Grodan A-OK 1.5 in plugs are tapered stone wool starter plugs used to germinate seeds and root cuttings before transplanting into a larger stone wool block or slab. Each plug measures 1.4 × 1.4 × 1.6 in (36 × 36 × 40 mm), with 98 plugs to a sheet. This listing is the full case: a case of 30 A-OK sheets, which is the maker’s 2,940 plugs per case. The maker gives a nutrient-solution operating window of pH 5.5 – 6.5. This page gives Grodan’s published dimensions, case count, operating window, and application guidance; explains how stone wool performs as a propagation substrate; shows best uses, how it compares to other starter media, where it fits, and who it is not for — all cited.
What it is
Grodan A-OK plugs are tapered stone wool plugs on a sheet, designed for propagating plants from seed. Stone wool (also called stonewool or mineral wool) is made from basalt — solidified lava — so it is a mineral-based medium, not the carbon-containing organic matter found in coconut husks, peat moss or composted-wood potting soils. The A-OK fibre itself is finely woven stone wool with uniform density, firmness and fibrous structure. This format is sold under the maker’s A-OK 1.5 inch nominal name; the measured plug size Grodan publishes for it is 1.4 × 1.4 × 1.6 in (36 × 36 × 40 mm), slightly under the round-number name. This listing supplies the plugs as a full case of sheets rather than a single sheet.
Specifications
| Product type | mineral-based stone wool propagation plug |
|---|---|
| Plug dimensions (measured) | 1.4 × 1.4 × 1.6 in (36 × 36 × 40 mm) |
| Plugs per sheet | 98 plugs per sheet |
| Case composition | case of 30 sheets |
| Plugs per case | 2,940 plugs per case |
| Nutrient-solution pH operating range | 5.5 – 6.5 |
| Nutrient-solution EC at propagation | 1.5 EC (mS/cm) |
| Pre-formed hole | yes — a pre-formed hole receives the seed or cutting |
| Tray fit | the 1.5 in A-OK plugs fit snugly into the Grodan Gro-Smart tray, which sits inside a standard propagation dome |
Why choose Grodan A-OK plugs
- Published, precise geometry. You know the exact measured size — 1.4 × 1.4 × 1.6 in (36 × 36 × 40 mm) — which matters when you slot-match plugs to a block or slab with a receiver hole of a set size.
- Full-case volume for production. The case holds 2,940 plugs across 30 sheets of 98 plugs, sized for operations moving meaningful propagation volume each cycle.
- Clean, uniform mineral fibre. Grodan describes its stone wool as pathogen-free and residue-free at production, and much less likely than carbon-based organic media to be contaminated by fungi, yeasts and bacteria, so seedlings across a fresh case start on a comparable, low-contamination footing.
- Sheet format built for trays. Each sheet fits snugly into the Grodan Gro-Smart tray inside a standard propagation dome, so it drops into existing propagation racks and dome systems.
Best uses
- Seed germination. Drop a seed into the pre-formed hole, keep the plug moist at the maker’s target of pH 5.5 – 6.5, then transplant once true leaves show and roots emerge from the plug base.
- Cutting propagation. Grodan’s cutting guidance directs using a cutting no more than 14 days old and inserting the stem about half an inch into the saturated A-OK plug.
- Greenhouse and controlled-environment propagation. Because a sheet of 98 plugs seats in a Gro-Smart tray and dome, and the case carries 30 such sheets, the format integrates with standard commercial propagation workflows.
- First stage into a block system. The tapered plug is a propagation-stage medium, transplanted into a larger stone wool block or slab once rooted.
How stone wool performs as a propagation substrate
Stone wool is an inorganic mineral-fibre matrix whose open pore structure holds water by capillarity while retaining air channels between the fibres. How much of each the roots get depends on how the substrate is irrigated. Peer-reviewed research on stone wool used in commercial horticulture reports that root-zone water content in stone wool varies widely with drainage design — a median of about 75% by volume (first-to-third quartile roughly 70–80%) under low-drainage conditions versus about 26% under high drainage — and that reducing the root-zone volume lowers its buffering capacity, so it requires more stringent management of system parameters. In practice that means the volume and frequency of your feed directly govern how much air the roots reach. Because the medium buffers pH and EC weakly, nutrient-solution chemistry acts quickly on the root zone, so monitoring matters more than it does with organic media.
Compatibility and operating range
- pH: the maker’s published nutrient-solution operating range is 5.5 – 6.5.
- EC at propagation: Grodan’s guidance gives 1.5 EC (mS/cm) as the nutrient-solution strength for the propagation stage.
- Irrigation trigger: the maker directs beginning fertigation once the plug’s wet weight reaches 50% of its saturated weight, which avoids both waterlogging and drought stress in propagation.
- Cutting age: for cuttings, the maker limits insert age to no more than 14 days old.
- Inorganic and clean: because stone wool is mineral-based and pathogen-free at production, with no carbon-based organic matter for microbes to feed on, it carries no native biology and does not buffer chemistry the way peat or coco can — introduce any beneficial inoculant separately if you want it.
