What size container do I need?

 

Putting it into practise: smaller pots on the shelf above the back door for salads. The biggest pot on the left has a courgette and tomato plant © Sarah Cuttle 2010

Putting it into practise: smaller, shallower pots on the shelf above the back door for salads. The biggest pot on the left has a courgette and a tomato plant! NB the wooden boxes all have water reservoirs in the bottom – so are 10 – 15cm shallower than they look. © Sarah Cuttle 2010

If you’re planning to get more containers for the new growing season, you may well be asking: ‘what size of container do I need?’ There’s no straight forward answer – but here’s the information that will help you decide.

If you’re reading this post, you may also be interested in what’s the best material for containers.

Is depth or volume more important?

The general rule is: the bigger the pot, the bigger the crop. The volume of the pot is more critical than the depth – for everything except root vegetables.  30cm (12 inches) is deep enough for most crops. Salads and herbs will be happy in less – 10cm – 15cm (5 – 8 inches) is usually enough.

Pros and cons of big pots

Big pots have big benefits

  • They dry out less quickly. This makes watering easier. It also reduces the stress on your plants. Most crops don’t like rapidly fluctuating water levels – it slows their growth and often triggers premature bolting (this is when a plant goes to seed before you want it to – rocket and coriander do this a lot, for example). 
  • They hold more nutrients, ideal for hungry vegetable crops. 
  • They’re better at sustaining beneficial soil life, like worms. This helps you to mimic natural soil growing conditions in your pots. This in turn makes for healthier and more disease resistant crops.

But also drawbacks

  • They cost more (in both pot and soil)
  • They take up precious space
  • They weigh more. Weight is a particular issue if you are growing on a balcony or rooftop – you must take care not to overload the weight bearing capacity. (Check with a structural surveyor if unsure).

Pros and cons of smaller pots

Smaller pots are better choice for some situations because:

  • They’re cheaper to buy and fill
  • They’re lighter and easier to move around,
  • They take up less space.

The downsides are

  • They dry out faster – creating more work and more stress on the plant. You can get round this problem by using a container with a water reservoir or setting up a self watering system.
  • They need more regular feeding – observe plants carefully.
  • You’re more limited in what will grow in them. Don’t try big hungry crops in them like courgettes. However, light feeders – like most salads and herbs – will do fine.

So which is best?

If space, budget and weight is no issue, then large pots win every time (I’d probably go for 30 – 40cm deep, 1 meter long and 60cm wide – equivalent to 12 – 16 inches deep, 3 feet long, 2 feet wide).

But as urban growing IS usually constricted by space, weight or budget (usually all three!) a mix of sizes is often the best solution. Here’s what I recommend you try – but as always, experiment to find what works best for you:-

Choose smaller pots for your salads, Asian greens and herbs. Cheap plastic window boxes 15cm (6 inches) deep, 20cm (8 inches) wide are fine. Plastic trays from grocery stores or polysterene boxes from fish mongers do the job equally well.

Choose medium sized pots – at least 30cm (12 inches) deep and 30cm (12 inches) in diameter – for larger leafy veg like chard or kale, baby root vegetables like carrots, beetroot or turnips, and for the smaller fruiting vegetables like chillies or small hanging basket type tomatoes. Old plastic food waste buckets, large empty cooking oil tins (see pic below), and the buckets on flower stalls (often available free) are good recycled options.

Choose large potsat least 30cm (12 inches) deep and 40cm (16 inches) in diameter (50cm or 60cm diameter is even better) – for larger fruiting crops like vine tomatoes, aubergines, peppers, courgettes and squash, bigger root crops like potatoes. I also like to grow peas and beans in large pots. Old 50 litre recycling bins are a perfect size. Old hessian sacks or compost bags can also be used – great for potatoes. If you’re thinking of investing in your growing, check out Earthboxes, (NOT an affiliate link) a good size for most veg.

What’s your experience?

What size pots do you use, and which size works best for you?

