The short answer: coco coir is a good fill for potato grow bags because it stays loose, so tubers form without being squeezed and come out clean when you tip the bag. Use a bag or container at least 30 cm wide and deep, plant one seed potato per 30 cm of width, start with 10–15 cm of coir under the tubers and keep topping up as the shoots grow. First earlies in a bag are usually ready 8–12 weeks after planting, around the time the plants flower.
Key facts
- Container guidance: at least 30 cm wide and deep, one seed tuber per 30 cm of diameter. For deeper containers, about 19 litres of mix per plant.
- Tubers start best with the root zone at 15–20 °C. Above about 20 °C, tuber numbers fall sharply.
- Earth up when shoots reach about 8 cm, adding roughly 10 cm at a time until the bag is full.
- Common scab is worst above about pH 5.2–5.5, and steady moisture from just before emergence through early tuber set reduces it.
This is about table and new potatoes for eating. Producing seed-potato minitubers in coir is a different job with its own densities and feed programme, covered in our article on coco peat for seed potato minitubers.
Why grow potatoes in coir
A potato grown in a bag lives or dies on three things: drainage, air in the root zone and even moisture. Garden soil in a container fails on all three. It compacts, waterlogs at the bottom and dries into a crust at the top. Extension advice is clear on this: don't fill potato containers with garden soil.
Coir doesn't compact the way soil does. It holds water but drains, and it stays loose enough that tubers swell into round, smooth shapes. At harvest you tip the bag out and the potatoes come out clean. It's also light, which matters if your bags are on a balcony or need moving.
Two things to know before you start. Coir has almost no nutrients of its own, so every gram of food comes from you. And washed coir that isn't buffered takes up calcium and magnesium from the first feeds. Our article on buffered vs unbuffered coco peat explains how to handle that.
Bag and container size
| Container | Seed tubers | Notes |
|---|---|---|
| Pot or bag 30 cm wide and deep | 1 | The minimum for one plant |
| Grow bag about 40–45 cm wide | 1–2 | One tuber gives bigger potatoes. More tubers give more, smaller ones |
| Potato sack or large bag, roll-down type | 3–5 | Common garden practice for earlies |
| Tub or bin 38–57 litres, 60–90 cm tall | 2–3 | About 19 litres of fill per plant |
| Long coir grow bag laid flat | Not ideal | Too shallow to earth up. Use it standing or choose a deeper bag |
You'll see anything from one to five seed potatoes per bag recommended, depending on the bag and the grower. More tubers per bag gives more potatoes but smaller ones. Fewer gives bigger potatoes. The total weight changes less than you'd expect, because the bag's volume and the feed set the limit.
Don't go too tall. A container much over 90 cm is hard to water evenly, and the bottom stays soggy and rots tubers while the top dries out.
Any opaque bag or pot with drainage holes works. Light reaching tubers turns them green, so clear containers are out. Pale bags stay cooler in sun than black ones, and that matters for tuber set, as you'll see below. Our coir grow bags and open-top bags can be filled with straight coir or a blend.
Planting step by step
- Use certified seed potatoes. They're virus-tested. Supermarket potatoes are often treated to stop sprouting and can carry disease.
- Chit earlies. Stand seed tubers in a light, cool, frost-free place for about four to six weeks before planting, until they have short, sturdy sprouts.
- Charge the coir. Wet the coir with a full-strength vegetable feed that contains calcium and magnesium, and let it drain.
- Roll the bag down. Fold the sides down to about a third of their height.
- Base layer. Put in 10–15 cm of moist coir.
- Plant. Set the tubers sprouts up, about 30 cm apart, and cover with 8–10 cm of coir.
- Water once. Water in well, then hold back until shoots show. Wet, cold coir is where seed tubers rot.
Cut large seed tubers into pieces of 2.5–5 cm with at least one eye each, and let the cut faces dry for a day or two before planting. Small tubers go in whole.
Topping up
When shoots reach about 8 cm, add roughly 10 cm of coir, leaving the tips showing. Repeat every couple of weeks, unrolling the bag as you go, until the coir is just below the rim. Some growers keep about 15 cm of foliage exposed each time rather than burying to the tips. Both work.
Topping up does two jobs. It keeps forming tubers covered, so they don't turn green. And in varieties that set tubers along the buried stem, it gives them more stem to set on. Don't expect a tall bag to fill with potatoes from bottom to top, though. Most tubers form in a band, and early varieties set their crop over a shorter period than later ones.
Feeding
Coir gives you a clean start, but it means you feed from day one. Garden advice is to start liquid feeding about six weeks after planting in peat-free composts. In coir, start earlier, from the time shoots emerge.
- Pot growers: a balanced soluble feed every two weeks from emergence, or a weaker feed at every watering. Extension advice favours a feed with more phosphorus and potassium than nitrogen once the plants are up.
