The short answer: courgettes (zucchini) and summer squash grow well in coco coir, and in greenhouse comparisons coir-grown plants started cropping earlier and set more fruit than plants in soil or rockwool. Give each plant at least 19 litres in a pot, or one to two plants per grow bag, feed at EC 1.5–2.0 mS/cm with pH 5.5–6.5, and pick every day once the plants are cropping. Pumpkins and winter squash will grow in coir too, but they need far more root volume than a courgette and only small-fruited varieties make sense in a container.
Key facts
- In greenhouse work comparing coir, rockwool, perlite, pumice and soil, courgettes on coir cropped earliest and outyielded rockwool and soil, mostly through more fruit per plant.
- Greenhouse courgette is fed at about 1.5–2.0 mS/cm and pH 5.5–6.8.
- Fruit is ready 3–4 days after the flower fully opens, at 15–20 cm and roughly 110–230 g. Above 20 cm it's usually unsaleable.
- Recommended greenhouse density is about 1.8–2.0 plants/m², up to 3 plants/m² in double rows, with smaller fruit at the higher figure.
Why coir suits courgettes
Courgettes are thirsty, hungry and fast. A plant goes from transplant to first harvest in roughly 40–60 days and then wants the same steady supply of water and feed for weeks. Coir handles that well. It holds a lot of water but still drains, so the roots stay wet without sitting in a sump, and you can water hard on a hot afternoon without drowning them.
On the organic substrate the root zone stayed warmer overnight than in rockwool, and that warmer minimum temperature is the likely reason the first fruit came sooner. If you're planting in a cold greenhouse or a spring tunnel, that's a useful head start.
Coir is not buffered unless you buy it that way, and washed coir will pull calcium and magnesium from the first few feeds. Charge the bag with a full feed containing calcium before planting. Our article on buffered vs unbuffered coco peat explains what happens and what to do about it.
Bags, pots and how many plants
Rooting depth for courgettes in soil is 45–60 cm. You can't give them that in a container, so you give them width and volume instead.
| Crop | Container | Plants | Notes |
|---|---|---|---|
| Courgette, bush summer squash | Pot of 19 litres minimum, 20–30 cm deep | 1 | Extension minimums run from 11 to 19 litres. 40–57 litres crops longer in hot weather |
| Courgette | Pot about 50 cm across | 1 | A common recommendation for one plant |
| Courgette | Standard grow bag or coir slab | 1–2 | One plant gives bigger, longer cropping. Two is the most a standard bag will carry |
| Greenhouse courgette | Long bag, roughly 90 × 30 × 8–10 cm | Set by house density | 1.8–2.0 plants/m² across the house |
| Trailing summer squash | 40 litres or more | 1 | Train up a cane or string to save floor space |
| Small pumpkin, winter squash | The largest container you have | 1 | Needs several times a courgette's volume. Choose small-fruited, compact types |
You'll see anything from one to three courgettes per grow bag recommended. Three is too many: the plants crowd each other and the bag runs dry by mid-afternoon. Two will crop if you water twice a day. One per bag is the easier crop.
For bags and slabs that suit this crop, see our coir grow bags.
Mixing your own fill
Straight coco peat works for a single-season crop of courgettes. For pots that sit outdoors through rain, blend in a share of coarse husk chips or perlite. That keeps more air in the root zone after a wet week, which matters because courgette roots die quickly when they sit saturated in warm weather.
Feed, EC and pH
Courgettes don't need the high EC of tomatoes. The greenhouse figure is 1.5–2.0 mS/cm, with pH 5.5–6.8 in the feed. A working programme on coir looks like this:
| Stage | Feed EC (mS/cm) | pH | What to watch |
|---|---|---|---|
| Bag charge | 1.5, full calcium feed | 5.5–6.0 | Saturate, let it drain, then plant |
| Transplant to first female flowers | 1.5 | 5.5–6.2 | Keep it lean so leaves don't run soft |
| Full cropping | 1.8–2.0 | 5.5–6.2 | Raise potassium as fruit load climbs |
| Hot spells | 1.5 | 5.5–6.2 | More water, slightly weaker feed, so drain EC doesn't climb |
The nutrient balance in greenhouse recipes leans heavily on potassium and calcium, with both well above nitrogen. Courgettes prefer nitrate nitrogen, though they will take up some ammonium. Excess nitrogen gives you big soft leaves and more powdery mildew, not more fruit.
In pots fed by hand, a balanced liquid feed every watering at half strength, or a high-potassium tomato feed once flowering starts, does the same job. Check the run-off EC once a week. If it keeps climbing well above what you're feeding, water through with plain water once. Our guide to pH and EC management in coco coir covers the method.
Watering
A courgette in full crop can drink several litres a day in summer. In coir you want to water little and often rather than flooding the pot once a day. Small, frequent shots keep the substrate moist without leaving it waterlogged, and that's the moisture pattern that keeps fruit filling evenly.
- Pots and bags by hand: water until you see a little run-off, then again when the surface starts to feel dry. In hot weather that's morning and late afternoon.
- Drip: start with a few short shots a day and add more as the plant grows, so there's always a little run-off from each bag.
