The short answer: both substrates grow commercial crops well, and the trials that compare them head to head show coir matching or slightly beating rockwool in tomato, cucumber and strawberry. Rockwool (stone wool) is an inert mineral fibre: it holds a lot of water, has no cation exchange capacity, gives full control of the root zone, but does not break down and much of it still ends in landfill. Coco coir is an organic fibre with a moderate cation exchange capacity: it holds and exchanges ions, needs washing (and sometimes buffering) before use, and can be composted afterwards. Choose rockwool when you want a blank, repeatable medium and have a recycling route. Choose coir when you want a more forgiving root zone, lower disposal cost, or an organic-compatible medium.
Key facts
- Tomato yield in the same trial: 84.9 t/ha on coir vs 67.5 t/ha on rockwool (Xiong et al., 2017). Cucumber: 7.7% higher yield per plant on coir (He et al., 2022).
- Stone wool is basalt and limestone melted at about 1,500 °C; 5% fibre, 95% pores, no cation exchange capacity (Grodan).
- Grodan reports a 65% worldwide recycling rate for 2022 and says landfill is still often "the cheapest and easiest way" to dispose of stone wool (Grodan recycling FAQ, 2023).
What each substrate is
Rockwool, sold for horticulture as stone wool, is made by melting basalt and limestone at about 1,500 °C, pouring the melt onto spinning discs that fling it into fibres, pressing the fibres into a mat with a binder, and cutting slabs, blocks and plugs. Grodan describes the result as 5% stone fibre and 95% pore space, pathogen-free from the furnace, with no cation exchange capacity. Slabs ship dry in a plastic sleeve and must be saturated before planting.
Coco coir is the pith and short fibre left when coconut husks are processed for long fibre, a by-product produced mainly in India and Sri Lanka. For greenhouse use it is washed to lower salts, sometimes buffered with a calcium solution, dried and compressed into blocks, slabs and grow bags that expand on first irrigation. Products range from fine pith to blends with husk chips, which changes the water and air balance. Our grow bags and open-top slabs use washed, unbuffered coco peat and custom blends.
Water holding and air porosity
The one peer-reviewed study that measured both substrates in the same lab (Xiong et al., tomato, 2017) found them closer than the marketing suggests:
| Property | Rockwool | Coir |
|---|---|---|
| Bulk density (g/cm³) | 0.08 | 0.20 |
| Total porosity (%) | 89.2 | 85.6 |
| Air porosity (%) | 5.1 | 5.6 |
| Water porosity (%) | 84.1 | 80.0 |
| pH | 6.5 | 6.1 |
| EC (dS/m) | 0.06 | 0.10 |
Source: Xiong et al., Frontiers in Plant Science, 2017. Air porosity is low in both because it was measured at saturation; in the greenhouse the slab drains and air content rises.
Rockwool holds slightly more water and holds it loosely: the fibres do not absorb water, so it sits in the pores and drains quickly. That is why rockwool responds fast to each irrigation and dries back fast. Coir holds a little less total water, but part of it is bound inside the pith particles and released more slowly. He et al. (cucumber, 2022) likewise reported higher aeration porosity and lower water-holding porosity in coir.
Slab height matters more in rockwool. Van der Gaag and Wever (2005) found 7 cm rockwool slabs more conducive to Pythium root rot of cucumber than coir dust, pumice or perlite, and tied this to rockwool's much higher water content rather than temperature or oxygen; raising the slab to 14 cm greatly reduced disease. In coir, growers adjust wetness by blend instead: more chips for a drier, more generative slab, more pith for propagation and hot summers.
Buffering and cation exchange: inert vs active
This is the biggest difference between the two, and it drives everything about feeding.
Rockwool has no cation exchange capacity (CEC). Grodan states that its fibres "will not chemically bind fertilizer elements". Whatever is in the drip solution is what the roots see, within hours. That gives precise control, and means mistakes arrive at the root just as fast.
Coir has a moderate CEC. Published figures vary widely with coir grade and test method: trade and extension sources quote anywhere from about 10 to over 100 meq/100 g, and Griffin Greenhouse Supplies puts coir at roughly half the CEC of peat. The behaviour matters more than the number. New coir arrives with its exchange sites loaded with potassium and sodium. When a calcium-rich feed runs through, calcium displaces some of that potassium and sodium into the root zone and is itself held back for a while. So coir either needs a calcium buffering step before use, or a first-weeks feed with extra calcium and magnesium and less potassium. Washing versus buffering, and how to read an EC spec, are covered in buffered vs unbuffered coco peat.
