In short: Potatoes are one of the easiest crops to grow in a bag. Use an early variety, a 40-litre bag, three to four chitted seed potatoes and a light, moisture-holding mix — 1 part coco coir to 1 part peat-free compost plus a general fertiliser — planted from late March (earlier under cover) and topped up as the shoots grow. First earlies are ready to tip out from June, about 12 weeks after planting. One 5 kg coir brick and 75 litres of compost fills four 40-litre bags.
Why bags suit potatoes — and why earlies
The Royal Horticultural Society (RHS) points out that if you have no room for a row of potatoes you can grow a few in a large container, a sack or a bag, and that early varieties are the most suitable because the plants are smaller and mature more quickly. Earlies (first and second earlies) are the new potatoes: ready in as little as 12 weeks, out of the bag by midsummer, and — the RHS notes — less likely to be hit by potato blight, which arrives later in the season.
Bags have two practical advantages over the ground. You control the growing medium completely, so scab, slugs and heavy clay are largely designed out; and harvesting is a tip-out rather than a dig, so no tubers are speared and none are missed. The price is that a bag dries out faster than soil and holds only as much food as you put in it — which is exactly what the choice of compost is about.
What compost for potatoes in bags?
The RHS's baseline for containers is peat-free multi-purpose compost, and that works. The mix below improves on it in the three ways that matter in a bag: it holds more water, it stays light enough to earth up and tip out, and it does not slump into a brick by July.
| Ingredient | Share | What it does |
|---|---|---|
| Coco coir (expanded brick) | 1 part | Holds 8–10 times its own weight in water yet stays open and airy; light to lift; re-wets easily if the bag dries; no peat. |
| Peat-free multi-purpose or garden compost / well-rotted manure | 1 part | Nutrition and biology. Potatoes are hungry; coir alone contains almost no food. |
| General fertiliser (organic granular, e.g. a blood, fish and bone type, or a controlled-release feed) | Per the pack rate, mixed through | Feeds the crop through 12–16 weeks in a limited volume. |
Why not straight multi-purpose? Because a 40-litre bag in full sun in June can need water every day, and a peat-free compost that has dried out is slow to re-wet. Coir is the water-holding half of the mix — the same reason it is the base of most commercial growing media. Why not straight coir? Because it is washed, buffered and clean, but it is not a feed: on its own it will grow a plant with nothing to eat. Mix the two and you get the best of each.
Coverage, using the standard 5 kg brick that expands to about 75 litres:
| Bag size | Mix needed (filled in stages) | Coir + compost per bag | Seed potatoes per bag |
|---|---|---|---|
| 30 L bag / 30 cm pot | ≈ 25 L | ≈ 12 L + 12 L | 1–2 |
| 40 L bag (the common size) | ≈ 35 L | ≈ 18 L + 18 L | 3–4 |
| 50–60 L bag or dustbin | ≈ 45 L | ≈ 22 L + 22 L | 4–5 |
| One 5 kg brick (75 L) + 75 L compost | ≈ 150 L of mix — four 40-litre bags. The 3-brick pack does twelve. | ||
On numbers per bag: most grow-bag instructions allow three to five seed potatoes in a 40-litre bag. The RHS is more conservative — one seed potato per 30 cm of container diameter — and the trade-off is simple: fewer tubers per bag gives larger potatoes; more gives a bigger count of smaller ones. Three in a 40-litre bag is a good middle.
When to plant potatoes in bags (UK)
The RHS planting calendar for the ground applies to bags outdoors, and bags can go earlier under cover — the RHS suggests a large container in a greenhouse or a bright, frost-free spot for an early crop, which also keeps the plants away from blight. Plant later in colder regions.
| Type | Chit from | Plant (outdoors) | Under cover | Harvest |
|---|---|---|---|---|
| First earlies | Late January (mild areas) to early March (cold) | Late March | From late February | June–July, about 12 weeks after planting |
| Second earlies | As above | Early to mid-April | From March | July–August |
| Maincrop | Not needed | Mid-April to early May | — | Late August to October (better in the ground or a very large container) |
Chitting means standing the seed potatoes eyes-up in egg boxes in a bright, frost-free place until the sprouts are about 5 cm long — four to six weeks. The RHS calls it not essential but worthwhile for earlies, because it brings the crop forward.
Step by step
- Buy certified seed potatoes of an early variety in January or February, when the choice is widest, and chit them (above).
- Expand the coir. Put the 5 kg brick in a large tub with about 20 litres of water and leave it 20–30 minutes, then fluff it up — how to expand a brick. Mix 1:1 with compost and add the fertiliser at the pack rate. Keep the rest of the mix in a lidded tub or a tied compost bag for the top-ups.
- Check the bag drains. Purpose-made grow bags have holes; if you are reusing an old compost sack or a bucket, the RHS reminder is to pierce drainage holes in the base first. Roll the sides of a fabric bag down to half height so light reaches the young shoots.
- Fill the bottom 15 cm with mix (the RHS's half-fill), set the seed potatoes on it sprouts-up, spaced evenly, and cover with about 10 cm of mix. Water well.
