The short answer: both crops do well in washed coco coir slabs or grow bags of 15–20 L per metre, but you run them very differently. A cucumber is in fruit three weeks after planting and drinks hard, so you feed it at EC 2.0–2.5 mS/cm in frequent small shots with 10–25% drain and watch for wilting under fruit load. A sweet pepper takes about 20 weeks from seed to first pick, wants a feed of 2.5–3.0 mS/cm with the root zone at 3.5–4.0, fewer and smaller shots, and drain that starts near 5% and rises to about 30% in summer. In both, keep pH at 5.5–6.2 and push calcium in the first weeks, because unbuffered coir takes it out of the first feeds.
Key facts
- Cucumber on a 50:50 fine-and-coarse coir slab yielded 7.7% more per plant than on rockwool at 2.8 plants/m².
- Pepper on coco slabs held a root-zone EC of 3.0–5.0 mS/cm where rockwool swung from 3.0 to 8.0 under the same irrigation.
- Cucumber: 45 days from seed to first harvest in summer, 60 in winter. Pepper: about 20 weeks, then 7–9 weeks from fruit set to harvest.
- Pepper blossom-end rot fell from 21.5% to 12.6% of fruit when the feed was diluted by about a quarter.
Why both crops suit coir
Cucumbers and peppers sit at opposite ends of the greenhouse fruit crops. One grows fast, carries fruit for weeks at a time and is replaced two or three times a year. The other is slow and woody and sits in the house for a year. Coir handles both because it holds a lot of water and still drains, and because it buffers the root zone against the mistakes each crop punishes.
For cucumber, the evidence is a direct comparison on 1 m slabs at the same feed, light and irrigation. The coir slab was 100 × 20 × 8 cm, half 0–6 mm pith and half 10–20 mm chips, EC under 1 mS/cm. Plants on it had a larger leaf area from day 16, and yield per plant was 7.7% higher than on rockwool. Leaves on coir carried more calcium and magnesium at the end of the crop, not less.
For pepper, the useful finding is stability. On cocopeat slabs the root-zone EC stayed between 3.0 and 5.0 mS/cm across shot sizes from 75 to 150 ml per plant. On rockwool under the same regime it ranged from 3.0 to 8.0. Peppers on coco grew better, set larger fruit and did it with less water per shot: 112 ml held 65% water content in coco where rockwool needed 150 ml.
The chemistry to manage is the same as for tomatoes. Washed, unbuffered coir carries potassium and sodium on its exchange sites and takes calcium and magnesium from the first feeds. Our article on buffered vs unbuffered coco peat explains what washing and buffering each do.
Slab and bag sizes, mixes and plant density
| Cucumber | Sweet pepper | |
|---|---|---|
| Unit | 1 m slab or bag, 15–20 L | 1 m slab or bag, 16–20 L |
| Mix | 50:50 pith and chips in the trial above. 70:30 is common from suppliers | 100% pith or 60:40 pith to chips gave the best yields |
| Plants per unit | 2–3 | 3–6, depending on row spacing |
| Density | 1.25–1.5 plants/m², often twin-headed to about 3.2 stems/m² by spring | 3.0–3.5 plants/m² at 2 stems, or 2.5 plants at 3 stems, for 6.5–7.5 stems/m² |
| Coir per plant | 5–8 L | 3–7 L |
Pepper tolerates a small root volume. When 7, 14 and 21 L of coir per plant were compared, volume made no difference to yield, and the 7 L bag used 13% less water over the season. Six plants on a 20 L coir bag is standard Canadian practice. Cucumber is the opposite. It drinks so hard in summer that a small volume runs dry between shots, so keep it to three plants per 1 m unit.
Mix matters differently for each crop. Cucumber wants the open 50:50 blend so the slab re-oxygenates between frequent shots. Pepper gave its highest yields on pure pith and on 60:40, but the 60:40 mix also showed the most blossom-end rot late in the season. The lesson is that peppers on a chip-heavy mix need more frequent irrigation to keep the water content steady, not that chips are wrong.
Both formats are available as grow bags and open-top slabs, with the pith-to-chip ratio set to your order.
Pre-saturation
The procedure is the same as for tomatoes, and the tomato article has the full step-by-step: fill, stand 24 hours, flush if drain EC is high, cut slits low, then charge with nutrient solution the day before planting. The only crop-specific choice is the EC you charge at.
- Cucumber: your young-plant feed EC, around 2.0–2.5 mS/cm. The plant needs to root out fast, and a slab saltier than the block holds it back.
- Pepper: 2.5–3.0 mS/cm. The young pepper is steered generative from the start, and a root zone of 3.0–3.5 at planting is normal. Growers lay pepper blocks on their side so the stem base stays drier.
A slab with drain EC well above the charging solution wasn't washed enough. Flush again before planting rather than fighting it for six weeks.
Irrigation: the fast crop and the slow crop
Cucumber needs more water, more often. Pepper needs less, less often, and gets into trouble when it's kept too wet.
