Propagation Mediums

Choosing a propagation medium

At the propagation stage you are not feeding a plant, you are keeping a cutting alive long enough to root. That means holding moisture without drowning the stem, and holding air so the new roots have something to 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 it is the single property that separates a propagation medium from the medium you will grow in afterwards.

Rockwool

The propagation standard, and the biggest part of this range. Grodan describe it as mineral wool made from molten basalt and limestone, spun into fibres and compressed into cubes, blocks and slabs. It is sterile and inert, holds its shape when wet, and its hydrophilic fibres wick evenly. Speedgrow's Multigrow rockwool is the sustainability option — it uses an all-natural, formaldehyde-free ECOSE binder.

Coco

The organic alternative, and easier to overwater forgivingly. Bio Diesel coco plugs come pH-buffered with a starter solution already in them, which removes the buffering step 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 — dry, so they store indefinitely and you wet only what you need.

Peat

Jiffy-7 pellets are the simplest thing on the page: 42mm compressed discs that you add water to and watch expand into a plug with its own netting. Sold in 30-packs and 1000-pellet cartons. No tray of loose medium to fill and nothing to mix.

Sizing, and potting on

Propagation media come in a ladder, and the point is that you move up it. Grodan's own instruction on their cubes is explicit: once roots start to emerge through the cube, plant it on into a larger Grodan block. Starting a cutting in a block far bigger than it needs is the most common way to lose one — too much wet medium around too few roots.

The ladder here runs: 40mm wrapped cubes and 38–40mm plugs to strike in, available 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 370 x 550 x 150mm for running 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. Rooting in rockwool and potting into rockwool or clay pebbles keeps one system. Rooting in coco and potting into coco does the same. Mixing families works, but it changes how you water.
2. Pick the pack size honestly. A 12-pack of cubes suits a hobby run; the 1386-cube Grodan cartons and 1000-pellet Jiffy boxes are commercial quantities, not a starter buy.
3. Choose hole or no hole. Cubes with a pre-formed hole are for cuttings. No-hole cubes are for seed, where you want the seed enclosed.
4. Note what needs conditioning. Coco plugs from Bio Diesel 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. Plugs dry out fast in open air. A humidity dome is not optional at this stage.

What else the strike needs

Medium is one of three variables. Bottom heat from a heat mat drives the strike rate, humidity from a dome stops the cutting transpiring faster than it can drink, and a rooting gel or mycorrhizal inoculant from propagation and cloning additives does the rest. If you would rather buy the combination in one box, propagation kits pairs them up. Full-size media for potting on are in grow mediums — rockwool and coco each have their own page.