Hydro grains buying guide: choosing the right bag
All hydro grains in this collection are clay granules from COMPO, so the main choice is not the material but how much you need and how you plan to use it. Work out the volume first, then compare the price per litre. Larger bags cost more upfront but are clearly cheaper per litre, and unused grains keep well.
| Product | Volume | Best for | Price from | Approx. price per litre |
|---|---|---|---|---|
| COMPO BIO Granuplant Indoor Hydro Granules | 3 L | One or two houseplants, a top layer | €8.07 | €2.69 |
| COMPO BIO Granuplant Hydro Granules | 5 L | A few medium pots, drainage layers | €9.89 | €1.98 |
| COMPO Hydro grains | 6 L | Repotting several houseplants | €7.60 | €1.27 |
| COMPO BIO Granuplant | 10 L | Several pots or one large planter | €11.95 | €1.20 |
| COMPO BIO Granuplant Hydro Granules | 25 L | Large planters, many plants | €20.29 | €0.81 |
| COMPO BIO Granuplant | 40 L | Balcony troughs, large drainage jobs | €23.60 | €0.59 |
How many litres do you need?
For full hydroculture, you need enough grains to fill the whole inner pot. For a drainage layer, a few centimetres at the bottom of the pot is usually enough, so a small bag goes a long way. The volume of a plastic pot is often printed on the bottom; add around ten percent extra for rinsing losses and topping up. If you are converting several plants at once, one 25 or 40 L bag is usually cheaper than several small ones.
Hydroculture: growing without soil
In hydroculture, the plant grows in hydro grains in an inner pot with holes, which stands in a watertight outer pot. The grains draw water upwards while the spaces between them let air reach the roots.
- Rinse all soil from the roots before moving a plant from soil to hydro grains.
- Use a water level indicator and let the level drop to the minimum before topping up, so the roots get enough air.
- Feed with a fertiliser made for hydroculture and follow the dose on the label.
Drainage layer and top layer
A layer of hydro grains at the bottom of a pot helps prevent waterlogging, because excess water collects below the soil instead of around the roots. This works best in pots with a drainage hole. You can also mix some grains through potting soil to make it more airy. As a top layer, hydro grains look tidy and keep the soil surface drier, which makes it less attractive to fungus gnats.
Tips for use and reuse
- Rinse new grains in a sieve or bucket to remove dust before use.
- Soak them in water for a few hours before planting so they are evenly moist.
- To reuse hydro grains, remove old roots, rinse them thoroughly and let them dry; white limescale deposits can be loosened with water and a little vinegar.
- Store unused grains in the closed bag in a dry place.
Frequently asked questions
What are hydro grains and what is hydrokorrels in English?
Hydro grains, called hydrokorrels in Dutch and Blähton in German, are small balls of expanded clay. They are also known as hydro granules, clay pebbles or LECA. They are porous and light, hold some water and leave space for air, which makes them suitable for hydroculture, as a drainage layer in pots and as a decorative top layer on potting soil.
What is the difference between COMPO Hydro grains and COMPO BIO Granuplant?
Both are clay hydro granules from COMPO for houseplants, hydroculture and drainage. In this collection, the main practical difference is the bag size and price: COMPO Hydro grains come in a 6 L bag, while COMPO BIO Granuplant is available in 3, 5, 10, 25 and 40 L. Check the product page of each item for its exact description.
How many hydro grains do I need for a drainage layer?
For a drainage layer, a few centimetres of hydro grains at the bottom of the pot is usually enough. A 3 or 5 litre bag is therefore enough for several medium-sized pots. For full hydroculture you need enough grains to fill the entire inner pot, so check the pot volume and choose a 10, 25 or 40 litre bag for larger plants.
Can I reuse hydro grains?
Yes. Clay hydro grains do not decompose like potting soil, so you can use them again. Remove old roots, rinse the grains thoroughly in a sieve or bucket and let them dry. White limescale deposits can be loosened by soaking them in water with a little vinegar and rinsing them well afterwards. Store dry grains in a closed bag.
How fast are hydro grains delivered and what does shipping cost?
Orders placed before 16:00 CET on a working day are shipped the same day. Delivery usually takes 1–2 working days in the Netherlands and 2–4 working days in Germany. Shipping is free in the Netherlands and Germany on orders from €39.99; below that it costs €5.95. You can return products within 30 days.