Choosing potting mix for Brisbane's heat and humidity
If your potted plants keep drying out by lunchtime or rotting after a storm, it's worth looking at what they're growing in. The mix matters as much as the pot.
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Don't fill pots with garden soil
It's tempting to dig some soil out of the yard and fill a pot with it. In a garden bed, soil works because it's part of a huge system full of worms, fungi, drainage paths and deep moisture. Put it in a pot and it compacts, drains poorly and either sets like concrete or turns to mud. A lot of Brisbane's backyard soil is heavy clay, which makes this even worse. Roots need air as much as water, and compacted soil gives them neither.
Potting mix is made for containers. It's usually a blend of composted bark, coir (coconut fibre) or peat, sand, and added nutrients, and it's designed to stay open and drain well for a long time.
What the ticks on the bag mean
In Australia, potting mixes can be certified to Australian Standard AS 3743. Look for the Standards mark on the bag:
- Regular grade (usually shown with black ticks) meets the basic requirements for drainage, pH and so on, but may have little fertiliser added.
- Premium grade (usually red ticks) meets the same physical requirements and also contains enough nutrients to get plants established.
For pots on a balcony, a premium mix is usually worth the extra few dollars. Cheap unbranded mixes can be mostly fine bark that breaks down quickly and shrinks away from the sides of the pot.
Search for a premium potting mix and check the bag for the Standards mark before you buy.
What SEQ conditions ask of a mix
Our climate pushes potting mix in two directions, so you want a balance:
- Free drainage for the wet season. When a summer storm drops 50 mm in an hour, water needs to run straight through.
- Some water holding for hot, dry spells. Coir holds moisture well and rewets more easily than peat once it has dried out.
- A mulch layer on top. A couple of centimetres of sugar cane mulch or fine bark on the surface cuts evaporation noticeably and keeps the top of the root zone cooler.
"My pots dry out every afternoon"
If you're watering every day and plants still wilt by 3 pm, it's usually not about forgetting to water. Common causes:
- The pot is too small for the plant. There just isn't enough mix to hold a day's worth of water. Move up a size.
- The mix has become hydrophobic. Once potting mix dries out completely it can repel water, which runs down the sides and out the bottom without soaking in. Stand the pot in a bucket of water for 20–30 minutes to rewet it, and consider a soil wetting agent to help it take up water again.
- The mix is old. After a year or two the organic material breaks down, shrinks and compacts. Repot with fresh mix.
- The pot is baking. Dark pots in afternoon sun heat up badly. Shade the pot itself, or move it.
Feeding after the first couple of months
Even premium mix runs out of nutrients eventually, faster in summer when plants are growing hard and frequent watering washes nutrients through. After six to eight weeks, start a light feeding routine: a controlled-release fertiliser every few months, or a liquid feed every couple of weeks during the growing season. Follow the label rather than guessing. More is not better.
A safety note on potting mix
Potting mix bags in Australia carry a health warning for a reason. Potting mix can contain bacteria, including Legionella longbeachae, that can cause serious lung infection. Open bags carefully and away from your face, dampen the mix to reduce dust, wear gardening gloves and a P2 dust mask when handling it, and wash your hands afterwards.