Potting a cactus Potting Cacti and Succulents

When to re-pot?

Take a look at the cactus on the right. It's filling the (2") pot totally - the soil surface cannot be seen. Now is the time for a new pot - if you want your cactus to grow that is.

Of course, in the wild, this plant may well be growing in a crack between rocks, with even less soil than this. Who is going to repot it? The answer is that it will of course survive without re-potting at all, but it will be starved of nutrients and growth will slow right down.


Do we want our cacti to just survive? This Mammillaria would survive quite well in this pot for another 10 years or so if not potted on (If given some water and feed from below - in a saucer) but if put in a larger pot, it could double in size again in the next 12 months.

By not re-potting we are treating our plants as bonsai subjects and keeping them small. This is fine if you are limited by space and really don't want them to grow bigger. Potting them on, say every 2-3 years or so, into pots (just one size larger each time), will allow your plants to grow and will encourage more flowers as the plants are able to mature.

Don't over pot. A pot just a little larger than the plant is best. Repot generally when the plant touches the plant rim (or with a tall plant, the day before it falls over!) into a one-size larger pot. A good, gritty, porous compost is needed, in the UK an easy mix is to use is John Innes no. 2 with 50% sharp sand added or Multipurpose compost with added John Innes - again I like to add about a half extra grit to this as well.

To make your own mix, I would recommend 50% (sharp) horticultural sand (= 3 mm grit) , 30% sterilized loam or garden soil, 15% pumice or perlite, 5% moss peat or leaf mould. A slow release fertilizer such as Osmocote (NPK 18-16-12) can be added, which provides nutritional requirements for the first season .

Many combinations of soil work well. Each book will give you a different recipe. Experiment for yourself, with locally available materials, bearing in mind that the soil must be well-aerated, free-draining and with a slightly acid pH of between 6-7 (for best absorption of nutrients).

The compost should also maintain this structure over a period of time. The trend today is to move away from composts with large humus (peat/leaf mould) content as this tends to break down over a year or two. Coir (coconut fibre) maintains a better structure and is a more renewable resource.

Pumice or other similar “inert” materials are increasingly used, soil structure and aeration are maintained and, as with humus, the pumice can hold on to nutrients and make them available to the plant.

The above notes apply to the vast majority of succulents, although there are always exceptions.

This mammillaria might like a new pot
This mammillaria could do with a new pot, but if you asked it, it would say it's quite happy!

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