Cucumber. A common rule is 150–200 ml per m² for every 100 J/cm² of radiation. In the coir-versus-rockwool comparison each plant got one 100 ml shot per 80–100 J/cm², with drain near 20%. A 30 × 96 ft house uses 500–1,000 US gallons a day in summer, roughly 3–8 L per plant. Run the first shot soon after sunrise and the last 1–2 hours before sunset. Later than that builds root pressure overnight, which shows up as leaf-edge damage and vertical scars on fruit. If fruit look dark in the morning, yesterday's irrigation was short: add late-day shots or one or two night shots of 100–150 ml.
Drain is less of a lever in cucumber than in tomato. In coconut fibre, 10% and 30% drain gave the same yield and fruit count in wet and dry seasons, and 10% gave higher sugars. Run 10–20% and raise it only if drain EC climbs.
Pepper. Shots are small, 75–112 ml per plant on coco slabs, and fewer. Start half an hour after sunrise, aim for first drain around 10 am, and stop 1–1.5 hours before sunset. Drain starts at about 5% in the first weeks, when the crop is small and you're steering it generative, and climbs to about 30% by summer. In coconut fibre, 30% drain gave more fruit and more first-grade yield than 10% in a dry season, while 10% was enough in a wet one.
Pepper water use in summer is about 3.5–4.0 L per plant per day on a northern crop. A tropical crop in coir bags peaked at 1.2 L per plant, about 4.9 L/m² at 2.4 plants/m². The gap is light and leaf area, so size irrigation to your own radiation sum.
| Cucumber | Sweet pepper | |
|---|---|---|
| Shot size | about 100 ml per plant | 75–112 ml per plant |
| Trigger | 80–100 J/cm² per shot | fewer shots, steered by slab water content |
| Last shot | 1–2 h before sunset | 1–1.5 h before sunset |
| Drain | 10–25%, 10–20% typical | 5% early, about 30% in summer |
| Peak use | roughly 3–8 L per plant per day | 3.5–4.0 L per plant per day |
EC and pH targets by stage
Cucumber. Standard advice is feed EC no higher than 2.2 mS/cm and root zone no higher than 3.0, pH 5.5–6.5. Extension figures for the feed run 2.0–2.5. In a coir-bag winter crop the feed was held near 3.0 before fruit set to keep the plant compact, then dropped to 2.0–2.3 once fruit set, with daily volume rising as fruit swelled. Under stress, feed at 1.8–2.5 with the bag at about 2.8.
| Cucumber stage | Feed EC (mS/cm) | Root zone EC | Notes |
|---|---|---|---|
| Planting to first fruit set (about 3 weeks) | 2.2–3.0 | up to 3.0–3.5 | higher in low light to hold the plant back |
| Fruit set to first harvest (7–10 days) | 2.0–2.5 | about 2.8–3.0 | raise volume, not EC |
| Full harvest | 1.8–2.5 | up to 3.0 | drop toward 1.8 under heat or heavy load |
Pepper. Pepper runs saltier all season. Blocks go onto the slab at a root-zone EC of 3.0–3.5. In the first weeks the feed is 2.5–3.5 and the root zone is allowed to rise to 4.0–4.5, then eased back to 3.5–4.0 as light improves. For the rest of the crop the feed is 2.5–3.0 to hold the root zone at 3.5–4.0. A Spanish autumn-winter crop ran slab EC 3.2–3.5 with 35% drain at about EC 2.5. The general soilless figure of 2.2 for pepper feed is a low-light starting point, not a season target.
| Pepper stage | Feed EC (mS/cm) | Root zone EC | Notes |
|---|---|---|---|
| Charging the slab | 2.5–3.0 | – | 24 hours before planting |
| First 4–6 weeks | 2.5–3.5 | 4.0–4.5 | generative steering, 5% drain |
| Setting and filling fruit | 2.5–3.0 | 3.5–4.0 | drain rising with light |
| High summer | 2.5 or a little below | 3.5 | higher EC means more blossom-end rot |
Keep both crops at a feed pH of 5.5–6.2. In coir, drain pH drifting down is a salt signal rather than an acid-dosing error, and the fix is more drain.
Calcium and magnesium in unbuffered coir
Everything in the tomato article about the first four weeks applies here: raise calcium and magnesium above your standard recipe, trim potassium, and keep ammonium below 10% of nitrogen until the exchange sites have charged.
Cucumber recipes already carry 150–300 ppm calcium against 300–500 ppm potassium, and the coir comparison ended with higher leaf calcium on coir than on rockwool. The early dip is real but short.
Pepper is where you see calcium trouble. In bell pepper, 150 mg/L calcium in the feed and 12 irrigations a day gave the lowest blossom-end rot, and the frequent irrigation mattered most on coarse substrates. A tropical coir crop ran 140 ppm calcium at establishment rising to 181 ppm in production. Root-zone temperature drives it too: a warmer root zone gave more calcium-deficient fruit than any other factor tested, and the Canadian target is 20 °C with problems above 23 °C. Start the calcium high and don't let the slab warm up.