 

Large olive oil drums like this can make pretty ‘medium sized’ containers – perfect for say a chilli or small pepper plant. (The nasturtiums in this pot are a bit straggly because they’re not getting enough sun!). I’ll do another post on the variety of recycled containers you can use soon.

18 comments… add one

  • Great article Mark. I just composted a pot grown courgette plant because it never produced fruits of more than 3 inches long and 1 inch in diameter. On emptying out the plant I found that the bottom 6/7 inches of the soil was as dry as dust though I watered daily and the top 5/6 inches of compost was very moist ( the pot was 14 inches in diameter and 14 inches deep). How can I be sure next year that the bottom roots of the courgette plants receive enough water and thus produce better sized fruit? Should I water from underneath with a saucer? Thanks, love your website.

    Reply
  • I find it helps to stand blueberry pots in saucers if they are in a sunny/windy position because they require constant dampness. I do concentrate on ornamentals at work because the place has to look nice for visitors and I am only there 3 days per week, I often mix edibles with the ornamentals though. I’m going to try doing more of that this year because GYO has become so popular. I have grown small pumpkins and squash in the pots, but not yet courgettes, mainly because they tend to become such enormous plants – and if they get mildew they look awful! I am probably going to try them this year though.
    I would never recommend lining our pots with plastic, it stops the roots from binding on to the pot walls (see my previous comment), is likely to impede drainage, means that less oxygen reaches roots, and encourages slugs and vine weevils. I think people need to consider the foliage/root balance of their potfuls instead of worrrying so much about the material of the walls. In my experience people expect large plants to thrive in small pots – if you look at the volume of leaf and compare it by eye to the volume of compost you can see why some potfuls never seem to be happy. This is hard to explain without being able to draw some sketches!

    Reply
  • Perhaps the biggest issue with containers, is the high temperature the soil can reach. Having the container shaded either articically or with plants, will reduce the stress dramatically. The choice of container, again is important. I cant find it on the internet, but there was a trial a while back that tested the various different container types. Black plastic, surprisingly was not the worst as it radiated heat as well as absorbed it. Clay I believe was the worst.

    The same can be said to be true of protecting the pots from the excesses of the cold. It would seem logical that wood might be the best material, as it is a poor themal conductivity. The greater the volume of compost will also provide a better buffer (likie the ground) to extremes.

    There is alwatys a temptation to over water plants in pots, though this in fact, can cause more problems than it releaves. Watering early morning when the compost is cool, reduces the speed at which compost heats up and there by keep the soil temperature at a level where the roots can function. A sodden compost at a high temperature, not only removes air available to the roots, but will also prevent the compost cooling as quickly at the end of the day. Perfect conditions for root dieseases and reduction of nutrient and water uptake.

    When watering pots, you should only really supply as much as the plant will need in one day, and in the ornamental industry, research has shown that watering 25% less than the rainfall received in the previous 24 hours, is the most efficient method to follow.

    Alternatively, use a water resevoir and take away the worry!

    Reply
    • Thanks for sharing your experience, Morris – I’d be fascinated to see the trial of different container materials if you come across it again. I was planning to write a post on the pros and cons different container materials soon – and it would be great to back my observations up with some hard facts! About watering pots – the other advantage of watering pots in the morning, of course, is that it is less conducive to slugs which love evening waterings!

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      • Cheap, thin terracotta may not be a good insulator, but sturdy, good quality terracotta of the correct porosity works beautifully. I find that the fine roots of plants grown in Whichford terracotta (and here I must declare that I am the gardener there) bind themselves to the terracotta, so that the pot and rootball almost become one – this prevents the compost from shrinking away from the pot walls (as it does in plastic pots), so that it is easy to water the potful without the water running uselessly down the gap. I can honestly say that I have not had problems with plants getting heat stress in our pots – but correct plant choice and positioning is important too…

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        • My experience with sturdy terracotta is quite limited so its invaluable to learn from your experience, Harriet. My experience with terracotta is that it can dry out rather quickly – or is this just the case with the cheap stuff? I did try some blueberries in large, very good quality terracotta pots last year and they did less well than they did in the plastic pots. Although there may be other reasons why this happened (eg I moved them to a more sunny spot as apparently it improves the flavour of the fruit). I’d be very interested to know if you would you grow a water hungry plant like a courgette in terracotta?

