The short answer: coco coir suits potted culinary herbs because the crop is short, the pot is small, and the product has to survive a week or two of neglect on a shelf. Coir rewets after it dries out, is light to ship, and looks clean in the pot. Use a fine grade, sow thickly, run basil warmer and at a lower EC than the rest, and keep finished basil above 12 °C from the packing bench to the till. The one thing coir won't do is feed the crop. Everything the plant eats has to come from you.
Key facts
- Retail herb pots run 9 to 12 cm. A 4-inch basil pot is sown with 5 to 15 seeds and is saleable in 4 to 6 weeks.
- European pot-basil growers sow far denser: 25 to 30 plants in a 9 to 10 cm pot, 40 to 50 in a 12 cm pot, or 30 seeds in a 0.4 L pot.
- Basil is injured below 12 °C and shows leaf spotting below 16 °C in the greenhouse. Keep the day at 18 to 21 °C.
- For parsley and coriander, biomass per cell climbs steeply up to about 15 seeds and then flattens. 16 to 20 seeds for parsley and 18 for coriander is the sweet spot.
- In a pot trial of basil in peat, coir and wood fibres, peat and coir gave the tallest plants and the most biomass.
Why potted herbs and coir get on
A supermarket herb pot is grown as a crop for four to nine weeks, sold as a plant, then treated as a cut herb in the kitchen. The medium has to do three jobs.
In the greenhouse it has to drain fast enough for a dense stand of seedlings to root without damping off, yet hold enough water that a bench of 70 pots per square metre isn't watered six times a day. In a pot-plant trial, pure coir measured about 67 percent water-holding capacity and 26 percent air-filled porosity, a better air figure than the two peat-based mixes beside it.
At dispatch, weight matters, and coir is light. On the shelf the medium has to survive drying out. A peat pot that goes bone dry on a retail bench shrinks from the pot wall and sheds water when someone tops it up. Coir rewets even after the plant has wilted, which is why the pot often recovers once the customer waters it at home. And washed fine coir is an even brown that stays put when the pot is tipped in a basket.
What coir doesn't bring is nutrition. A trial of pot plants in 100 percent coir found growth comparable to peat mixes with fertiliser and poor growth without it. Feed from the first true leaf.
Pot sizes and what goes in them
| Pot | Where it sells | Seeds or plants per pot | Time to sale |
|---|---|---|---|
| 9 to 10 cm (0.3 to 0.4 L) | Supermarket "cut once or transplant" pot | Basil 25 to 30 plants, or 30 seeds. Coriander and parsley similar | 4 weeks in summer, up to 9 in winter |
| 4 inch / 10 cm (US retail) | Garden centre, farm stand | Basil 5 to 15 seeds for a looser, longer-lived plant | 4 to 6 weeks |
| 12 cm (0.6 to 0.8 L) | Kitchen pot meant to be kept for weeks | Basil 40 to 50 plants | 5 to 9 weeks |
| 15 cm and 1 gallon | Patio herbs, mint and chives | 1 to 3 plugs, or a rooted cutting | 3 to 4 weeks from plug |
The two seed counts are two products. A pot stuffed with 30 seedlings looks full at three weeks and is meant to be cut once. A pot with 8 seedlings takes longer to fill but keeps growing on a windowsill.
Fill loose to the rim, tap the tray once, don't press. Compacted coir loses the air you chose it for. A 9 cm pot takes about 0.3 L and a 12 cm pot 0.6 to 0.8 L, so a 5 kg block, usually quoted at 60 to 75 litres once expanded, fills around 200 small pots or 90 large ones. Both 5 kg blocks and 650 g briquettes are sold for this. Washing matters more for herbs than for tomatoes because seedlings meet the medium at their most salt-sensitive stage.
Fine or medium grade
For standard herb pots, use the finer end of the range. Small seed needs a fine surface, or it sits over air gaps and dries out between waterings. A 9 cm pot is shallow, so the lower third stays wet anyway and what you need is capillary rise to the seed, not extra drainage. And fine pith expands further per kilo, roughly 15 L/kg against 13 to 14 for medium grades and chips.
A medium grade earns its place in the 12 to 15 cm pot for mint and chives that will be watered for months. There the extra air at depth helps, and mixing 20 to 30 percent perlite into fine coir does the same job. Pure fine coir can also slump and stay wet in a cold, dull winter house, so basil going out between November and February gets a 10 to 20 percent perlite share. In a bright summer house it isn't needed. The ratios and what each adds are in the article on coir, perlite and vermiculite mixes.
Sowing: density and multi-seed pots
Almost every retail herb pot is a multi-seed pot. The question is how many.
For basil the European answer is dense and the North American answer is moderate, and both fit their markets. A trial on sub-irrigated pot basil compared 9, 12, 15 and 18 plants per 10 cm pot and found 12 gave the best yield and leaf quality, with thinner stems as the count went higher. Northern European growers still go to 25 to 50 because the pot is judged by how full it looks in a sleeve, not by stem diameter.