- Don't let the bag dry out completely. Bone-dry coir can be slow to take water back. If it happens, see rewetting dry coco coir.
Pollination and fruit set
Male and female flowers are separate on the same plant. The pollen is heavy and sticky, so it doesn't move on the wind. Something has to carry it, bees outdoors and you or a bumblebee hive under cover.
The first flowers are often all male. That's normal. Female flowers have a small fruit behind the petals. If fruit swells for a day or two and then yellows and rots from the tip, it wasn't pollinated.
To hand-pollinate, pick a freshly opened male flower in the morning, strip the petals and rub the pollen onto the centre of the open female flower. Parthenocarpic varieties set fruit without pollination at all, and they're the easiest choice for a closed greenhouse or a balcony with few bees, though there aren't many of them.
Training and spacing in a greenhouse
Greenhouse courgettes are grown in bags at about 1.8–2.0 plants/m². That spacing helps hold back Botrytis and Sclerotinia. Double rows can go to about 3 plants/m², which raises yield per square metre but gives smaller fruit.
Train each plant up a string to a wire and remove the old lower leaves as you harvest past them. Good airflow at the base is your main defence against grey mould on the stems and the fruit ends. Day temperatures of 24–29 °C, nights of 16–18 °C and a minimum of 10–13 °C keep the crop moving. Trained crops in an extended season have reached 67–78 t/ha.
Picking
This is where most container courgettes go wrong. Fruit is ready 3–4 days after the flower fully opens, sooner in hot, sunny weather. Pick at 15–20 cm, around 110–230 g. Leave it three days longer and you'll have a marrow.
Pick every day once the plant is cropping. In a field comparison where courgettes were picked three times a week, the gap over the weekend gave oversized fruit every Monday. Good varieties have given about 4 kg per plant over a season in field conditions. Expect a little less from a crowded grow bag and more from a well-fed, trained greenhouse plant.
Summer squash and pumpkins
Pattypan and yellow summer squash are handled exactly like courgettes.
Pumpkins and winter squash are a different crop. One plant can run several metres and carries its fruit for months. They'll grow in a large coir-filled container or a coir-amended bed, but give them a much bigger root volume than a courgette, a strong support for the vine and a sling under each fruit if it's trained upward. For in-ground growing, see coco coir in soil and raised beds.
Problems you'll see
| Problem | Likely cause in coir | Fix |
|---|---|---|
| Small fruit yellows and rots from the tip | No pollination, or the plant is overloaded | Hand-pollinate, pick daily, check feed |
| Blossom-end rot | Calcium not reaching the fruit: dry periods, high EC, uncharged unbuffered coir | Charge with calcium feed, water little and often, keep drain EC down |
| Powdery mildew | Dry roots plus humid air, soft growth from excess nitrogen | Steady watering, airflow, less nitrogen, remove worst leaves |
| Grey mould on stems and fruit ends | Dense canopy, wet leaves at night | Space plants, remove old leaves, water the bag not the plant |
| Wilting at midday with wet bag | Root loss from saturation | Fewer, smaller shots, more air in the mix |
| Plant stops setting fruit | Marrows left on the plant, too few bees, cold nights | Pick everything at size, pollinate, warm the greenhouse |
Getting coir for courgettes
CVM Fiber and Industries in Kanyakumari, Tamil Nadu, India, makes washed coco peat in 5 kg blocks, 650 g briquettes, grow bags and open-top slabs, plus husk chips and custom blends. Our coco peat is washed with water to reduce EC, with no calcium nitrate or other chemicals, so plan to charge it with your feed before planting. If you're planning a courgette or squash crop in coir, contact us and tell us the format and volume you need.
FAQ
How many courgette plants can I put in one grow bag?
One or two. One plant per bag gives the plant enough water and root room to crop for longer, and it's the safer choice in hot weather. Two will crop if you water at least twice a day in summer. Three is too many for a standard bag.
What size pot does a courgette need in coco coir?
At least 19 litres and 20–30 cm deep, with one plant per pot. A pot about 50 cm across or 40 litres or more keeps the plant going longer in a hot summer, because it doesn't dry out between waterings as fast.
What EC and pH should I use for courgettes in coir?
Feed at 1.5–2.0 mS/cm with pH 5.5–6.5. Start at about 1.5 until the first female flowers, then raise to 1.8–2.0 when the plant is cropping hard. Courgettes don't need the higher EC used for tomatoes.
Why are my baby courgettes rotting in coco coir?
Usually because the flower wasn't pollinated, so the fruit starts to grow and then aborts. Hand-pollinate in the morning with a fresh male flower, or grow a parthenocarpic variety under cover. Blossom-end rot from a calcium shortage looks similar, so charge unbuffered coir with a calcium feed and keep watering even.
How often should I water courgettes in coir bags?
Little and often. In hot weather that means at least morning and late afternoon by hand, or several short drip shots a day. Water until a little drains out, and don't let the bag dry out completely.
Can I grow pumpkins in coco coir?
Yes, but they need much more root volume than courgettes, a long season and a strong support if trained upward. In containers, choose compact or small-fruited varieties and the biggest pot you can manage. For large pumpkins, coir is better used to improve a bed than as the whole root zone.