          I’ve occasionally lined terracotta with plastic to reduce the water loss – is this ever something you’d recommend?

          Reply
  • You are spot on – it is much easier to look after a large container than a small one and yields will be better. My tip (I have 200-500 pots on display all year) is to group pots together – creating a more humid microclimate – thus the pots dry out much less quickly. This way you can also insert pots of flowers among the group to attract pollinators and move them to hide empty spaces where crops have been harvested.

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    • Thanks for sharing your experience and tips, Harriet. I love growing a few flowers in between crops, too. And very interesting observation about grouping pots to create a humid microclimate – that’s a great idea. I know that some gardeners say it’s important to ensure there is a good air flow between plants to help prevent pests – do you ever find this is an issue when you place lots of pots together?

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      • I know that’s potentially an issue, but the pots don’t need to be crammed together to make a difference, in fact because our pots are decorative I arrange them so that they can all be seen, this often involves putting some of them up on brick plinths or walls. It’s trial and error, really, and of course with containers it’s easy to put spacing errors right! I also think that because plants are raised up in pots the airflow can actually be better than on the ground. A lot of gardening involves getting to know your own patch – where the stagnant, snaily corners or the wind tunnels are – and making adjustments to optimise the use of your space.

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  • This website is a true inspiration! Please, keep it going. I’m interested to find out more about your wooden boxes. Are the water reservoirs on the bottom fully water tight and are they connected to your water butt? Also, for a similar size balcony garden in your experience would you say a 50 litter water butt connected to a down pipe would be big enough.

    Reply
    • Yes, the water reservoirs in the wooden boxes were fully watertight – they were basically empty plastic boxes linked together with plumbers piping, which in turn were linked to the water butt. If you buzz me an email I can send you a link to the design. About water butt size – I guess it depends on a few things like how much rain fall you get in your area, how regular it is and how big a roof area you are collecting from. Water is always handy to have so if you can fit / afford a slightly larger one (or two linked together) it might be useful. But a 50 litre could be enough (much, much better than no water butt!) – maybe give one a try and see how it goes? (The one on my balcony was 80 litres – which was enough for most of the year).

      Reply
  • I always find the bigger the container the better (within reason) and your analysis explains very clearly the reasons why. Deep metal planters are often available from garden centres and DIY stores and these are lighter than terracotta and last longer than wooden containers. They can add up to a united display on a roof space.

    Reply
  • Very topical, I’ve just been outside looking at what’s survived the cold temperatures & planning which edibles to grow in which pots/bags…
    My challenge is finding veg/herbs/fruit that do well in containers that are mostly in the shade. Do you have any recommendations?

    Reply
    • Most leafy veg usually do OK in less sun – so things like salad leaves and oriental leaves (like pak choi) usually do well. Also many herbs do well, particularly things like mint, parsley, chives, sorrel, and lovage. Even some of the more Mediterranean herbs like rosemary and bay will do OK with just a few hours of sun. As long as you don’t have total shade (you really need a minimum of 2- 3 hours a day) all these things should do OK. I’ll be putting together a more comprehensive list in the near future.

      Reply
  • This is a really useful post – thank you. However, I was sorry to see the Earthboxes are made of plastic and contain a peat-based growing medium. Do you think it is essential to used peat in containers? I really try to avoid it if possible.

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    • Thanks for sharing these thoughts, Joanna, important issues. Like you, I don’t use peat in containers, either, and don’t think it’s necessary. The Erathboxes I’ve seen for sale have always been empty – so you can choose which compost to use. I usually use municipal compost or New Horizon peat free – and add some loam if I can find it. Plastic is also a thorny issue and, to be honest, I’m not too sure where I stand on it. On the plus side for Earthboxes, the plastic used is very strong and resilient (as well as being ‘food grade’), designed to last for ten years or more.

      Reply

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