The upside of CEC is stability. In the sweet pepper trial by An et al. (2009), root-zone EC stayed within 3.0 to 5.0 dS/m in cocopeat but ranged 3.0 to 8.0 dS/m in rockwool under the same irrigation treatments. He et al. (2022) measured higher calcium, magnesium and sulphur in cucumber leaves and fruit on coir. The downside is that coir can also hold the wrong things: sodium and chloride from poorly washed material, or salts that accumulate when drain is too low.
pH behaviour
Rockwool does not buffer pH, so root-zone pH follows the drip solution and plant uptake. Grodan and Hort Americas both recommend saturating new stone wool with nutrient solution adjusted to pH 5.5. Grodan's handling guide adds that slabs should be saturated at least 48 hours before planting and left full for at least 24 hours to reach full capillary capacity, and never planted dry.
Coir is slightly acidic: pH 6.1 against 6.5 for rockwool in the Xiong trial, and Abad et al. (2002) found coir dust from 13 origins slightly acidic across the board. Because coir has some buffering, root-zone pH drifts more slowly, but high-bicarbonate irrigation water can still push it, so drain pH should be checked on the same schedule as drain EC.
Irrigation strategy
Both substrates are drip-irrigated to a drain target. The targets and shot pattern differ.
Rockwool. Wageningen UR's tomato guide recommends irrigating by radiation sum at 2 to 3 ml per J/cm², "the smallest possible quantity with the highest possible frequency (at least more than 10x per day)", and a drain of 15 to 30%: below 15% root-zone EC climbs, above it water and fertiliser are wasted. Papadopoulos et al. (2008) found that starting irrigation at 70 to 80% slab water content gave the highest root growth and marketable yield in tomato on rockwool. Grodan has published that a daily water-content swing under about 8% steers the plant vegetative and 10% or more steers it generative, which is why rockwool growers use dry-back as a steering tool.
Coir. Coir runs on the same radiation-sum logic but usually needs fewer, slightly larger shots, because it releases water more slowly and holds EC more steadily. An et al. (2009) maintained 65% slab water content with 112 ml per plant per event in cocopeat against 150 ml in rockwool. He et al. held both substrates at about 20% daily drain with 100 ml shots every 80 to 100 J/cm². Dutch Plantin reports that its double-layer coir slabs needed 10 to 20% less water and feed than rockwool slabs of similar size for the same tomato yield; that is a supplier-commissioned trial, but it points the same way as the independent data. Coir's slower dry-back makes it harder to steer generatively through water content alone; growers do it through EC and blend choice instead.
Griffin Greenhouse Supplies' advice for growers moving from rockwool to coir is to expect to modify the irrigation strategy and probably the fertility programme. A sensor in the slab, a calibration bucket under one dripper and a week of adjusting shot size until drain lands on target is the normal route.
| Irrigation variable | Rockwool | Coir |
|---|---|---|
| Typical drain target | 15–30% (WUR tomato guide) | ~20% in trials; 10–25% commonly cited |
| Shot pattern | Small, very frequent (10+ per day) | Fewer, moderately larger |
| Water to hold 65% slab WC (pepper) | 150 ml/plant/event | 112 ml/plant/event |
| Root-zone EC range, same trial | 3.0–8.0 dS/m | 3.0–5.0 dS/m |
| Dry-back as steering tool | Strong, fast | Weaker, slower |
| First weeks | Saturate at pH 5.5 with nutrient solution | Extra Ca and Mg, less K unless buffered |
Root-zone temperature
Direct comparisons are scarce. Jo and Shin (2022) grew strawberries on coir and rockwool with and without root-zone heating and reported that heat was better maintained in coir than in rockwool. Grodan warns that root-zone temperatures above 26 °C raise Pythium risk in stone wool and advises saturating slabs the night before planting in hot weather so the solution does not heat up. Van der Gaag and Wever measured substrate temperatures above 30 °C for more than six hours on sunny June days in both media. Neither insulates strongly; coir's higher bulk density gives it a little more thermal inertia, an advantage under root-zone heating and a mild disadvantage when you want to cool the slab quickly.
Reuse and disposal
Rockwool does not decompose. Grodan's recycling FAQ says that "in many cases, the cheapest and easiest way to dispose of the product is via landfill" because of infrastructure, logistics and volume constraints. It reports a 65% worldwide recycling rate for 2022 through partners, lists brick manufacture, compost, potting mix and engineered soils as outlets, and says it composts over 5,000 tons a year in North America. The material is classed as non-hazardous solid waste. The route depends on where you are: Vineland Research (Henry and Grygorczyk, 2026) estimates that 10 to 15% of the roughly 150,000 tonnes of greenhouse waste landfilled each year in Ontario's Leamington region is stone wool from vegetable production, with land application and composting pathways still being built.
Coir is compostable. Once the sleeve is removed, spent coir can be composted, added to potting mixes or spread on fields. Hort Innovation's project on coir waste from hydroponic berries (MT17016, 2018) found on-farm reuse as a soil amendment without composting suitable, on-farm composting effective when co-composted with a high-nitrogen material such as manure, and regional organic recyclers often the most cost-effective outlet. It also warns that pests and pathogens can survive in spent substrate and may need treatment before reuse in a crop.
