This common potting mix mistake could be the reason your plants aren’t thriving

Potting mix is key to a thriving container or raised planter garden, giving the plants almost everything they need to grow. But when planting out seedlings or some new blooms, it’s common to reach for the bag that’s been sitting around at home from your last trip to the garden centre.

This isn’t always a good idea, because while potting mix doesn’t expire, factors like incorrect storage can degrade vital elements in the mix. Anna-lize Pretorius, seed production manager at The Diggers Club, told nine.com.au plants rely on the product to provide them with everything they’d get in the ground, so it needs to be in good condition.

“Potting mixes contain organic materials such as composted bark, peat alternatives, composts and fertilisers, which will slowly change over time. While the mix does not suddenly ‘expire’, some of the nutrients and biological activity can decline the longer it sits,” she said.

“The main changes that occur are nutrients become depleted as organic components break down, the structure can decline, causing the mix to become compacted and less able to hold air around plant roots, beneficial microorganisms may reduce if the mix has been stored for a long period or exposed to heat, and if it becomes wet and remains sealed, it can develop unpleasant anaerobic conditions, which may cause a sour smell.”

Correct storage of potting mix is the most important factor in keeping it in good condition. This means keeping the bag sealed, raised off the ground, in a spot that’s cool and shaded, and away from full sun, rain and extreme temperatures.

Unopened bags are more likely to retain quality than opened bags and can last a long time if stored correctly. Open bags, meanwhile, need to be used within six to 12 months, but they should be checked for quality first.

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“The mix should smell earthy and fresh, not sour or musty, and it should still have a good structure that allows air and water to move through it,” Pretorius said.

If the condition of opened potting mix has declined while sitting around at home waiting to be used, it is possible to revive it with small amounts of things like compost, worm castings, fertiliser and even fresh potting mix, but it’s often better to just get a fresh bag.

“Compost alone will not restore the full balance of a degraded potting mix. Potting mixes are specially formulated to provide the right balance of drainage, water-holding capacity and nutrients, so if the mix has become compacted, smells unpleasant or has broken down significantly, it is best replaced with a new mix,” Pretorius said.

Potting mix that has been used in a pot or planter also needs refreshing from time to time, as the nutrients get used up by the plant within three to six months.

“Container plants use nutrients much faster than plants growing in the ground because their roots are restricted to a small volume of mix. Regular watering also leaches nutrients from containers over time,” Pretorius said.

Fertilising regularly helps with mitigating this loss of nutrients, but other elements in potting mix also break down over time so it’s good to refresh it every one to two years.

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“Signs it needs refreshing include water running straight through the pot instead of soaking in, the mix shrinking away from the sides of the container, plants growing poorly despite regular feeding, roots becoming crowded or circling around the bottom of the pot and the mix becoming hard and compacted,” Pretorius said. 

To refresh potting mix in containers and pots, it’s recommended to remove old roots and debris from the container before loosening the existing potting mix with a hand fork. Then remove about 20 to 30cm worth of the existing potting mix and add fresh potting mix and fertiliser and mix before putting the plant back in.

“One of the biggest mistakes gardeners make is treating potting mix like garden soil. Containers are a completely different growing environment; plants rely entirely on the mix in that pot for water, nutrients and air around their roots,” she said.

“Starting with a quality potting mix and maintaining it throughout the growing season will make a huge difference, particularly for spring and summer vegetables. A little regular care, feeding, topping up and refreshing the mix, can keep containers productive for many years.”