For parsley and coriander the data is cleaner. A controlled-environment trial sowed 1, 5, 10, 15 and 20 seeds per cell. Fresh weight per cell rose about three-fold from 1 to 15 seeds and then levelled off. The recommended rates came out at 16 to 20 seeds for parsley and 18 for coriander. Sage plateaued at 10. More seed just makes thinner, shorter plants and costs money.
Sowing straight into the final pot is normal for basil and coriander. A quart pot direct-sown with 20 basil seeds can be ready to ship four weeks from sowing, while 288-cell plugs with 2 to 5 seeds per cell were ready to transplant at two weeks and then needed finishing.
Parsley takes 21 days to germinate at 21 °C, so many growers plug it rather than tie up the finishing bench. Chives take 7 to 14 days and are plugged as a clump. Mint seed doesn't come true, so root cuttings or divisions straight into the coir pot. Seed depth, covering and the first water are covered in coco peat for seed germination.
EC, pH and temperature for the main herbs
Washed coir sits at a pH of 5.5 to 6.5 and a low EC, so you're adjusting rather than fighting the medium. The ranges below are what seed companies and trials give. A 1:2 media extract and a feed solution EC aren't the same number, so note the method.
| Herb | Feed EC (mS/cm) | Target pH | Temperature °C | Notes |
|---|---|---|---|---|
| Basil | 1.0 to 1.4 in hydroponics. Dense pots fed from below run 2.2 to 2.8 | 5.5 to 6.2 | 18 to 21 day, 16 to 18 night | Below 16 °C leaves spot red-brown or yellow. Winter crops want the higher EC |
| Coriander | Low to moderate | 5.8 to 6.2 | Cooler than basil | Heat and long days push it to bolt. Sow weekly |
| Parsley | Low to moderate | 5.8 to 6.2 | 21 to germinate, cooler to grow | Slow start, then steady |
| Mint | Tolerates more salt and more water than the others | 5.8 to 6.5 | Wide range, hardy | Keep it damp. Cuttings, not seed |
| Chives | Low to moderate | 6.0 to 6.5 | Cool-season, cold-tolerant | 60 days seed to first cut, 30 from a plug |
The basil EC split makes sense once you see where the figures come from. The 1.0 to 1.4 figure is hydroponic guidance where roots sit in solution all day. The 2.2 to 2.8 figures come from sub-irrigated pots carrying 12 to 30 plants, where each sip has to carry more. In pots, 2.8 mS/cm came out best, and 3.1 was already showing salt stress. If you're ebb-and-flooding dense basil pots in coir, start around 2.0 and watch drain EC. Culture sheets for pot basil give a media EC of about 1.0 by the 1:2 method and 100 to 175 ppm nitrogen in the feed.
Mint is a wet-ground plant, so "coir holds too much water" doesn't apply. Coriander and chives are the ones that sulk in a cold, saturated pot. And because unbuffered coir holds potassium and sodium and gives up calcium slowly, a feed with calcium nitrate from the first week avoids the pale, stunted fortnight that puts a basil crop behind.
Watering a four-week crop
- Sow into wet, not soaked, coir. Hydrate, drain, fill. First water from below or a fine rose. Basil pots are often kept dark for the first 7 days, which also stops the surface drying.
- Feed from the start at low strength. Under 100 ppm N to the first true leaf, then 100 to 175 ppm.
- Water from below where you can. Basil hates wet leaves, and downy mildew and botrytis both start there. Ebb-and-flood or capillary matting also keeps the surface drier.
- Let the top dry between waterings on coriander, parsley and chives. Not on mint.
- Ease off in the last 3 to 4 days. A pot that leaves at field capacity is heavier, sweats in the sleeve and goes mouldy. One that leaves slightly dry travels better, and the customer's first watering brings it back.
In summer a 9 cm basil pot with 30 plants wants water daily by week three. If you miss one, coir wets up again without channelling.
Shelf life after dispatch
The herb that fails is basil, and the cause is cold. Basil is chilled below 12 °C. Below 10 °C the damage is severe, and leaves blacken in a day at 0 °C or three days at 5 °C. A supermarket chiller for cut herbs is set well below that. If a potted basil goes into the same cabinet as the bagged coriander, it's dead by Monday. It belongs on the ambient produce shelf, and the lorry needs to be above 12 °C too. Cut basil keeps about 7 days at 20 to 25 °C, 12 to 13 days at 15 °C and 8 days at 10 °C. A living pot does better because it's still growing, but the temperature rules are the same.
Coriander, parsley, chives and mint all take a cool shelf. What kills them is drying out and then being drowned by a well-meaning staff member. Coir rewets without the water running down the pot wall, and it drains, so a pot left in a tray of water for a day isn't finished.
