Each week gardening expert Sabrina Hahn responds to The West Australian readers’ most pressing problems.

This week she’s helping with a bay tree, viburnum, privet hedge and mandarin, and confirming whether a mate’s handy hint is actually a good idea or not.

To submit your own question to the Green With Envy column, published inside Saturday’s The West Australian, write to Ask Sabrina, GPO Box D162, Perth 6001, or email home@wanews.com.au.

Include your full name and suburb. Due to the volume of questions, not all of them will be answered.

Needle drop

Q I have a large pine tree in the front garden which I think is a Cook Island pine. (The branches curl upwards at the ends). The tree sheds loads of needles which form a deep covering on the ground. Someone has suggested that the needles are acidic and would make good mulch for azaleas, of which I have four in pots in various parts of the back garden. Is this correct and should I collect them up so the other plants in the front garden are not covered in needles?

Vincent Shaw, Winthrop

A Pine needles make an excellent mulch for acid-loving plants such as azaleas, provided they are not applied too thickly. Aim for a layer of about 5cm, which will help retain moisture around the root system without smothering the soil. Definitely remove the needles from the front garden if they have built up into a thick layer. Some plants may not tolerate them well and a deep covering can stop rainfall from reaching the soil properly.

Spot luck

Q My bay tree seems to have white spots. Not sure if this is normal or perhaps a bug? Some leaves are damaged.

Susan Miles, Carlisle

A The spotting on the trunk of the bay trees is quite normal and nothing to be concerned about, however, the leaves appear to have a nutrient deficiency. Apply liquid trace elements to the entire plant and drench the soil. Bay trees are very resilient and will bounce back pretty quickly.

Rust riddle

Q I have a problem with my Viburnum tinus, with rust covering all leaves and any regrowth. They have been growing well for a few years and now they are having problems. I have experienced this with other plants along the same stretch. Cottonwoods suffered the same fate. It was suggested that maybe it could be “millea”. I am not sure of the spelling but apparently it travels underground and effects what is growing above. I am not sure whether I should chop them out.

Josette Morgan, Esperance

A You may be referring to Armillaria root rot disease but a soil test would need to confirm this. One of the symptoms is a honey-coloured fungus that appears at the base of the plant. There is no fungicide at this stage to treat armillaria. Improving the soil with minerals and microbes, compost and manures might help in improving the soil to support better nutrient uptake by the plants and becoming more resistant to disease. Drench the soil with Soil Dynamics Jurassic Juice, which has humic and fulvic acid.

Time to repot

Q I never miss your Saturday articles, they have been a great help over the years. However, the attached problem I cannot solve. Potted 10-year-old grafted mandarin tree has delivered this problem for the first time. Foliage started dropping about a month ago. As they ripen they soften and taste very nice.

Kevin Edgar, South Yunderup

A Citrus trees should be repotted every five to six years because their root systems can fill the entire volume of the pot, leading to a shortage of moisture and nutrients. Additionally, the roots can block the drainage holes, preventing water from draining properly, which can cause the soil in the bottom of the pot to become anaerobic. Before repotting, it’s advisable to prune the tree by one-third. Once the tree is out of the pot, you can also do some root pruning. To improve drainage, consider drilling extra holes in the sides of the pot. It’s a big task and will require an assistant to help get the tree out of the pot.

Privet problem

Q I have a privet hedge that is turning black. I have tried Mancozeb on it, used Seasol but still it’s the same. I don’t want to lose it as it blocks the bareness of the front garden.

Vicky Smith, Dawesville

A Privet is a terrible weed that spreads quickly and is highly invasive because it can produce up to 10 million seeds annually. It can live up to 100 years which is why it becomes a menace. The reason it have sooty mould is because of sap suckers like scale, aphis or mealy bug secreting honeydew. Prune it back heavily then spray with eco oil, and repeat spray a fortnight later. Good replacements for your hedge that are less invasive are Escallonia, Hibiscus Apple Blossom, Callistemon Western Glory, Hakea laurina and Viburnum Anvi.