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Debunked: crocks in plant pots helps drainage

Pot crocks

We seem to have caused rather a controversy in the normally sedate world of gardening by debunking a gardening myth that’s been around for years…

Turn on any gardening TV show or open any gardening magazine and you’ll find gardeners religiously putting a layer of ‘crocks’ at the bottom of their patio pots before filling them with compost.

A pile of crock

The idea behind this activity was to improve drainage. But Which? Gardening research found that crocks made no difference to how well our plants did. Research by soil scientists has proved that water does not flow freely from fine-textured materials such as compost into coarser material such as crocks. In fact, it only does this when the compost above the crocks is saturated. So, in wet summers, crocks can prevent water draining out of the pot and do more harm than good.

Yet, so many of us do it. We found that 62% of Which? Gardening members always use crocks in large pots – only 6% never do. And 71% of crock-users opt for broken terracotta pots. But with plastic pots now being more commonly used than terracotta ones, it becomes an expensive business breaking perfectly good pots just to use as crocks.

Will you still use crocks?

So what will you do this spring? There is an argument that crocks help reduce the volume of compost you need to fill pots and so you could save you money. Using broken polystyrene instead of terracotta crocks can also help to reduce the weight of pots, which is useful if you struggle to carry them or are going to put them on a balcony or somewhere else where you don’t want excess weight.

For me, my nostalgic side will miss the ritual of finding crocks and putting them in, but debunking myths is what makes working for Which? Gardening so fascinating. Therefore, if not using crocks is one less job to do and it’ll also help the success of my pots, then I’ll be happy to give it up.

Do you put crocks in plant pots?

Yes (53%, 591 Votes)

I did, but will stop thanks to Which? Gardening's research (28%, 318 Votes)

No (19%, 216 Votes)

Total Voters: 1,125

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Using crocks helps prevent compost escaping through the drainage holes and by adding weight it can help prevent pots falling over if the plant makes them rather top heavy. That’s my argument for continuing to use them.

Can the “crocks” be recycled? Or will they just end up in the general rubbish? So using them at the bottom of flower pots would be a good way to avoid unnecessary landfill use.

My crocks have probably been reused many times because new ones are less easy to obtain thanks to the move to plastic pots. I have a plant pot with my stock. No doubt you have heard of the term: “A load of old crocks”. 🙂

I stopped using crocks 25+ years ago. Instead, I use polyester wadding (the stuff my wife uses for stiffening and backing in sewing). It’s thin, so doesn’t steal room from the growing medium, it stops the groewing medium falling through the drainage holes and its capillary allows for good drainage. It is resistant to nasties crawling into the pots through the drainage holes too.

Buy the cheapest, thinnest wadding and simply cut a small circle (or square) to fit the bottom of the pot.

This sounds like a brilliant gardening tip. Thank you.

Steph says:
3 May 2014

Yep – I think so too, and so will I! Thanks

Flibertigibit says:
4 May 2014

Similarly for many years I have used cut up pieces of cheap versions of those checkered throw away J cloths cut into roughly the right size and usually double thickness. I especially do this for plants I know will be moving into bigger pots soon or planted out. It does the job well and seems to be biodegradable so I don’t need to bother disturbing the roots, just leave it to rot away.

Old j-cloths sound like a good idea to me too. As you say, they’ll eventually biodegrade (rot), so many be even better than wadding.

Margaret Mccadden says:
13 March 2015

This is a brilliant idea! You’re a genius, Thank you

Moolie says:
30 March 2018

I have for years used squares or circles cut from old tights or stockings for the same purpose. For me even cheaper than wadding and reusable too. I use broken crocks as well, when I want to add weight to the bottom of a pot.

Sally says:
25 April 2014

I use anything as long as it`s not going to harm the plant. However, will now not worry so much about drainage, particularly in small pots. Thank you Which? Mind you if your plants are keeling over in the wind because they are top heavy perhaps you should put them in a wider base pot.

