Germinating Using Rock Wool

Germinating Using Rock Wool Cubes

Germination using rock wool is a common practice for commercial growers and anyone growing hydroponically.


The best aspect of germinating using rock wool is its convenience. There is no need to venture outside to fill seed pots or trays with soil. It is also very easy to do and reliable.

Rock wool cubes are man-made, a combination of rock and chalk or sand. They contain no plant nutrients – the nutrients are added through liquid feed – but make an excellent growing medium.

Using Rock Wool Cubes

See my step by step video on using rock wool cubes, or if you prefer, continue reading below:

Rock wool cubes are supplied dry and need to be soaked in water before use. They come in a wide variety of sizes, but I decided to use the smallest cubes as my intention is to grow the seedlings on in compost before transplanting to the allotment.


Rock wool cubes come in plastic trays which provide a convenient support for the cubes. The trays can be cut to the size you need. I have tried two approaches: placing the cubes inside our heated propagator, and placing the trays on tin foil covered with plastic food wrap, as a cheap way of creating a warm growing environment for our seedlings.

Before using the cubes they need to be soaked in water. Rock wool makes an excellent moist growing medium, and naturally provides enough air pockets to avoid the root systems of plants becoming unhealthy. What the medium lacks is nutrients. I diluted some plant nutrient into the water we soaked the cubes in, to provide the ideal start for our seedlings (some people prefer not to do this, adding the nutrients after germination).


After soaking the cubes, it is a quick task of making a small hole in the cubes using a pencil or pair of scissors. Simply place one seed per hole per cube, and cover the seeds by stretching over a few fibres of rock wool.

After 5 days I was very impressed with the results. The picture below is the result I achieved with cucumber and squash seeds in our heated propagator:


The germination rate was 100%. The germination rate of marigold and lettuce seeds was nearly as good, and these were placed on a sunny windowsill under plastic food wrap.


When the roots are visible through the rock wool they are ready for potting on. I placed our seedlings in small pots and returned them to a sunny windowsill. In a week or two, my plan is move them over to our polytunnel for a few days before planting out.