- Earth up as they grow. When the shoots are 15–20 cm tall, add mix around them so only the top few centimetres of leaf show. Repeat every couple of weeks, unrolling the bag as you go, until the mix is a few centimetres below the rim. Earthing up protects shoots from frost and keeps forming tubers in the dark so they do not turn green.
- Frost. Shoots that are above the mix on a frosty night need fleece, a cloche or a bucket over them; a light frost blackens leaves but the plant usually recovers.
- Water and feed. Once the bag is full, water so the whole depth is moist — the RHS notes that potato leaves act like an umbrella, so bags can be dry underneath even after rain. In hot weather that can mean daily. Give a balanced liquid feed every couple of weeks from the time the bag is full, at the pack rate; when flower buds show, a high-potassium (tomato) feed helps tuber swelling.
- Harvest. The RHS guide: earlies are ready when the flowers open or the buds drop, with tubers the size of hens' eggs. Scoop away some mix by hand to check, and cover them again if they are too small. Then tip the whole bag out onto a sheet, or harvest a few at a time by hand from the side.
Problems with growing potatoes in bags — and the fixes
| Problem | Usual cause | Fix |
|---|---|---|
| Small crop of tiny potatoes | Too many tubers per bag; the bag dried out while tubers were swelling; not enough food | Three per 40 L bag; keep the mix evenly moist from flowering onward; feed fortnightly |
| Green potatoes | Tubers exposed to light | Earth up more generously; discard green ones — the RHS warns they are potentially poisonous |
| Rotting seed potatoes or soggy mix | No drainage, or planted into cold wet compost too early | Pierce the base; wait for the mix to be at least cool rather than cold; plant chitted tubers rather than soft ones |
| Wilting on hot afternoons | Bag dried out — fabric bags lose water through the sides too | Water deeply in the morning; a coir-based mix re-wets quickly, a dried-out peat-free compost does not. Move the bag out of afternoon sun if you can |
| Rough, corky patches on the skin (scab) | Dry conditions when tubers were forming; alkaline conditions | Even moisture; coir is slightly acidic (pH 5.5–6.5), which suits potatoes; do not lime |
| Brown blotches spreading on leaves in warm, humid weather (blight) | Blight spores, from July onward | Grow earlies and be finished by then; cut the foliage off at the first sign and harvest what is there |
| Long pale shoots | Chitted or grown on in too little light | Chit in a bright window or a cold greenhouse, not a cupboard |
| Slugs and slug-holed tubers | Slugs sheltering in the bag | Stand the bag on a hard surface, harvest earlies promptly, do not leave maincrops sitting in bags |
After harvest: use the mix twice
The spent mix is 150 litres of good structure with the fertiliser used up. Do not grow potatoes or tomatoes in it again (the RHS advises rotation to avoid a build-up of pests and disease), but refreshed with a fresh dose of compost and fertiliser it is ideal for a second crop of salads, dwarf beans or autumn-planted garlic, or for topping up a raised bed — see reusing coco coir.
For Christmas new potatoes, cold-stored seed potatoes are sold in summer to plant in bags in August and grow on in a greenhouse or porch; the same mix and method apply, with the bag brought under cover before the first frost.
Frequently asked questions
How many seed potatoes do I plant in a grow bag?
Three in a standard 40-litre bag is the reliable middle ground. Suppliers' instructions typically allow three to five; the RHS's rule for containers is one seed potato per 30 cm of diameter. Fewer gives bigger potatoes.
Do potatoes grow well in grow bags?
Yes — earlies in particular. Expect a colander or two of new potatoes from each 40-litre bag of three tubers, more with generous feeding and watering. Maincrops do better with the room of a bed or a very large container.
When are potatoes grown in bags ready to harvest?
First earlies about 12 weeks after planting, in June and July; second earlies in July and August. The RHS's cue is open flowers or dropped buds, and tubers the size of a hen's egg when you scoop a look.
Can I grow potatoes from supermarket potatoes?
You can, but it is not recommended: shop potatoes are not certified free of virus and disease and are often treated to stop them sprouting. Certified seed potatoes cost little and crop reliably.
What is the best compost for potatoes in bags?
A moisture-holding, free-draining, well-fed mix: 1 part coco coir to 1 part peat-free compost plus a general fertiliser. Straight peat-free multi-purpose works but dries out faster and is heavier to lift.
Can I grow potatoes in coco coir on its own?
Only if you feed from the start: coir carries almost no nutrients. The 1:1 mix with compost is simpler and gives the plants something to draw on between feeds.
Can I use an old compost bag as a grow bag?
Yes. The RHS suggests repurposing old sacks or large compost bags, as long as you pierce holes in the bottom for drainage and roll the top down so light reaches the shoots.
One washed, calcium-buffered 5 kg brick — about 75 litres of the water-holding half of a potato-bag mix, enough for four 40-litre bags, delivered free across the UK.
Shop the 5 kg brick → 3-pack for a dozen bags