I stopped using crocks after being so advised on one of the TV gardening shows (I think). Now I don’t know what to do with all those broken pots (lots after this winter, for some reason). But putting crocks at the bottom of containers doesn’t seem to make any difference – and, in fact, they do get embedded in the compost, and prevent drainage. So I’m no longer putting them in.

I’m confused. Article above states:

“water does not flow freely from fine-textured materials such as compost into coarser material such as crocks. In fact, it only does this when the compost above the crocks is saturated. So, in wet summers, crocks can prevent water draining out of the pot and do more harm than good.”

Water only flows freely when compost saturated but in wet summers crocks prevent drainage? Contradictory statement.

I use old bricks but this is more to make the pots heavy and harder to steal.

Hi Fat Sam

The crocks stop water draining from the compost until the compost is absolutely saturated.

So having crocks in the bottom of the pots keeps the compost wetter than you’d want it in a wet summer, a bit like having a pot with no holes in the bottom. If there is a thunder storm and the compost becomes awash, water will then flow through the crocks, but not before, leaving you with constantly soggy compost.

I hope that explains things a bit more clearly.

I do not claim to have much expertise in growing plants but I don’t have a problem with constantly soggy compost if I use crocks.

I have been putting crocks in pots for many years and have just learned something new : )
However I find putting a few crocks in the bottom also helps when taking a plant out if its become a bit pot-bound. It just takes something as thin as a pencil to push the crock up and the plant out.
There`s something comforting in old crocks or an old beach pebble or two. I once dropped a
favourite blue and white cup on the floor of the greenhouse and used pieces of it instead of crocks. There they stayed for years until I forgot about them, it was like seeing old friends again `Oh, hello, there you are!`
I guess I`m just an old crock too : )

Lessismore says:
27 April 2014

I use old broken mugs as well. I would never consider breaking up anything new to use. They stop the soil falling through the holes and make it easier to remove a plant from the pot.

Ozzie George says:
8 June 2014

I agree, been using crocks for years, always found they work perfectly , never had any trouble with drainage or root rot. If using good friable compost you won’t have any trouble

The RHS says “Where potting media might be washed out of the container, place drainage material over the hole(s) in the bottom of the container, using broken up polystyrene, stones or broken terracotta (crocks). Use a minimum of material as it is important to have as much rooting area as possible”. This is why I have used crocks, large or small depending on the pot or container size. Nothing to do with aiding drainage, just preserving the compost (not usually a problem though with peat-free fibrous stuff).
I also stand containers on spacers off the patio in an effort to keep out the ants – usually in vain. Anyone got a solution?

Hi Malcolm: as I posted above, I stopped using crocks and use a piece of Polyster Wadding cut to the shape of the pot bottom as a lining. This is just to keep the soil from washing out and (perhaps my naive thinking) to keep the slugs out.

Use the thinnest polyester wadding which you can buy in metre lengths from most haberdasheries. It is the white stuff they use for backing quilting and such in sewing. I’m sure your wife will know what to get. It is cheap and even a metre lasts a long time. Here’s an example of it http://www.amazon.co.uk/Polyester-Wadding-75gm-100cm-40in/dp/B00AXA4FIA/ref=sr_1_1?ie=UTF8&qid=1398598881&sr=8-1&keywords=polyester+wadding

Malcolm: To get rid of the ants try soaking the whole pot in water for about 30mins. If the pot is too large to lift you could try stuffing the drainage hole with plastercine and filling it with water for 30 mins.

I have always used crocks because I thought that they improved drainage. But I like the idea of putting something like the wadding Terfar suggests across the bottom of the pot instead.
The big reason I can see for not putting the crocks in, apart from drainage is that whenever I empty pots out I always find a collection of horrid slugs nesting in among the crocks, so I have provided them with a lovely sheltered position to hide in. Bah!

Sounds like a load of old crocks to me !!!!! What did they do….cement the crocks together???

