Week Three – Rockwool

There are so many ways to grow tomatoes when using hydroponic methods!

One option is to use rockwool, which is a bit like loft insulation. It can hold plenty of water and air, which is great for tomato plant roots!

Heartbreaker (Dwarf Variety) Growing in a 3×3 inch Grodan Rockwool Block.

One of the big issues when growing in small containers is support. Soil provides plenty of weight and is great for holding support canes but a small pot of perlite or a Grodan block is very light and plants will topple-over.

Growing under cover, indoors or greenhouse, means that it is usually possible to use a support from above such as string!

The seedlings in the media above fit perfectly into the large hole Grodan 3 inch (7.5cm) blocks below.

How to repare a rockwool block

To prepare a block, soak it in water for a couple of minutes, remove it,  then rinse through with water at the desired pH level. This water should also contain a nutrient feed. It’s important not to squeeze the rockwool so that it maintains its open structure.

It’s amazing how much water these can hold – compare the weight of a dry one with one that has just been watered!

The Cost

Rockwool is not expensive … the main expense when growing hydroponically is the cost of the nutrients.

Seedling feed such as Rootit First Feed isn’t expensive but nutrients given later on, as our plants grow, are expensive over time.

If you grow tomatoes to save money, it may be best to stick to soil methods. If growing tomatoes is a keen interest, like any other hobby, you probably won’t mind the extra cost.

Rockwool Slab

This is a hydroponic grow bag filled with rockwool rather than soil – about the same length as a grow bag but slimmer!

The Grodan blocks sit on the slab, rather like a bottomless pot sits on a grow bag so that the roots can grow into the media below.

If growing three tall varieties, the slab is ideal as it is about the same length as a grow bag – about 1 metre. Holes are cut so that the rockwool blocks can sit on the slab without the rockwool being exposed to the light. If exposed to light, algae tends to form which isn’t healthy for the roots. This can also happen on the surface of soil too, but it is not as noticable on soil.

Of course if you are growing a dwarf variety, you really don’t need the slab – just the block!

This week …. give your seedlings a feed with the Wilco feed at 1ml per half litre or use Rootit First Feed as per instructions on pack. Also, adjust the pH with lemon juice as recommended here.

I can send you a free sachet of First Feed that will make up 2 litres of feed if you send me a stamped addressed envelope.

Email me for address – [email protected] or Rootit First Feed is available here.

Next week we’ll have a look at some of the nutrients and additives available in the world of hydroponics.

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