Container Gardening: How to Grow from Seed in Pots

A pot is not a small garden. It is a closed system with a fixed volume of compost, a fixed reservoir of water and a fixed store of nutrients, all of which run out on a schedule that has nothing to do with what the weather is doing. That is the single idea behind every piece of advice below, and it explains why the same packet of seed that thrives in a border can sulk in a container three metres away. Get the volume, the watering and the feeding right and pots will outperform open ground for salads, herbs and half the flower list — with no digging, no weeding to speak of, and complete control over the soil. This guide covers sowing straight into containers, choosing the right size, and keeping the crop going once the compost has given everything it has.

• Best for salads, herbs, dwarf and fast crops, most annual flowers, alpine strawberries
• Minimum size 5 litres for salad and herbs, 10–15 litres for a fruiting crop, 30 litres-plus for a climber
• Compost multipurpose for annuals; soil-based (John Innes type) for anything staying more than a season
• Feeding starts about six weeks after sowing — fresh compost holds roughly that much fertiliser

What actually changes when you grow in a pot

Four things, and they compound. Water leaves a container far faster than it leaves the ground: there is no reservoir below and the sides lose moisture too, so a 10-litre pot in July can go from wet to bone dry in a day. Nutrients are finite and are washed out of the bottom every time you water — bagged compost carries about six weeks of feed and then simply stops. Roots sit in a container that heats up in sun and freezes through in winter, so they experience temperature extremes that soil buffers away. And volume caps the plant: a crop can only build as much root as the pot allows, and root volume sets top growth.

None of that makes containers harder, but it does make them less forgiving of neglect. What you gain in return is real: perfect soil from day one with no need for the ground preparation in our soil preparation guide, the ability to move a crop into sun or out of wind, far fewer slug problems than a bed, and a growing space that works on a balcony, a doorstep or a flat roof. If you are entirely new to raising plants from seed, read seed growing 101 first — the fundamentals are the same, only the container changes.

Choosing pots: size is the whole game

Almost every container failure traces back to a pot that was too small. Work in litres rather than centimetres, and when in doubt go one size up — a bigger pot is more forgiving of a missed watering, which is the mistake everyone actually makes.

As a working guide: 3–5 litres suits cut-and-come-again salad, radish and a single herb plant; 10 litres takes a bushy cherry tomato, a pepper or a group of short-rooted carrots; 15–20 litres is the minimum for an aubergine, a cucumber or a full-size tomato; and 30 litres or more is what a climber such as a runner bean or a row of peas needs to crop properly. Window boxes are shallow, so treat them as the 3–5 litre category however long they are.

Drainage holes are non-negotiable; crocks in the bottom are a myth that reduces the compost volume and does nothing for drainage. Choose material for how much watering you want to do — terracotta is handsome and dries fastest, glazed and plastic pots hold moisture far longer, and a light-coloured pot keeps roots cooler than a dark one in full sun.

Sowing in containers, step by step

1. Decide: sow direct, or start in modules? Fast, cheap and root-sensitive crops — salad, radish, carrots, dill, nasturtium — go straight into the final pot, following the principles in our direct sowing guide. Slow or tender crops such as tomatoes and peppers earn their space better if started in modules indoors using our indoor seed starting guide, then moved on.
2. Fill to within 3 cm of the rim and firm lightly That gap is the watering reservoir — without it, water runs off the surface instead of soaking in. Use fresh compost rather than last year's, which has neither structure nor feed left. Check the packet for depth and spacing; understanding seed packets decodes the rest.
3. Water the compost before you sow, not after Watering a dry pot from above washes small seed into the corners and down the sides. Wet the compost, let it drain, then sow onto an evenly damp surface — the conditions that matter for germination are the same in a pot as anywhere else.
4. Sow thinly and thin ruthlessly Crowding is worse in containers because the root volume is shared. Thin to the final spacing while seedlings are small; overcrowded, under-lit pots are the classic cause of leggy, weak seedlings, and poor air movement invites damping off. Nothing coming up at all? Work through why seeds are not germinating.
5. Put the pot in the right place — then leave it there Six hours of sun for fruiting crops; three or four is plenty for salad, and light shade actually suits it in summer. Anything raised indoors needs a week of hardening off before it goes out. Tall crops need support from the start — see plant supports and trellising, and remember a pot with a climber in it catches wind like a sail.

Pots or open ground? An honest comparison

Containers are not a compromised version of a border. They are better at some things and worse at others, and knowing which is which saves a season of disappointment.

A useful middle case is fruit. Full-size garden strawberries want a bed, but alpine strawberries are close to the perfect container crop — neat, runner-free, happy in part shade and cropping from June to the frosts. Our guide to harvesting alpine strawberries covers what to do with them once the pots start producing.

