The short answer: washed, low-EC coco peat is a workable substrate for growing minitubers from tissue-culture plantlets in a net house or greenhouse, and it is already the standard medium in several Indian seed programmes. In trials it produces as many or more tubers per plant than peat, but they come out smaller, and because coir carries almost no nitrogen you must fertigate from the first watering. Plant 66 to 100 plantlets per square metre, hill with fresh coir every week or two, hold the feed at EC 1.5–2.0, cut the haulm at about 90 days and let the bed dry before you lift.
Key facts
- EU pre-basic seed rules require minitubers to be produced in a protected facility and in a growing medium free from pests. Indian net houses moved from soil to cocopeat partly because soil limited them to one crop per season.
- In a glasshouse comparison, coir gave more tubers per pot than peat and the same total yield, but smaller, lighter tubers.
- Plantlets at 66 per m² in cocopeat beds gave 381 seed-size tubers (3–30 g) per m², against 189 at 22 per m².
- Weekly hilling changed tuber size but not tuber number. Feed held at EC 1.5–2.0 and pH 6.0–6.5.
- Steam-sterilised, refertilised substrate matched new substrate for minituber yield.
Why minitubers are grown in a soilless substrate
A minituber is the first tuber generation out of the laboratory. Virus-free plantlets are multiplied in vitro, hardened, then planted under insect-proof netting so aphids can't reinfect them. The point is health, so the substrate has to be clean. Pre-basic tissue-culture seed in the EU must be produced in a protected facility and in a growing medium free from pests, and lots must be free of rots, black scurf, common scab and powdery scab. Other national schemes use similar wording. Field soil can't meet that. It may hold Rhizoctonia, powdery scab spore balls, Fusarium, blackleg bacteria and nematodes, and there's no practical way to prove it doesn't.
Soil also costs you cycles. In northern Indian net houses the season runs from October to April, long enough for two crops, but programmes growing in soil took only one because a second crop in the same soil carried too much soil-borne disease risk. Cocopeat can be sterilised between crops, so the bed space gets used twice.
Peat was the default soilless medium because it is acidic, light, open and essentially pathogen-free. Coco peat is replacing it for three reasons: peat-free policy in Europe, local availability and price in India and East Africa, and decent trial results. The caveats are below. For what can and can't be in coir, see is coco coir sterile.
Coir against peat, sand and other media in trials
The systems differ (pots, trays, beds), so compare directions rather than absolute numbers.
| Comparison | What happened |
|---|---|
| Coir vs peat, glasshouse pots, two UK varieties | Coir produced more tubers per pot than peat and the same total weight. Individual tubers were smaller and lighter. Coir-grown plants showed chlorosis from week 12. |
| Coir vs peat, small controlled-environment microcosms | Coir alone gave pale, stunted plants that senesced by day 70. A 60:40 peat–coir blend stayed healthy to day 84. |
| Peat–perlite vs industrial substrate, in vitro plants, Romania | 7.8 minitubers per plant on the industrial substrate against 6.9 on peat–perlite. |
| Coconut fibre vs vermiculite vs autoclaved soil vs commercial mix, Brazil | Coconut fibre and the commercial mix were both judged viable. |
Two patterns matter. First, coir pushes tuber number rather than tuber size. Seed programmes usually want numbers, so that suits most people, but if your buyers want 20–35 mm tubers you'll manage size with density and hilling, not the substrate. Second, every poor coir result above came from nutrition, not the material. Coir tested with significantly lower nitrate, phosphorus, calcium and magnesium than the peat mix beside it, and higher potassium and sodium. Plants in lightly fed coir went pale and stopped. Plants in coir with a proper nutrient solution did not.
Processed coco peat beats raw product for this crop, which is the usual story: lowering salts pays. Washing lowers EC. Buffering also displaces the potassium and sodium held on the fibre, and whether you need it is covered in buffered vs unbuffered coco peat. For minitubers, well-washed coir with full fertigation from day one is the common route.
Beds, trays and substrate depth
Minitubers are grown in polythene-lined raised beds, HDPE troughs on benches, plastic trays or pots. Published set-ups include 700-micron HDPE troughs of sterilised cocopeat on raised benches in India, 51 × 34 cm trays lined with perforated black plastic in Zimbabwe with 5 cm planting holes and substrate added at each hilling, and 15 cm deep boxes holding 6 kg of substrate over a 10 cm drainage layer in Brazil.
Work on the basis that you need room for hilling. Start with 10 to 15 cm of coir and leave 5 to 10 cm of freeboard for the top-ups that follow. Beds need drainage holes or a fall to a drain. A saturated coir bed in a warm net house is how you lose plantlets to rot in the first fortnight.
