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Question Time – Tasmanian blue gums/black spot/onion weed

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Question Time – Tasmanian blue gums/black spot/onion weed

Tasmanian blue gums

Don was asked whether it was a good idea to plant Tasmanian blue gums (Eucalyptus globulus) in the garden. Eucalyptus globulus is the floral emblem of Tasmania. Don said that this lovely tree is far too big for an ordinary suburban backyard. However, there is smaller variety of the same tree called Eucalyptus globulus ‘Compacta’. For details on availability ring your local government forestry nursery.

Black spot on rose

Many members of the audience were having trouble with black spot on their roses. Black spot is a fungal disease that thrives in hot, humid conditions. Symptoms are dark spots on leaves, yellowing leaves and leaf fall (older and low leaves are affected first). Some rose varieties, such as ‘Iceberg’, are more resistant to black spot than others. To control black spot:

Spray in spring and summer with a fungicide formulated for black spot. This will be necessary every few weeks, particularly if the weather is hot and humid.
Spray the plants and the ground around them with lime sulphur or bordeaux after winter pruning and before bud swell. This will help to control fungal spores overwintering on the plant.
Gather up fallen foliage and put in the garbage (not the compost).

Onion Weed

Don was asked how to get rid of onion weed. He said that the only way was to spray with a glyphosate-based product, such as Roundup or Zero. Trying to dig out onion weed will only make the problem much worse. There may be thousands of bulbs in the soil and they’ll keep coming up, so persistence is the key. It’s a good idea to have some spray ready so that you can repeat spray whenever you see new shoots appearing. If there are other plants nearby it may be best to apply the glyphosate with a small paintbrush, to ensure it only makes contact with the onion weed and not plants you want to keep growing well.

Approximate product costs


Yates Rose Gun, $10 for 750ml
Yates Lime Sulphur Spray, $9 for 250mL
Chemspray Bordeaux, $16 for 200g
Glyphosate sprays such as Roundup or Zero, $40-$45 for 1 litre concentrate

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  3. Don’s Expert Answers: Large Lemon Scented Gum in large backyard. Planted when neighbours all had large backyards, but now houses have been built close to the tree. Is this dangerous? I recently had to cut down and grind the stumps of two other Lemon scented gums as they were very close to the fence line and the new house built in the neighbour’s backyard. The remaining gum is a few metres from where the trees were. Will the dying root systems from the other trees make the remaining tree unstable? Last question are these trees a fire hazard because of the eucalyptus oil?
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