Putting stones, pebbles, crocks, etc does help drainage. If there is water in the the saucer, the compost soon gets smelly if there are no crocks in the bottom to raise the compost.

Crocks also give the pot weight to stop it falling over.

Crocks also help roots to stay in the pot.

Crocks also reduce the amount of compost needed.

Sorry, you haven’t persuaded me to give up using crocks.

John Plumb says:
2 May 2014

I will do my own test trial this summer.I have two large identical tubs either side of the steps
in the garden.One will have terracota crocks the other nothing.I will report the outcome in
due course.

I cut circles of fine aluminium mesh from Halfords. They allow easy drainage and prevent insects getting into the pots.

Annabat says:
3 May 2014

Interesting – I use pebbles, crocks, polystyrene, newspaper (not all at the same time!) to stop the compost blocking up the drainage holes & slipping out. I don’t suppose it matters to any great extent whether thee does, or thee doesn’t .

I place a layer of weed control fabric over the holes in the base. This allows drainage but prevents slugs, worms and other insects from getting in via the back door!

Flibertigibit says:
5 May 2014

Now that is a good idea too! Gardeners are nothing if not resourceful!

Chris says:
11 May 2014

” Research by soil scientists has proved that water does not flow freely from fine-textured materials such as compost into coarser material such as crocks. In fact, it only does this when the compost above the crocks is saturated. So, in wet summers, crocks can prevent water draining out of the pot and do more harm than good.”

Doesn’t make sense to me, or am I missing some profound truth here?

I always thought the whole idea of putting crocs in a pot was to enable the water to drain BETWEEN the irregular spaces in the crocs and not just through them. It would also depend on whether the pot is made of clay of plastic. I always understood that water retention was greater in a plastic pot and that water in a clay pot dried out at a much faster rate being of a more porous nature.

If the water only drains freely when the compost above the crocs is saturated then surely in a wet summer the compost is more likely to be saturated and yet “crocs can prevent water draining out and do more harm than good!” Surely this is a contradiction in terms or am I also missing something here!!!

The reason we put a few pottery crocks in the bottom of pots is because the pots on the terrace stand in large saucers or trays so that when watering them any run-through is captured and reabsorbed as the soil dries out. This seems to work so we don’t think we’ll be making any changes. We don’t use fibrous compost for more than about a third of the fill mixing it with garden soil which is on the sandy side and prone to running through the drainage holes. This seems to suit the plants very well and we usually have healthy growth and good blooms or foliage as appropriate. I share Chris’s and Beryl’s reaction to the research report; it’s strange what they think is crying out for such earnest research these days.

I am a trifle upset that this research apparently has not been extended to considering the benefits of terra-cotta on stabilising the temperature of the roots.

” And, young or old, the roots are usually not has hardy as the plant’s top. American holly (Ilex opaca) is hardy to USDA Zone 5. The top part (stems and foliage) of the plant will survive to a temperature of about -20 F, but immature roots die at 23 degrees above zero, and mature roots at nine degrees. In the ground and insulated by the earth, that’s usually no problem for the roots of hollies in Zone 5 where the average minimum temperature is -10 to -20 F. But in a container, root damage in American holly would begin to occur at 23 degrees if left unprotected — a drastic difference from -20 degrees.”

Fortunately we do not have these temperatures but the principle remains the same that excessive heat and cold are not good for roots.

My belief is that the crock provides thermal benefits besides stabilising the plant pot. There may also be ,on very hot days evaporative cooling, which will also be of benefit.

So as it stands the Which? research looks partial and to be honest does not instil me with any confidence as there is no methodology or source research to link to. Perhaps Which? can provide a link please.

Ozzie George says:
8 June 2014

There is nothing wrong in using old crocks, been doing it for years and never had any problems with drainage or root rot. If using good friable compost you wont have any trouble. Using plastic milk containers and other plastic materials is also very good in very large pots. saves having to fill the pots with lots of compost and easier on the pocket too