Your gardening questions answered: Why are my trees dying?

Q: I don’t know the name of these trees but they seem to be dying, with casting of leaves getting worse and worse. Can you help? BMcW, Co Donegal

A: Your trees are a decorative, non-native, sun-loving evergreen species that hails from New Zealand and is known as the cabbage tree, or Cordyline australis. Despite its tropical, somewhat truffula tree-like appearance, it’s generally considered hardy in Ireland, except in very cold, inland parts of the country where it can sometimes struggle. Although it can happily tolerate temperatures as low as minus 7C, last winter’s sustained, exceptionally cold weather meant that even mature plants growing in coastal Irish gardens suffered.

I’m guessing this is the cause of your cordyline trees’ unhappy appearance. To help them recover, I’d suggest cutting back all of their dead or badly damaged swordlike leaves. Then use a sharp secateurs to prune back any dead or damaged sections of the trunks. In some cases, this might mean cutting them right back to the base but they should soon reshoot. To help them along, spread a 5cm layer of organic mulch around the base of the plants and then top dress with a slow-release organic granular fertiliser in spring.

Occasionally cabbage trees that have been badly affected by extreme winter cold will also develop a condition known as cordyline slime flux, where both the frost-damaged crown of the plant as well as its frost-damaged leaves subsequently become infected with bacteria. When this happens, the affected parts of the plant have an unpleasant smell and an orange or white liquid might ooze from them. Again, the only solution here is to prune away the affected parts of the plant, which will usually soon recover. Just make sure to protect any subsequent new soft growth from frost damage until it’s properly hardy.

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[ Your gardening questions answered: What is my mystery plant? ]

One last word of warning: in exceptionally cold winter weather, you may still need to occasionally offer even established plants addition protection in the shape of a few layers of horticultural fleece, something that’s increasingly likely, given the increased temperature extremes caused by climate change.