Watering and feeding: the two things that go wrong

Judge water by weight, not by the surface. Lift the pot, or push a finger in to the second knuckle — the top centimetre dries out constantly and tells you nothing. When you do water, water until it runs from the bottom; a splash that wets only the top 2 cm trains roots upwards into the part of the pot that dries first. In high summer that may mean daily, and twice daily for a big plant in a small pot, which is exactly why the general advice in our watering guide needs adjusting upwards for containers. Standing pots in saucers through a hot spell helps; standing them in saucers through a wet winter drowns them.

Feeding begins around week six, when the compost's built-in fertiliser is exhausted. A balanced liquid feed every fortnight suits leafy crops and herbs; once a fruiting crop flowers, switch to a high-potassium feed weekly, following our guide to feeding and fertilising. Yellowing lower leaves on a plant that is otherwise healthy almost always means hunger, not disease. Pests are fewer in pots but not absent — aphids find containers just as easily, and the controls in common garden pests apply. A mulch of gravel or bark on the compost surface cuts evaporation noticeably and keeps splash off low leaves, much as it does in a bed (see mulching and weed control).

What to sow in containers now

Late August and September are quietly one of the best container windows of the year: the compost is warm, germination is fast, and a pot can be moved under cover when the nights turn. Sow winter lettuce, corn salad, mizuna, pak choi, spinach and radish now for picking well into winter, and keep a tray of microgreens going on the windowsill for the months when nothing outdoors is growing. Hardy annual flowers sown in pots this month and overwintered under a cold frame flower weeks earlier than a spring sowing — the reasoning is set out in autumn sowing of hardy annuals. See the full month lists in what to sow in August, and browse everything tagged for now at sow in August and sow in September. Sowing a short row every fortnight rather than one big batch keeps a small collection of pots productive — see succession sowing.

One winter note: roots in a pot are several degrees colder than roots in the ground. Move containers against a wall, raise them on feet so they drain, and wrap the pot rather than the plant — it is almost always the roots that are lost, not the top growth.

Common container mistakes

• A pot that is too small — the commonest failure by far. Root volume sets plant size; go one size up.
• Reusing last year's compost — no structure, no feed. Fresh compost for anything grown from seed.
• Filling to the rim — leave 3 cm so water can pool and soak in instead of running off.
• Watering little and often — wets only the top few centimetres and draws roots upwards. Water until it runs out of the bottom.
• Forgetting to feed after week six — the compost is empty by then and growth simply stalls.
• Crocks and gravel in the base — they reduce compost volume without improving drainage. Just use pots with holes.
• Leaving saucers full in winter — waterlogged compost freezes solid and kills more container plants than cold air does.

Frequently asked questions

Can you sow seeds straight into a large pot?
Yes, and for fast crops it is the better method — salad, radish, carrots, dill and nasturtium all resent transplanting and do best sown where they are to grow. Slow or tender crops such as tomatoes, peppers and aubergines are worth starting in modules indoors, because a big pot of compost occupied by three seedlings for six weeks is wasted space.

What size pot do I need?
Think in litres: 3–5 for salad, herbs or radish; 10 for a cherry tomato or pepper; 15–20 for an aubergine, cucumber or full-size tomato; 30 or more for beans and other climbers. Depth matters as much as width for root crops, while width matters more for shallow-rooted salads.

How often should I water containers?
There is no fixed schedule — check daily and water when the pot feels light or the compost is dry a finger's depth down. In midsummer that is usually every day, and twice a day for a large plant in a small pot; in autumn it may be twice a week. Always water until it runs from the drainage holes.

Which vegetables grow best in pots?
Anything fast and shallow-rooted: cut-and-come-again lettuce and other salad leaves, radish, spring onions, mizuna, pak choi and most herbs. Among the fruiting crops, cherry tomatoes, peppers and chillies do well, and dwarf beans crop reasonably. Avoid parsnips, maincrop carrots, sprawling squash and hungry brassicas unless the container is very large.

Start with three pots rather than thirty: one of salad, one of herbs and one of something that flowers. Keep them where you will walk past them daily, because containers reward attention more than any other way of growing. Browse our vegetable seeds, herb seeds and flower seeds, the compact favourites in lettuce, basil and nasturtium, our organic seeds, organic vegetables and organic herbs, pollinator-friendly choices for a balcony in bee-friendly flowers and edible flowers, or read our beginner's guides to growing vegetables, herbs and flowers from seed. Not sure what to buy? Start with how to choose the right seeds, browse all our seeds, or dig into the rest of our growing guides.