How it compares to other starter media
Several media are used for germination and cutting propagation. This table is factual — it does not claim any one out-performs another; which suits your operation and transplant system is a grower’s decision.
| Factor | Grodan A-OK plug (stone wool) | Peat/perlite plug | Coco coir plug |
|---|---|---|---|
| Material | inorganic stone wool (mineral fibre from basalt) | organic peat blend with perlite for aeration | organic coconut coir fibre |
| Cleanliness / pathogen load | pathogen-free and residue-free at production; no carbon-based organic matter for microbes to feed on | typically pasteurised but not fully sterile | typically low microbial load but not fully sterile |
| pH operating range | 5.5 – 6.5 (maker’s published range) | grower-managed; peat is acidic and usually needs buffering | grower-managed; coco is near-neutral but can vary |
| Packaged volume | 2,940 plugs per case | varies by brand and pack | varies by brand and pack |
Where it fits
The A-OK plug is the propagation stage in a stone wool production line, and this case suits growers who start large plug batches each cycle. For other starter plugs, propagation media, and containers, browse the rockwool starter plugs category. Growers comparing plug media often also look at iHort Q-Plug Cubes as a bound-media alternative, and pair propagation with a SunBlaster LED Mini Greenhouse Kit for light over the tray or Jump Start Soil Heating Cables for bottom heat.
Frequently asked questions
How many plugs are in this case? The case holds 2,940 plugs per case, supplied as 30 sheets of 98 plugs.
What are the exact plug dimensions? The maker’s published measured size is 1.4 × 1.4 × 1.6 in (36 × 36 × 40 mm) — a little under the round-number name the format is sold under.
What pH and EC should my nutrient solution be? Grodan’s published range is pH 5.5 – 6.5 at 1.5 EC (mS/cm) for propagation.
When do I water? Begin fertigation once the plug’s wet weight reaches 50% of saturated weight.
Who this is for — and who should skip it
Suited: commercial and semi-commercial growers running stone wool propagation who want a uniform, clean starter plug that seats in standard trays and start large plug batches each cycle. The full-case format — 30 sheets, 2,940 plugs — is aimed at operations moving meaningful propagation volume.
Not for / its limits: hobby growers who need only a sheet or two per cycle will find a full case far more than a small operation requires. Because stone wool is mineral-based and pathogen-free at production, with no carbon-based organic matter, it carries no native biology and needs active pH and EC management — growers used to peat or coco should expect a shorter window between parameters drifting and roots showing stress. Stone wool is not an organic-certified input, so operations requiring certified-organic media should not use it.
HELD — could not source, not stated as fact in page content: any warranty term (held per M-05); the live retail price (the storefront price field is the source of truth at deploy); shelf life or storage guidance; OMRI or certified-organic status (not found on Grodan’s published material); the exact water-to-air fill percentage at saturation for this plug size; fibre-density values; the out-of-bag rinsate pH for this plug format; a numbered tray size such as a ten-by-twenty count (Grodan states the sheet fits its Gro-Smart tray inside a standard propagation dome, rather than giving a numbered tray size); the receiver-hole plug-in transplant geometry into a specific Gro-Block (the maker AO page fetched this session states seed propagation but does not restate the receiver-hole transplant detail); per-crop germination or propagation success rates; and any yield or performance claim. Identity note: this listing’s slug uses the round-number nominal name, while Grodan’s published measured size for the A-OK format is slightly smaller than that nominal name — the round number is the product name, and the measured dimensions are given above.
Ready to add it to your propagation workflow? Add to cart, or talk to a grower about whether the A-OK case format fits your system and volume.
Sources
- Grodan101 — Grodan Starter Plugs product page (A-OK 1.5 in: measured plug dimensions 1.4 × 1.4 × 1.6 in / 36 × 36 × 40 mm; 98 plugs per package/sheet; 2,940 plugs per case; nutrient solution 1.5 EC, pH 5.5–6.5; cutting no more than 14 days old inserted ~½ in into the saturated plug; fertigation trigger at 50% of saturated wet weight; the 1.5 in A-OK plugs fit snugly into the Gro-Smart tray inside a standard propagation dome; finely woven stone wool with uniform density, firmness and fibrous structure; HTTP 200, retrieved 2026-07-08): https://www.grodan101.com/product-overview/grodan-starter-plugs/
- Grodan — AO Plug product page (tapered stone wool plugs on a sheet for propagating plants from seed; retrieved 2026-07-08): https://www.grodan.com/global/solutions/product-overview/vegetable-solutions/grodan-ao-plug/
- Grodan — Introduction to Stone Wool (made from basalt / solidified lava; mineral-based, not carbon-containing organic matter; pathogen-free and residue-free at production; much less likely than carbon-based organic media to be contaminated by fungi, yeasts and bacteria; retrieved 2026-07-08): https://www.grodan.com/about/introduction-to-stone-wool/
- Kim, Y. et al. — stone wool substrate cover-incision study on root-zone water content and buffering capacity (peer-reviewed, PMC, 2022): median root-zone water content 75% by volume (Q1–Q3 70–80%) under low-drainage conditions vs. 26% under high drainage; reducing root-zone volume lowers buffering capacity and requires more stringent management (HTTP 200, retrieved 2026-07-08): https://pmc.ncbi.nlm.nih.gov/articles/PMC9218559/
Reviews
There are no reviews yet.