Potting mix: cheap doesn’t mean cheerful for plants and gardeners

An experiment shows you only get what you pay for with potting mixes. Avoid the supermarket and hardware products.

Sydney gardening expert Tim Pickles has experimented with a variety of potting mixes.

I’m always campaigning about not using cheap potting mixes because plants cannot grow well in them.

Tim Pickles, owner of multi-award-winning Tim’s Garden Centre in Sydney, is on the case too. He runs simple trials by planting the same seedlings into bags of rival potting mixes. The differences can be outstanding (see above), with almost no plant growth in the bags from two supermarket and hardware chains compared with healthy plants in a premium quality mix.

“New gardeners are buying these cheap mixes then blaming themselves when the plants die,” he says. “We will lose these people from gardening.”

So pay the extra for top-quality potting mix.

READERS’ Q&A

My 2m-tall kaffir lime has been crucial for my Thai curries, sporting luscious leaves with amazing flavour. But after four years is hasn’t grown any fruit. Can I speed up the process? - Sonali Patel, by email

The usual causes of not flowering and fruiting are too much shade or excess high-nitrogen fertiliser. Use Dynamic Lifter Plus for Citrus or Gyganic for Fruit and Citrus. Fruit is secondary to the leaves, both in production and uses. The flowers are tiny and once small fruits set, regular watering is critical to stop them being shed. The fruit is warty and bitter, with little pulp or juice, but the rind is used in cooking. Makrut lime is becoming the preferred name, as kaffir has racist connotations.

We removed some large cedars more than two years ago, then composted and mulched for a year before planting natives. After about eight months, healthy plants, mostly grevilleas, turned up their toes. The soil tested 5.5 pH. Is there a residual chemical left from the cedars? - Denise Chadwick, Ballarat

Any allelopathic toxins will have leached out or dissipated by now. The pH is good. Sudden death of established plants can indicate phytophthora root rot, a common soil-borne pathogen. There is no cure but phosphoric acid (Yates Anti-Rot) is the best treatment if applied early enough. The roots could have been eaten by curl grubs, or could have become waterlogged if they reached a clay layer not far below. Examine the roots and check soil at depth to aid diagnosis.

My dwarf beans are falling over from the weight of the beans. Should I stake them? At what size should I pick them? Something is munching on the leaves. Should I spray? - Linda Stebbean, Melbourne

Dwarf beans are prolific when young. Stake plants if needed. Pick beans when they start showing fullness as the seeds develop. Some beans can be short. They’ll get tough if left too long, and picking encourages more production. Tolerate minor leaf damage. Check for snails at night. Use Success Ultra or organic Dipel on caterpillars.

When is the best time to cut back hydrangeas and azaleas? - Suzie Hope, Hobart

Prune hydrangeas in autumn, or winter if you like the faded flowers. In frosty areas, delay until spring. Prune azaleas immediately after flowering, and again lightly if needed in summer.

Send your questions to: [e-mail] or Helen Young, PO Box 3098, Willoughby North, NSW 2068. Website: helenyoung.com.au. The best question for January wins a set of five elegant, magnetic vases for your dining table, worth $72.95 from Independence Studios (isgift.com).