The plastic sleeve is the same problem for both. Grodan notes that biodegradable films are not yet suitable for greenhouse conditions and are often refused by composters.
Cost considerations
Prices vary too much by region, volume and product to quote figures that would stay true, so here is the structure of the comparison:
- Purchase and freight. Coir slabs are generally cheaper per litre and ship compressed; stone wool ships at full volume.
- Volume per m². In the Caté trials in Brittany (Grodan, 2021 to 2023), tomatoes used 8.43 L/m² of stone wool against 11.08 L/m² of coir. Coir growers typically use more volume per plant, which narrows the price gap.
- Preparation. Rockwool needs pH 5.5 saturation and 24 to 48 hours of lead time. Unbuffered coir needs a buffering step or a modified early feed; buffered coir costs more up front and saves that work.
- Disposal. Tipping fees and haulage for stone wool are a real annual line for large greenhouses. Composted or land-applied coir can be close to zero cost.
- Yield. Independent trials favour coir slightly; the supplier-run Caté trial favoured stone wool by about 6%. Both are within normal grower-to-grower variation, so run your own comparison bay.
When growers choose each
Rockwool is the usual choice when the grower wants a fully inert, repeatable medium with fast root-zone steering through water content; a recycling or takeback route exists nearby; propagation is already in stone wool plugs and blocks; or the whole growing programme is written for stone wool.
Coir is the usual choice when disposal is expensive or landfill restricted; the grower wants a buffered root zone that forgives irrigation gaps, hot days or short pump failures; the operation is organic or wants the option (washed, unbuffered coir only, since calcium-nitrate-buffered coir is not accepted in organic production); or the crop is one where coir has a strong record, such as tomatoes, cucumbers, peppers and soft fruit, across a range of hydroponic systems.
Kagome, Japan's largest hydroponic tomato grower, began on rockwool and moved entirely to coir slabs, after early all-pith slabs made the crop too vegetative and a blended coir product fixed it (HortiDaily). Coir is not one material; the blend has to match the crop.
Side-by-side summary
| Rockwool (stone wool) | Coco coir | |
|---|---|---|
| Origin | Basalt and limestone melted at ~1,500 °C | Coconut husk pith and fibre, by-product |
| Structure | 5% fibre, 95% pore space | Organic particles, blendable with chips |
| Total porosity | ~89% | ~86% |
| Water held at saturation | ~84% | ~80% |
| CEC | None | Moderate; holds K, Na, Ca |
| Starting pH | Follows solution; saturate at pH 5.5 | Slightly acidic (~6) |
| Starting EC | Very low (0.06 dS/m) | Low after washing (0.10 dS/m in trial) |
| Pre-treatment | Saturate 24–48 h before planting | Washing (by supplier); buffering or Ca-rich start |
| Root-zone EC stability | Lower | Higher |
| Dry-back steering | Fast and strong | Slow and weak |
| Pythium risk at low slab height | Higher (7 cm slabs) | Lower |
| Root-zone heat retention | Lower | Higher |
| End of life | Landfill or recycling programme; does not degrade | Compost or soil amendment |
| Organic production | Not applicable | Washed, unbuffered coir only |
| Independent trial yield | Equal or slightly lower | Equal or slightly higher |
If you are specifying coir for the first time, buy to a written EC specification with the test method stated, pick the blend for the crop and climate, and plan the first three weeks of feed around calcium with drain analysis twice a week. For grow bags, open-top slabs or custom blends in washed, unbuffered coco peat, contact us with your crop, slab size and target EC.
FAQ
Is coco coir better than rockwool for tomatoes?
Independent trials have come out level or slightly in favour of coir. Xiong et al. (2017) recorded 84.9 t/ha on coir against 67.5 t/ha on rockwool with heavier fruit on coir, while a supplier-run trial in Brittany found stone wool about 6% ahead. Both results sit within normal grower-to-grower variation, so the choice usually comes down to disposal, cost and how you want to manage the root zone.
Can I use the same irrigation schedule on coir as on rockwool?
Not without adjusting it. Coir holds and releases water more slowly, so it usually needs fewer, slightly larger shots to reach the same drain percentage; in one pepper trial, 112 ml per event held 65% water content in cocopeat where rockwool needed 150 ml. Put a moisture sensor in the slab and a calibration bucket under one dripper for the first week and adjust shot size until drain lands on target.
Why does coir need buffering and rockwool does not?
Rockwool has no cation exchange capacity, so nothing is held on the fibre and nothing is released. Coir has exchange sites that arrive loaded with potassium and sodium; calcium in the feed displaces them into the root zone during the first weeks. Buffering swaps those ions for calcium before the coir reaches you; the alternative is a calcium-rich, lower-potassium feed at the start with regular drain analysis.
