Propagation Mediums

Choosing a propagation medium

You are not feeding a plant yet. At this stage the medium only has to keep a cutting alive until roots form. Moisture has to stay in the cube without drowning the stem, and air has to stay in it so those new roots can breathe. Every product here is built around that ratio. Bio Diesel describe their coco plugs as holding the ideal air-to-water ratio for fast root development, and that is the property that sets a propagation medium apart from the medium you grow in afterwards.

Rockwool

This is the usual starting point, and the largest slice of the range. Grodan describe it as mineral wool made from molten basalt and limestone, spun into fibres and then pressed into cubes, blocks and slabs. The fibre is sterile and inert, keeps its shape once wet, and wicks evenly because it is hydrophilic. Speedgrow's Multigrow line is the sustainability pick: an all-natural ECOSE binder with no formaldehyde.

Coco

If you want an organic plug, this is the more forgiving family if you overwater. Bio Diesel coco plugs come pH-buffered with a starter solution already in them, so you skip the soak rockwool needs. Eazy-Plug coco peat trays come in 12, 24 and 77 cubes, and ROOT!T dry coco plugs come as a 24-cell tray with 60-plug refills — they stay dry on the shelf, so you wet only what you use.

Peat

Jiffy-7 pellets are the simplest format on the page: 42mm compressed discs that swell into a netted plug once you add water. Sold as 30-packs and 1000-pellet cartons. Nothing to mix, and no loose medium to fill a tray with.

Sizing, and potting on

These media sit on a size ladder, and you step up it. Grodan tells you, on their own cubes, to plant on into a larger Grodan block as soon as roots start to emerge through the cube. Starting a cutting in a block far bigger than it needs is the usual way to lose one — too much wet medium around too few roots.

The ladder on this page runs from 40mm wrapped cubes and 38–40mm plugs to strike in, sold as 12 and 30-packs or as bulk trays and cartons; then 75mm cubes with or without a hole; then 100mm Delta blocks and 150mm Hugo blocks; then crop box slabs at 370x550x150mm if you want to run the crop out in the same material. Cubes come with a hole for a cutting or without one for seed.

How to choose

1. Match the medium to what you grow in next. Strike in rockwool and pot into rockwool or clay pebbles and you stay in one system. Strike in coco and pot into coco does the same. Mixing families is fine, but it changes how you water.
2. Pick the pack size honestly. Twelve cubes is a hobby run. A 1386-cube Grodan carton or a 1000-pellet Jiffy box is a commercial quantity, not a first buy.
3. Choose hole or no hole. A pre-formed hole is for cuttings. A solid cube is for seed, so the seed sits enclosed.
4. Note what needs conditioning. Bio Diesel coco plugs arrive pH-buffered. Rockwool does not come pre-conditioned — follow the maker's soaking guidance before you use it.
5. Buy the tray and the dome with it. Open air dries a plug out quickly. At this stage a humidity dome is part of the setup, not an extra.

What else the strike needs

The medium is only one of three variables. Bottom heat from a heat mat pushes the strike rate up, humidity under a dome stops the cutting losing water faster than it can take it up, and a rooting gel or mycorrhizal inoculant from propagation and cloning additives does the rest. If you would rather take the combination in one box, propagation kits pair them. Full-size media for potting on sit under grow mediums — rockwool and coco each have their own page.