How to make the perfect potting mix

For gardeners who enjoy DIY and experimentation, it’s easy to buy ingredients separately and create custom mixes to suit different plants. The main ingredient for almost all plant groups (the exceptions are cacti and bonsai) is composted bark, but you could also use standard potting mix. Other possible ingredients include scoria, gravel or washed sand, coir peat, pumice, perlite, vermiculite, horticultural charcoal, lime, fertiliser and a wetting agent.

You don’t need to buy every ingredient, as some play the same role. Understanding what each brings to a potting mix is key, as it can help or hinder plant health. Every ingredient performs one or more of these functions – drainage, aeration, moisture retention, pH, weight, bulk and nutrition.

Depending on what you’re growing, you can tweak the ingredients and their ratios. While every plant has its preferences, here’s a general guide to what matters most in mixes for groups with more specific needs:

• Cacti and bonsai – excellent drainage
• Orchids and epiphytes – excellent drainage and airflow
• Succulents – good drainage
• Ferns – slightly moist mix
• Azaleas, camellias, roses, etc – slightly acidic pH
• Aussie natives and South African plants – low to no phosphorus

It’s easy to get caught up in the details – ingredients, ratios and endless potting mix recipes – but simple is sometimes best. The two experts below rely on a few key ingredients in their blends, proving that you don’t need a complex mix to grow healthy, thriving plants. Just keep in mind that plants growing in simpler mixes may need more frequent feeding and, depending on your climate, extra watering.

Mix it like a pro

Horticulturist Craig Wilson, the owner of Gentiana Nursery, Victoria, swears by a simple two-ingredient mix tailored to the needs of the garden plants he grows: “I use only pine bark and gravel. I avoid coco peat. It retains moisture for too long, and when it eventually does dry out, it’s difficult to re-wet. Indoor plants should be allowed to slightly dry out as they’re in a still environment. The best way to determine if your plant needs watering is by the weight of the pot – if it’s really light, water it! My mix lasts for about two years before it starts really breaking down.”

Craig’s mix

• 75% 3–6mm pine bark (medium-grade orchid mix) 
• 25% 2–4mm gravel. He then adds a 3–6 month slow-release fertiliser for outdoor potted plants, and recommends using a liquid fertiliser for indoor plants.

Attila Kapitany has a renowned dry-climate garden at Narre Warren in outer Melbourne and is one of Australia’s top succulent and cactus experts. He says that although the two groups are often thought to have similar requirements, they are quite different: “Most common cacti are desert-adapted and need excellent drainage. Scoria is the main component in my cactus mix, and often that’s all I use. Many common succulents – like aeoniums, echeverias, agaves and aloes – can grow in general potting mix.”

Attila’s list of go-to ingredients includes general potting mix, scoria, washed river sand, horticultural charcoal, slow-release fertiliser and compost, which are blended in varying ratios depending on the plant’s requirements. Scoria forms the backbone of his mixes, making up as much as 90 per cent of the mix for cacti – notably for small, rare and slow-growing varieties. He often includes charcoal to absorb pathogens and help keep the mix fresher for longer, and to boost aeration, especially when used as crocking to improve drainage at the bottom of containers.

For all potting mixes:

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