Trees make our city cooler, improve air quality and provide more habitat for birds and animals. They also improve health and wellbeing, creating a calmer and safer environment.
Here are some top tips to help you plan and care for trees in pots.
Plan your pot sizes
The City’s Tree Management officers say “it’s a good idea to check the expected size of the mature tree and choose a container with enough space for the growing tree and its roots.”
The bigger the pot or planter bed, the more room there is for roots to grow and the better the plant will perform.
“For the best results, pot your new plant in stages over time,” they say “And expect to re-pot as the tree grows.”
A general guide for trees in pots is to start with a pot twice the width and depth of the root ball and finish with a large pot of at least 60cm wide at the top and 55cm deep.
Some other tips to consider:
- 20cm and 25L pots are not suitable for growing large plants long term. You’ll need to re-pot your new tree/shrub into a larger container within 2 months
- 65L pots are suitable for large shrubs with root balls up to 45cm wide and 45cm deep
- 200L pots are suitable for trees with root balls up to 60cm wide and 55cm deep
- Remember that a large pot with soil can be heavy. If you plan to place a large pot on a balcony or rooftop, check that the structure can bear the weight.
Caring for trees in pots
It’s important that there are adequate drainage holes in the bottom of your container. Excess water needs to flow out so that the root system doesn’t become sodden. Adding gravel to the bottom doesn’t improve the drainage.
Trees in pots dry out quickly and this retards their growth. Water often and thoroughly. Your tree will appreciate good quality potting mix. Look for the Australian Standard symbol on the bag. Everyday garden soil won’t do the same job. It doesn’t drain well in containers and is more prone to weeds, insects and diseases.
Frequent watering can leach nutrients out of the soil, so you need to fertilise regularly in small doses. Use a slow-release organic fertiliser or a water-soluble liquid type. Follow the instructions on the packet and check the dosage specifically for plants in containers.
Signs that it’s time to re-pot your tree
The size of a tree is usually proportional to its root system. If the roots have reached their growth limit in a certain container, the tree’s external size will be restricted. Re-pot to allow growth.
Every couple of years or so, you’ll notice your tree dries out more quickly and lacks vigour. This tells you that attention is required.
Un-pot the plant and trim the outer 10mm of the root ball with a sharp knife or your secateurs to stop the roots from growing around the root ball. Re-plant into the same or a larger container with enough potting mix so the top of the plant sits about 3 or 4cm below the rim. Then fill with more potting mix and water.
Explore what the City is doing to grow our tree canopy.
Published 26 March 2019, updated 5 January 2024
