This Old Crock Doesn’t!

There’s long been a debate about whether you should put “crocks” in the bottom of your containers or not. My view is that it’s a lot of hot air. This post was in my “whenever” pile until I came across a tweet (on Twitter – where else?) that suggested adding polystyrene to containers to aid water retention! Polystyrene has become the modern crock of choice for many people who seek a way to utilise the remnants of those six-packs they bought at the local garden centre/supermarket. It’s never going to aid water retention. And it’s never going to aid drainage. At best, it’s going to save you a bit of compost.

If you have plastic containers, usually they’ll have arrived at your place without any holes in the bottom. But more often than not, the bottom will be split level. Have you ever noticed that decent flower pots have split level bottoms with holes in both levels? A lot of people don’t realise that the reason for this is that the lower level holes allow water to soak in from below when you stand the pots in water to irrigate them. Then when you stand the pots in shuttle trays, the lower level holes are impeded a bit which allows the pot to retain a little water. The higher level drainage holes ensure that not too much water is retained.


So if you buy plastic containers, first job is usually to make holes. Some people use a soldering iron to make the holes; others heat some sort of poker on the kitchen hob. A drill’s fine provided you don’t push too hard – this usually results in the bottom cracking rather than getting punctured with holes. Even better if your drill bit collection includes some bits with a spike in the middle. There’s probably a name for them but I don’t know it. If you have a split level  bottom (in your container!), make the holes in the upper level – you’re likely to water from above now and having a small reservoir of water in the lower level will help on hot dry days. How big should the holes be? If it’s a decent-sized container, aim for a centimetre for each hole – your flower pots will have 5mm holes at least so a centimetre isn’t overkill for larger containers. 5 or six of these holes will be enough for a decent-sized container. I work on about 3 to the square foot of base area plus one for luck. And accepted wisdom is that you don’t need to crock plastic containers. My wisdom is that you only use plastic containers in places where you can’t see the containers, only the plants! Well, usually. I do have some rather in-your-face plastics though I’m gradually replacing with ceramics.

This is what I mean - see those spikes in the middle? A bit different from ordinary wood bits but great for drilling in plastic.
This is what I mean – see those spikes in the middle? A bit different from ordinary wood bits but great for drilling in plastic.

If you’re a terracotta or stone enthusiast, or indeed a metal one, usually you’ll have a single hole around the middle of the bottom of the container. This will be anything between the size of a 1p and 50p coin. If there’s no hole, don’t buy it as there’s a strong chance that as you drill into the teracotta or stone you’ll crack the pot. (If you’re buying a pot for an indoor location – conservatory or whatever – you may not want a hole at all though it’ll be a lot better if you get one with a hole and then stand it in a saucer/tray filled with an inch or so of gravel so you don’t end up with a waterlogged plant!)

For outdoors, there are probably three things to worry about. In increasing order of importance:

  • Compost exiting through the hole
  • Drainage
  • Ants (and slugs) getting in through the hole

Yep, I rate the incursion problem above the drainage one. An ants’ nest in a lawn or a border is fairly innocuous – there’s lots of soil around it and a quick dose of ant-killer will deal with the nest if you’re that bothered. A quick brush with a broom sorts the lawn. You could argue that ants aerating the soil are doing as much good as earthworms and keep brushing the mounds. It’s only when a nest has clearly grown way too wide that I aim to destroy it.

But in a container, it’s different. The disturbance to roots may well kill the plants. You don’t want ants in your containers.

At the other extreme, you have a hole. Yep, water will drain through it. And the water will carry anything it can with it. If you don’t believe me, make a little mound of soil in one of your borders. Then (using a rose so you don’t have a major torrent), water that little mound. It’ll soon disappear. Water draining from the container will take compost with it. If your container’s standing on pot feet, then the water will happily run sideways once it exits through the hole and the compost will be carried out of the way. So the next watering will mean a bit more compost gets carried out with the draining water. But, generally, the amount will be insignificant (unless you don’t top the container up for about 20 years!). The crocking advice dates from a bygone age when everyone made their own compost and multi-purpose was still a jewel in Mr Innes’s eye.

So we’re left with drainage as the middle problem. If your container’s standing on gravel or earth, there’s somewhere for the water to drain to. If it’s standing on a solid surface then you should lift it up using pot feet or something similar. But people worry that compost will wash down into the hole and block it.

So add some crocks. Then put the compost in the pot. Initially, you’ll have some air under the crocks and this, you assume, will aid drainage.

But as soon as you add water, the draining water will carry compost into the air holes under the crocks. It’s just like Wagner’s Ring Cycle, except you don’t have to sit through 20 hours of caterwauling between the Rhine Maidens kicking things off and then ending things. Whatever you do, the compost will wash down into whatever air spaces you construct.

Stop for a minute and think.

Above all this drainage stuff you’ve probably got a foot or so of solid compost.

It drains.

So don’t worry about drainage. Worry about ants. And, did you know, you can sort the problem of ants and, at the same time, stop all your compost running out of the drainage hole(s) along with the water?

Arm yourself with an offcut of landscape fabric and a few square inches of chicken wire. Any close-mesh wire will do. If all else fails, an old kitchen sieve will provide the necessaries.

  • Cut yourself two squares (or circles if you’re that way inclined) of the wire mesh which are about an inch bigger all round than the drainage hole. Cut a piece of landscape fabric about the same size.
  • Place one piece of wire mesh over the hole.
  • Place the piece of landscape fabric over the mesh.
  • Place the other piece of wire mesh over the landscape fabric.
  • Fill the container with compost.

The compost isn’t going to get out and ants aren’t going to get in. You’ll have a hole so you’ll have drainage.

Pretty simple, wot?

And, of course, not only ants will get excluded by the landscape fabric.

So will slugs!