The grade matters too. Fine, dusty pith holds a lot of water and packs down under repeated hilling and irrigation. Pith with some short fibre or a share of fine husk chips drains better and shakes off the tubers more easily at harvest. The trade-offs are in coco peat particle size grades.
Planting density
Density is your main lever. More plantlets per square metre gives more tubers per square metre and fewer, smaller tubers per plant. Both trials below used tissue-culture plantlets, not tubers.
| Plantlets per m² | Spacing | Seed-size tubers per plant | Seed-size tubers per m² | Setting |
|---|---|---|---|---|
| 22 | 30 × 15 cm | 8.6 | 189 | Cocopeat beds, net house, India |
| 44 | 15 × 15 cm | 6.5 | 287 | as above |
| 66 | 15 × 10 cm | 5.8 | 381 | as above |
| 100 | 13 per 51 × 34 cm tray | 7.0–7.9 (all sizes) | – | Soilless mix, greenhouse, Zimbabwe |
| 200 | 26 per tray | 3.1–3.4 | – | as above |
| 375 | 48 per tray | 1.5–2.3 | – | as above |
Seed size in the Indian work was 3–30 g, and nothing exceeded 30 g at any density. In the Zimbabwe trays, 375 plants per m² gave the most tubers under 20 mm and the worst survival. Both studies point the same way: 66 to 100 plantlets per square metre balances tuber count per square metre against usable size. For a 100 m² house that's 6,600 to 10,000 plantlets per crop.
Trim long adventitious roots, plant into pro-trays of fine cocopeat, harden for about 20 days, then move to the beds. Plantlets take two to three weeks to root in. Survival at 14 days in the trays above ran from 68 to 100 % depending on density and feed, so plan for some replanting.
Hilling and top-up
Stolons left in the light turn into stems. Covering them is what converts them into tubers, so hilling matters more in a shallow bed than in a field. In the Zimbabwe trays, four hillings with fresh soilless mix, starting two weeks after planting, were compared at weekly, fortnightly and four-day intervals. Interval changed tuber size, not tuber number. Weekly hilling with the higher-potassium feed gave 100 % survival and 7 tubers per plant at 100 plants per m².
Hill with fresh, sterilised coir of the same grade, 1 to 2 cm at a time. Plantlets buried too deep in one go die, and hand hilling at close spacing was the main cause of losses in that trial. Stop once the canopy closes.
Fertigation and EC
Coir comes with potassium and very little else. Plantlets in unfed coir yellow within weeks, so feed from the first irrigation after transplanting.
| Parameter | Range used | Context |
|---|---|---|
| Nutrient solution EC | 1.5–2.0 mS/cm | Cocopeat beds, India |
| Nutrient solution EC | 2.0–2.5 dS/m | Hydroponic minitubers on inert substrates, Romania |
| Solution pH | 6.0–6.5 | Cocopeat beds |
| Solution pH | 5.8–6.5 | Hydroponic substrates |
| Granular regime | 6:28:23 at planting, then sulphate of potash or ammonium nitrate at each hilling | Trays, Zimbabwe |
| Reused substrate top-up | 25 g of 04-14-08 per 15 × 40 × 50 cm box | Brazil |
A complete solution with calcium and magnesium matters more in coir than in peat, because coir supplies little of either and its exchange sites are loaded with potassium. Start at the low end of the EC range for the first two weeks, move up once plants are rooted, ease back in the last three weeks. Of the two Zimbabwe regimes, the one that supplemented potassium gave better survival and larger tubers than the one adding nitrogen at intervals. Run-off testing during the crop is the same job as for any coir crop and is covered in coco coir pH and EC management.
Irrigation, dry-back and harvest
Water little and often. Coir holds a lot of water and plantlets are shallow-rooted, so short drip or micro-sprinkler runs two or three times a day beat a long soak. Overhead sprinklers are common in net houses and work, but they wet the foliage.
Published crop durations agree: haulms cut at about 90 days in Indian net houses, 84 days in Zimbabwe, 16 weeks in the UK glasshouse. In each case water was withheld from haulm cutting and the tubers left in the dry bed for skin set, 15 days in the Indian programme. An earlier haulm cut gives more small tubers and fewer in the 20–35 mm class, so the date is a size tool as well as a calendar entry.
Lifting from coir is easy: tubers come out with little substrate attached, less than from bark or wool media. Grades differ by programme, so agree one with your buyer before planting.
| Programme | Size classes used |
|---|---|
| India (net house) | Below 3 g, 3–30 g seed size, above 30 g |
| Zimbabwe | Below 20 mm, 20–35 mm, above 35 mm |
| UK | Below 15 mm, 15–20 mm, 20–30 mm, above 30 mm |
| Brazil | Below 15 mm, 15–23, 23–30, 30–40, 40–50, 50–60 mm |
Undersize tubers are still viable seed and are recycled in the next cycle, though they shrink in store and the smallest may not survive. In the UK work no minitubers from any substrate showed scab, black scurf or silver scurf, and PCR found none of the target pathogens in coir-grown tubers.