- Fertigated bags: a complete vegetable feed through the drip, with potassium raised once the plants start to flower and tubers are bulking.
- Watch nitrogen. Too much gives big tops and a late, small crop of tubers.
Watering
Keep the coir evenly moist, never soggy. Water when the surface starts to dry, and water enough that some drains out of the bottom. Bags outdoors may need watering even after rain, because the leaves shed water off the top of the bag.
Even moisture matters most from just before emergence to several weeks after. Keeping the root zone moist for a week before emergence and eight weeks after reduced common scab in field potatoes. It also gives more evenly shaped tubers. Big swings between dry and wet are what cause growth cracks and knobbly potatoes.
Heat: the bag-grower's real problem
Potatoes form tubers best when the root zone sits at 15–20 °C. Above about 20 °C, tuber formation falls off fast. In controlled conditions, each degree of extra root-zone heat cost about 3.5 tubers per plant, a bigger effect than the same rise in air temperature.
A black bag in full sun can run well above air temperature. So:
- Plant earlies early, so tubers set before summer heat. In a frost-free greenhouse, chitted earlies planted at the start of March can be ready by mid-May.
- Use pale or white bags, or shade the bag sides while leaving the leaves in sun.
- Group bags together so they shade each other, and stand them off hot paving.
- In hot climates, grow in the cool season only.
Greening, scab and blight
Greening. Any tuber exposed to light turns green and bitter. Discard green potatoes, as they may be poisonous. Keep them covered while growing and store them in the dark after harvest.
Common scab. Rough, corky patches on the skin. It's worst when the root zone is warm and dry and the pH is above about 5.2–5.5, and undecomposed organic matter can make it worse. Fresh coir isn't field soil, so it doesn't start with the scab load an old potato bed can carry. Keep the coir moist through tuber set and don't add lime. If you recycle coir, don't use mix from a scabby crop for the next potato crop.
Blight. Early varieties are less likely to be hit, because they're out before blight season peaks. Growing under cover helps too.
When to harvest
Earlies are ready when the plants flower or just after the flower buds drop, with potatoes about the size of a hen's egg. In a bag that's usually 8–12 weeks after planting for first earlies, a few weeks longer for second earlies. Feel down the side of the bag first. If the potatoes are still small, give it another week.
For a later crop, plant cold-stored seed potatoes in a large container in July or early August and keep them in a frost-free greenhouse for a winter harvest. In cool climates, maincrops are harder in bags because they need longer and get caught by blight.
At harvest, tip the bag onto a sheet and pick through the coir. Brush off what's left and dry the potatoes out of the light. Don't wash them until you're ready to cook.
Yields to expect
Bag yields vary more with variety, season and feeding than with anything else. In one grow-bag comparison using a soil, vermicompost and coco peat mix, two table varieties gave about 0.9–1.0 kg per bag.
What to do with the coir afterwards
Spent coir from a healthy potato crop can go into raised beds or the compost heap. Don't plant potatoes or tomatoes into it again next season, since both share diseases. See what to do with spent coco coir for more options.
Getting coir for potato bags
CVM Fiber and Industries in Kanyakumari, Tamil Nadu, India, makes washed coco peat in 5 kg blocks and 650 g briquettes, grow bags and open-top bags, and husk chip blends. Our coco peat is washed with water to reduce EC, with no calcium nitrate or other chemicals. If you're planning potatoes in bags, for a garden range or your own growing, contact us.
FAQ
How many seed potatoes per grow bag?
One per 30 cm of bag width is the cautious figure, and many growers put three to five in a large potato sack. More tubers give more but smaller potatoes. For new potatoes from a standard bag, two or three is a good middle.
Can you grow potatoes in pure coco coir?
Yes, as long as you feed them. Coir has almost no nutrients, so start feeding when shoots emerge and keep going. Charge unbuffered coir with a calcium and magnesium feed before planting.
How deep should the bag be for potatoes?
At least 30 cm, and 60–90 cm for a tub you'll keep topping up. Much taller than that and the bottom stays wet while the top dries, which rots tubers.
When should I add more coir to my potato bag?
When the shoots are about 8 cm tall, add about 10 cm of coir. Repeat every couple of weeks until the bag is full to just below the rim. This keeps tubers covered so they don't go green.
Why are my bag potatoes small and few?
Usually heat or hunger. Tubers form best with the root zone at 15–20 °C, and a black bag in sun runs much hotter. Plant earlier, use pale bags or shade the sides, and feed from emergence.
How long do potatoes take in grow bags?
First earlies usually take 8–12 weeks and are ready when the plants flower, with potatoes about the size of a hen's egg. Second earlies take a few weeks longer. Maincrops take much longer and are less suited to bags.
Can I reuse coir from a potato bag?
Yes, but not for potatoes or tomatoes the following year. Use it to improve beds or in pots of unrelated crops, and don't reuse coir from a crop that had scab or blight on potatoes.