At the packing bench, sleeve every pot (it stops leaf damage, holds humidity and extends shelf life), dispatch slightly below field capacity, and let the surface dry so it doesn't go out green. Then label the basil for the ambient shelf.
Common problems
Basil leaf spotting in the greenhouse. Red-brown or bright yellow spots on basil in a cold house are a chilling symptom, not a disease. The threshold is around 16 °C. Check the night set point and whether the pots nearest the door or the glass are the ones affected.
Coriander bolting. Heat and long days send it to seed, and a house set up for basil at 20 °C is too warm. Grow it cooler, sow weekly so there's always a young batch, and use slow-bolt varieties.
Green surface on the pot. Algae on wet coir under bright light. It doesn't hurt the plant, but customers read it as old stock and it feeds fungus gnats and shore flies, already the commonest pest in pot basil. Top-water less, let the surface dry between irrigations, dispatch dry. The full treatment, including sciarid control, is in mould, algae and fungus gnats in coco coir.
Pale, slow seedlings in week two. Almost always nutrition. Coir has no reserve. Start the feed earlier and include calcium nitrate.
Slumped, sour pots or damping off in winter. Fine coir, low light, cold roots, too much water. Go a grade coarser or add perlite for winter sowings, water from below and lengthen the gap. If damping off persists in dense pots, the seed count is too high for your light.
Blends for herbs
| Fill | Where it fits |
|---|---|
| 100 % fine washed coir | Summer basil and coriander in 9 to 10 cm pots, bottom-watered. Lightest, best pot count per block |
| 80 to 90 % fine coir, 10 to 20 % perlite | Winter sowings, parsley, anything on the bench more than six weeks |
| 70 % fine-medium coir, 30 % perlite or husk chips | Mint, chives and 15 cm patio pots watered for months |
If you also run cut basil or leafy greens in channels or rafts, that's a different set of numbers, covered in coco coir for lettuce and leafy greens. The production side of potted herbs, plugs through finished pot, is on the herbs application page.
If you're moving a herb line from peat to coir, trial one bench first. For a sample of fine or fine-medium washed coir, or to talk through a blend for a specific pot size, get in touch.
FAQ
Is coco coir good for growing basil?
Yes, as long as you feed it. In a pot trial comparing peat, coir and several wood fibres, peat and coir gave the tallest basil and the most biomass. Coir holds water well, drains freely and rewets after drying, which suits a dense retail pot. It has no nutrients of its own, so start a balanced feed with calcium from the first true leaf.
How many basil seeds go in a supermarket pot?
It depends on the product. European-style pots meant to be cut once carry 25 to 30 plants in a 9 to 10 cm pot and 40 to 50 in a 12 cm pot, often from about 30 seeds. A looser North American 4-inch pot meant to keep growing on a windowsill is sown with 5 to 15 seeds. More seed means a fuller look sooner but thinner stems and a shorter life.
What EC should I run for herbs in coco coir?
For basil in hydroponic systems, 1.0 to 1.4 mS/cm. For dense basil pots fed from below, growers run higher, around 2.2 to 2.8 mS/cm, and salt stress shows above about 3 mS/cm. Coriander, parsley and chives sit in the low to moderate range. Mint tolerates more. Keep pH between 5.5 and 6.2 for basil and 5.8 to 6.5 for the rest.
Why did my potted basil die in the supermarket chiller?
Basil is chilling-sensitive. Below 12 °C the leaves wilt and discolour, below 10 °C the damage is severe, and at 5 °C the leaves blacken within about three days. Cut-herb cabinets run colder than that. Potted basil has to be transported and displayed above 12 °C, which usually means the ambient produce shelf.
Should I use fine or medium coir for herb pots?
Fine for the standard 9 to 12 cm retail pot. Small seed needs a fine surface, shallow pots don't need extra drainage, and fine coir gives more litres per kilo. Use a fine-medium grade or add 10 to 30 percent perlite for winter crops, for mint and chives in larger pots, and for anything that will be watered for more than six weeks.
How long does a coir herb pot last on the shelf?
Cut basil leaves keep about a week at room temperature and roughly 12 days at 15 °C, and a living pot in coir does better than cut leaves because the plant is still growing. The limit is usually drying out or cold, not the medium. Coir rewets well after a dry spell, so a wilted pot often recovers once it's watered at home.
How do I stop algae forming on the coir surface?
Algae need a wet surface, light and nutrients, and a top-watered herb pot under greenhouse lights gives all three. Water from below, let the surface dry between irrigations, and let pots dry down slightly before dispatch. The algae itself doesn't hurt the plant, but customers read it as old stock and it feeds fungus gnats.
Can I grow mint and chives in pure coco coir?
You can, and mint in particular is happy in a medium that stays damp. For pots that will be grown on for months, most growers add 20 to 30 percent perlite or husk chips to keep air in the root zone as the coir settles. Mint should be started from cuttings or divisions rather than seed if you want a named variety. Chives take about 60 days from seed to first cut, or 30 from a plug.