Substrate reuse and hygiene
Fresh coir every cycle is the safest practice and what most inspectors want to see. Reuse is possible if you can sterilise. Brazilian producers steam-sterilised used substrate in insulated boxes to at least 100 °C, then added 25 g of 04-14-08 per box. Reused, refertilised substrate matched new substrate for yield in every size class. Reused substrate without added nutrients did not. An Indian programme drenched cocopeat with 3 % nano-silver hydrogen peroxide and washed it with clean water before planting.
Test reused material either way. Salts build up, pH drifts, and powdery scab survives a lot: one UK sample of a peat–coir blend turned up Spongospora DNA in a single tuber. General cleaning rules are in reusing coco coir, but for pre-basic seed the bar is higher than for a tomato crop. If you can't steam, don't reuse.
Hygiene around the bed matters as much as the bed: footbaths, disposable overshoes, no soil on tools, netting checked for tears, and a cleaned, empty house between crops.
Specifying coco peat for a seed-potato programme
- Washed, low-EC pith. Put the EC limit and the extraction method on the purchase order rather than accepting "low EC" on a label.
- Ask for the pH of the lot. Coir normally sits between about 5.6 and 6.9, and your feed will hold the root zone where you want it.
- Particle grade: fine pith for hardening trays, pith with some short fibre or fine chips for production beds.
- Compressed blocks if you are shipping any distance. A 100 m² house at 15 cm depth needs around 15 m³ per cycle, before hilling top-ups.
- Clean packing, wrapped pallets, no contact with soil in transit.
- A sample from the actual lot for your own EC and pH test before the full delivery.
CVM Fiber and Industries, in Kanyakumari, Tamil Nadu, India, supplies 5 kg blocks and 650 g briquettes of coco peat that is washed with water to reduce EC, with no calcium nitrate or other chemicals added, so it is unbuffered. Husk chips and custom pith-to-chip blends are available where you want a coarser grade for production beds. Tell us the bed volume, the grade and the EC limit your programme works to and we'll put together a specification: contact CVM.
FAQ
Why can't I grow minitubers in good field soil inside the net house?
Seed schemes require pre-basic tubers to be grown in a medium free from pests, and soil can carry Rhizoctonia, powdery scab, Fusarium, blackleg bacteria and nematodes that you cannot test out with certainty. Soil also limits you to one crop per season in the same bed. A clean soilless substrate lets you certify the crop and take two cycles from the same space.
Does coco peat give more or fewer minitubers than peat?
In direct comparisons coir produced the same or a higher number of tubers per plant and the same total weight, but the individual tubers were smaller and lighter. Trials where coir did badly were ones where it was fed as if it were peat. With a complete nutrient solution from transplanting, tuber counts are at least as good.
How many plantlets per square metre should I plant?
Between 66 and 100 per square metre is where trials in India and Zimbabwe found the best balance of tuber number and usable size. At 22 per square metre you get more tubers per plant but far fewer per square metre, and at 375 per square metre most tubers are under 20 mm and plantlet survival falls.
What EC should the nutrient solution be?
Indian cocopeat programmes hold the feed at EC 1.5 to 2.0 mS/cm and pH 6.0 to 6.5. Hydroponic minituber systems on inert substrates run a little higher, at 2.0 to 2.5. Start low for the first two weeks after transplanting and ease back in the last three weeks before haulm cutting.
How deep should the coir bed be?
Published systems range from trays with 5 cm planting holes to boxes 15 cm deep. Start with 10 to 15 cm of coir and leave 5 to 10 cm of freeboard, because you will add 1 to 2 cm of fresh coir at each hilling. Make sure the bed drains.
How often should I hill?
Weekly or fortnightly, starting two weeks after planting, for a total of four hillings before the canopy closes. Hilling frequency changes tuber size rather than tuber number. Add a thin layer each time, because burying small plantlets deeply is the main cause of losses.
When do I stop watering and harvest?
Haulms are cut at about 84 to 112 days depending on the programme, and water is withheld from that point. Leave the tubers in the dry bed for around two weeks for skin set, then lift. An earlier haulm cut gives more small tubers and fewer in the 20 to 35 mm class.
Can I reuse the coco peat for the next minituber crop?
Only if you can sterilise it, ideally with steam to at least 100 °C, and then refertilise it. Treated that way, reused substrate matched new substrate for yield in trials. Without sterilisation the risk to a pre-basic seed crop is too high, and many certification inspectors expect fresh medium for every cycle.