade perlite and exfoliated vermiculite are used in gardening, especially seed sowing

Both are light, inert, non-organic (not derived from a living organism) substances that are good for maintaining aeration in the soil by maintaining space between the soil particles. However they function differently in terms of their effect on water retention in the soil.

What is Perlite ?

Perlite is mined in regions that have seen ancient volcanic activity. It is really a form of volcanic glass. The material is formed under great heat within volcanoes so that it expands a bit like exploding popcorn. Steam from within the rock forms a structure of tiny bubbles which harden as the ash is pushed from the heat then cools.

Perlite is whitish in colour and appears crumb like because of the bubbly structure inside. It feels hard but will crumble if enough force is applied between the fingers.

• Perlite absorbs considerable water into the tiny bubble holes, nooks and crannies throughout. However, this water is not retained very well. It tends to drain out quite quickly.
• Perlite should be incorporated into the seed sowing compost
• Perlite is best for plants that need well drained soil as because of its irregular surface shape it does help aerate the soil.
• Results for seed production using perlite are mixed, since the perlite does not absorb moisture.
• Perlite is available in various size grades.

There are various accounts of success and failure using perlite in a seed compost for specific plants.

• Perlite seems to work for starting salad greens and other fast growing seeds.
• Peppers seem not to grow in perlite at all.
• Tomato seeds appear to struggle.
• TIP: Pepper and tomato plants grow better in sand mixed with rice hulls than they do in perlite.

Perlite does cause algae to grow, owing to the water that is held on the surface, this can become a concern as it does not look very nice if you have a green carpet of algae on the top of the seed compost (normally you will only get this after a long period of time) but it has not been proved to do any real harm.

What is Vermiculite ?

Vermiculite is a silicon mica-like material, mined from the surface of the earth. It is formed of thin layers of silicon compounds. When heated, the layers separate (exfoliate) within the vermiculite. The spaces between the layers can hold considerable quantities of water which are bound into the space by water surface tension.

Vermiculite is biege/light brown in colour and has a soft texture flaky feel to it.

Horticultural grade exfoliated vermiculite is the best choice for seed production. When added to soil, it holds moisture and reduces the need for watering.

• Vermiculite acts like a sponge, holding moisture close to the roots of the plants.
• The horticultural exfoliated vermiculite is also able to absorb (soak up) excess water away from plants, this can help prevent mildew.
• When soil is sandy, mixing horticultural vermiculite into the soil will help maintain moisture and also help air circulate.
• Horticultural vermiculite is available in various grading sizes (Four). From very fine to course.
• Vermiculite can be incorporated into the seed sowing compost, in addition it can be used for seed covering and some have used it solely as a seed growing medium

Image by: hddod

Vermiculite can also be used as a seed covering material, as can be seen here on the left, especially if the seeds require light to germinate. Instead of using sieved compost to cover the seeds use a light covering of the horticultural vermiculite (fine grade). Excellent seed germination results have been achieved using this method for covering seeds.

To make a soil-free growing medium for seeds, combine vermiculite with peat (or alternative) or composted pine bark. This mixtures retains air, nutrients and moisture. Very little watering is required.

Some have even used vermiculite alone, if this is your chosen method then you should feed the seedling with diluted fertilizer when the first true leaves begin to appear.