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Potting mixes Updated: 21 Aug 2009

Poorer performers

Among the poorer performers were Results (North Island version), Tui Outdoor Container Mixture and Tui Vegetable Mix.

The North Island version of Results produced very poor plants – as it did in our 2005 potting mix test. The South Island version performed better, so presumably has a different formulation.

Tui Products told us that in its autumn growing trials – which occurred after we’d bought the potting mixes for our test – it had found problems with some of its mixes.

These problems were traced to bark that was too fresh. Woody material such as bark needs nitrogen to break it down into compost that the plant can use. If it’s too fresh it uses up the nitrogen in the mix as it composts, leaving little for plants.

Tui Products informed retailers as soon as it became aware of the problem. It says it has increased its quality-control checks and no longer uses the supplier who provided that bark.

Tomato plants that we grew in the Tui Vegetable Mix also showed signs of damage that may have been caused by contamination of the compost on which this mix is based.

Distorted leaves suggested the contaminant may have been clopyralid, a long-lasting herbicide ingredient used to kill broad-leafed weeds. It can enter the composting process in lawn clippings, hay, straw or animal manure.

In New Zealand, herbicides containing clopyralid may only be sold to an approved handler. They may not be used in home gardens. Tui Products sent samples of the mix to be tested for clopyralid. No trace of clopyralid contamination was found in the samples.