What pH should rockwool be soaked at?
Grodan and Hort Americas both recommend saturating new stone wool with nutrient solution adjusted to pH 5.5. Grodan's handling guide says to saturate slabs at least 48 hours before planting and leave them full for at least 24 hours so the fibre reaches its full capillary capacity. Never plant onto a dry slab.
How do you dispose of used rockwool slabs?
Spent stone wool does not decompose. Grodan lists brick manufacture, compost, potting mix and engineered soils as recycling outlets and reports a 65% worldwide recycling rate for 2022, but also says that in many regions landfill remains the cheapest and easiest option because of logistics and volume constraints. The plastic sleeve has to be removed and handled as a separate waste stream whichever route you take.
Can spent coir be composted or reused?
Yes. Once the sleeve is removed, spent coir can be composted, mixed into potting media or spread on fields as a soil amendment; Hort Innovation's Australian project found on-farm reuse without composting suitable and co-composting with manure effective. Pathogens and pests can survive in used substrate, so treatment is needed before any reuse in a crop, and most growers replace slabs each crop.
Which substrate is more stable in root-zone EC?
Coir. In the An et al. (2009) sweet pepper trial, root-zone EC stayed between 3.0 and 5.0 dS/m in cocopeat but ranged from 3.0 to 8.0 dS/m in rockwool under the same irrigation treatments. The exchange capacity that makes coir need buffering is also what smooths out EC swings.
Is rockwool or coir better in hot climates?
Both can overheat in the slab; one Dutch study measured substrate temperatures above 30 °C for more than six hours on sunny days in both. Rockwool's low mass and quick drainage mean it heats and cools fast, and Grodan warns that root-zone temperatures above 26 °C raise Pythium risk. Coir's slower water release and higher bulk density give the plant a little more margin on a hot afternoon, but neither substrate replaces shading and good drain management.
Sources
- Xiong, J., Tian, Y., Wang, J., Liu, W. & Chen, Q.: Comparison of Coconut Coir, Rockwool, and Peat Cultivations for Tomato Production: Nutrient Balance, Plant Growth and Fruit Quality. Frontiers in Plant Science (2017)
- He, L., Ding, X., Jin, H. et al.: Comparison of rockwool and coir for greenhouse cucumber production: chemical element, plant growth, and fruit quality. Heliyon (2022)
- An, C. G., Hwang, Y. H., Shon, G. M. et al.: Effect of Irrigation Amount in Rockwool and Cocopeat Substrates on Growth and Fruiting of Sweet Pepper During Fruiting Period. Horticultural Science and Technology (2009)
- Jo, W. J. & Shin, J. H.: Effect of root-zone heating using positive temperature coefficient film on growth and quality of strawberry in greenhouses. Horticulture, Environment, and Biotechnology (2022)
- Van der Gaag, D. J. & Wever, G.: Conduciveness of different soilless growing media to Pythium root and crown rot of cucumber under near-commercial conditions. European Journal of Plant Pathology (2005)
- Papadopoulos, A. P., Saha, U., Hao, X. & Khosla, S.: Irrigation management in greenhouse tomato production on rockwool. Acta Horticulturae 779 (2008)
- Abad, M., Noguera, P., Puchades, R., Maquieira, A. & Noguera, V.: Physico-chemical and chemical properties of some coconut coir dusts for use as a peat substitute for containerised ornamental plants. Bioresource Technology (2002)
- Blok, C., van Os, E. & Kromwijk, A. (Wageningen UR Greenhouse Horticulture) with EcoConsult: Cultivation of Tomato, Technical information sheet No. 8 (undated)
- Grodan: Frequently Asked Questions, stone wool production and inertness (web page, accessed 2026)
- Grodan: Stone Wool Recycling FAQs (2023)
- Grodan: 3-3 Handling and placing of the slabs, grower factsheet (undated)
- Grodan: Caté's extensive comparative trials with meaningful insights for tomato cultivation (2023)
- Grodan, Acta Horticulturae: water content steering in Grodan Master slabs (ISHS paper)
- Hort Americas, "Tech with Dr. Steve": Starting Seeds in Grodan (undated)
- Henry, J. & Grygorczyk, A. (Vineland Research and Innovation Centre): Turning waste into value: the promise and challenges of greenhouse substrate reuse in Canada, first published in Greenhouse Canada (2026)
- Hort Innovation / RMCG: Coir waste management for hydroponic berries, project MT17016 (2018)
- Van Gaal, T. (Griffin Greenhouse Supplies), HortiDaily: Hydroponic media: give slab production a second look (2017)
- HortiDaily: Japan: Kagome optimizes coco mix to achieve balanced growth
- Fruitnet / Fresh Produce Journal: Coconut coir holds tomato solution (Dutch Plantin DP Optima trial at PlantResearch)