This is done once a plant has outgrown its current pot, and the new pot is usually the next size up. Potting up provides the plant with extra room for root growth, fresh nutrients from the new compost used and stability for plants that are hopefully growing fast. This is slightly different to 'potting on' which is usually moving a seedling from a seed tray into it's first pot.
A good way to see if a plant is ready to be potted up is to check the base of its pot to see if roots are growing through the drainage holes. If they are, it's time to be potted up. If roots are not showing, try easing the plant out of the pot to check its root growth. If the roots have reached the sides and bottom, then pot it up.
There is a balance to be found between root growth, top growth and growing medium (compost). If there is too much root proportional to the pot size and compost, the plant will quickly dry out after each watering and become stressed (You may have seen plants like this on the garden centre sale table. They grow rapidly and form flowers prematurely (‘bolting’), attempting to reproduce before they die).
On the other hand, your plant is also unlikely to thrive if it is placed in too large a pot.
In this case the large volume of compost in relation to root volume means that the roots will not use enough of the water in the compost so that it will remain wet and the plant will suffer as a result. This is known as overpotting, and can lead to rotting of the roots and other diseases. The visible symptoms of too much water, such as yellowing or browning of leaves and wilting, are similar to the symptoms of too little water, so often the plant is given even more water, making matters worse.
When potting up it is important that the soil level comes to the same point on the stem as it was in the previous pot, though some plants are more tolerant of changes in planting depth than others.
The instructions given here are for potting up plants that will eventually be planted in soil outside. If you are potting up plants that will stay in pots (e.g. houseplants) you may want to give more thought to drainage (e.g. putting ‘crocks’over the drainage holes in the pot) and the composition of the growing medium (you may want to use a specialist compost or add sand/grit).
The instructions given below are for potting up from 9cm to 13cm pots, but can be used for other sizes too. This method works best using round pots.
You will need:
Take the larger empty pot and put a small amount of potting compost in the bottom.
Put the small pot (the same size as the pots your plants are currently in) into the centre of the large pot on top of the compost you have just added. Make sure that the top of the small pot is level with the top of the large pot, and not above. This will ensure that there is room at the top of the pot for watering to take place. You may have to add more compost or take some out to achieve this.
Hold the small pot still with one hand, then fill both the pots with compost. Make sure you keep the small pot central and level. Gently firm the compost around the small pot. You don’t want it to loose or too compact.
Tap the large pot gently on the bench to settle. Add more compost if there are gaps between the small and large pot.
Place fingers either side of the plant stem at soil level, then tip it upside down and tap gently on the bottom of the pot. Keep tapping and squeezing the sides gently until the plant comes out of its pot.
Take out the small pot from the large pot by twisting and then lift. These needs to be done carefully so that the compost doesn’t collapse.
Place the plant in the hole left by the pot. Tap the pot gently on the bench to allow the compost to settle next to the plant’s roots.
Label your plant and water using a watering can that has a rose.