Techniques
Potting on
Moving a growing plant into a larger pot once it has outgrown its current one, to give the roots more room.
Potting on is the simple act of giving a young plant a bigger home. A seedling or small plant happily fills its pot with roots, and once those roots run out of room it slows down, dries out faster and starts to struggle. Moving it into a slightly larger pot of fresh compost gives the roots new space to grow into and a fresh supply of nutrients, keeping the plant growing on strongly. It is one of the most common steps between sowing and planting out, and an easy one to get right.
The clearest sign a plant is ready is roots showing at the drainage holes in the bottom of the pot. Tip the plant gently out of its container and you may see a dense web of roots wound round the outside of the compost — this is "pot-bound", and it means the plant has outgrown its space. Other clues are growth that has stalled despite watering and feeding, or compost that dries out almost as soon as you water it because there is more root than soil left to hold moisture. When you spot these, it is time to pot on.
The golden rule is to go up just one pot size at a time — for example from a 9cm pot to a 1-litre, not straight into a bucket. A small plant sitting in a huge volume of cold, wet compost cannot use all that moisture, and the soggy soil around the roots can lead to rot and damping off. Stepping up gradually keeps the compost in balance with the size of the root ball. To do it, part-fill the new pot with fresh multipurpose or potting compost, settle the plant in so it sits at the same depth as before, fill round the sides, firm gently and water it in.
Potting on usually follows on from pricking out, where tiny seedlings are first moved from their seed tray into individual modules or small pots. From there a vigorous plant such as a tomato or chilli may be potted on once or twice as it grows. Keep it somewhere warm and bright between moves, then, before it goes outdoors, give it time hardening off so the change from cosy windowsill to open garden does not check it. For more on growing plants on in pots all the way to harvest, see our guide to growing food in containers.
In a UK garden
In the UK you'll mostly pot on from March to May, keeping plants growing steadily indoors or in a greenhouse until the weather warms enough to harden them off and plant out.
Example
A tomato seedling in a 9cm pot, roots showing at the drainage holes, is moved up to a 1-litre pot of fresh compost a few weeks before going outside.