Common problems
| Problem | Crop | What you see | Likely cause in coir | Fix |
|---|---|---|---|---|
| Blossom-end rot | Pepper | Sunken tan or black patch at the fruit tip | High EC or sodium, water-content swings between shots, warm root zone, low calcium in the first weeks | Feed EC at or below 2.5–3.0. More, smaller shots. Ca at 150 ppm or above. Raise drain if sodium builds. 400 ppm calcium chloride spray if it has started |
| Wilting under fruit load | Cucumber | Upper leaves wilt by day and recover at night, often when dull weather turns bright or at first heavy set | Roots can't keep up with leaves: root loss from Pythium, or too many fruit | Cut fruit load fast, about one fruit per four leaves and no more than 4–6 on the main stem in spring. Feed EC 1.8–2.5. Keep the climate active on dull days. Check roots |
| Pythium root and crown rot | Both, mostly cucumber | Orange-brown soft crown, brown root tips, stubby roots, sudden wilt in warm sun | Saturated slab with low oxygen, root zone over 28 °C, low light, high salts, dirty water, fungus gnats and shore flies | Drain slits low, overnight dry-down, irrigation water at 18–22 °C, white drip lines, treat recirculated water, control flies. Remove infected plants with their bags |
| Root zone too warm | Cucumber | Large light-green patches on lower leaves in summer | Slab over 25 °C | Cooler irrigation water, shade, white film on the slab |
| Poor fruit set or flat fruit | Pepper | Flowers abort within a week, flattened fruit | Air above 27 °C, nights at or below 14 °C, crop too vegetative | Shade or vent. Hold nights above 14 °C. Drop night temperature 1 °C to steer generative |
Re-using slabs between crops
Coir slabs can carry a second crop, and pepper has been grown successfully on coir that was already two years old. Whether you should depends on what came before. Cucumber is the high-risk crop: Pythium oospores survive on dead roots in the substrate and in standing water, and a slab that carried crown rot should leave the house in a sealed bag. A clean pepper crop after a clean cucumber crop is a different matter. Flush the slab back to raw-water EC, check that it still drains, and treat it if you can. Our guide to reusing coco coir covers testing, steaming and spent coir.
For the greenhouse formats and blends, see coir for greenhouse crops. CVM supplies washed, unbuffered coco peat and husk chips as grow bags, open-top slabs and compressed blocks, blended to the customer's ratio. If you're planning cucumbers or peppers in coir, contact us to talk through formats.
FAQ
What EC should I run for cucumbers in coco coir?
Feed at 2.0–2.5 mS/cm and keep the root zone at or below 3.0. Many growers hold the feed near 3.0 until first fruit set, about three weeks after planting, then drop to 2.0–2.3 and raise the volume instead. Under heat or heavy fruit load, come down to 1.8–2.5 with the bag at about 2.8.
What EC should I run for peppers in coco coir?
Peppers run saltier than cucumbers. Charge the slab at 2.5–3.0 mS/cm, feed at 2.5–3.5 for the first weeks while the root zone rises to 4.0–4.5, then settle at 2.5–3.0 to hold the root zone at 3.5–4.0. In summer stay at the lower end, because higher EC increases blossom-end rot.
How many cucumber plants per coir slab or grow bag?
Two or three per 1 m unit of 15–20 L, giving 5–8 litres of coir per plant. High-wire density is typically 1.25–1.5 plants/m², with every fourth plant twin-headed as light improves. Cucumbers drink hard in summer, so don't go below about 5 litres per plant.
How many pepper plants per coir slab?
Three to six per 1 m bag of 16–20 L, at 3–3.5 plants/m² with two stems each. Pepper tolerates small volumes: 7 L per plant yielded the same as 21 L and used 13% less water. Six plants on a 20 L bag is standard practice in Canada.
What drain percentage is right for cucumbers and peppers in coco?
Cucumber: 10–25%, and 10% has matched 30% for yield in coconut fibre. Pepper: about 5% in the first weeks for generative steering, rising to around 30% in summer. In a dry season 30% drain gave peppers more fruit than 10%. Watch drain EC rather than fixing on a percentage.
Why do my cucumbers wilt at midday in coco coir?
Roots can't keep up with the leaves. If upper leaves wilt by day and recover at night, especially after dull weather turns bright or at the first heavy fruit set, suspect root loss from Pythium or too many fruit. Cut fruit load to about one fruit per four leaves, lower feed EC, keep the slab from waterlogging and check the roots.
How do I prevent blossom-end rot in peppers on coir?
Keep feed EC at or below 2.5–3.0 mS/cm, give many small shots so slab water content doesn't swing, run calcium at 150 ppm or above in the first weeks on unbuffered coir, and keep the root zone near 20 °C. Diluting the feed by a quarter cut blossom-end rot from 21.5% to 12.6% of fruit in one closed system.
Can I reuse a coir slab from cucumbers for a pepper crop?
Only if the cucumber crop was clean. Pythium survives on dead roots in the substrate, and a slab that carried crown rot should be disposed of. A clean slab can be flushed back to raw-water EC, checked for drainage and used again. Peppers have been grown on coir that was two years old.