Plant Doctor Archive

Silk tree

I have a silk tree (not the deciduous type), approximately 5 feet high, and still in its infancy. We live in Christchurch, which is being hit by some "cracker frosts" recently, leaving the leaves and fronds on our silk tree rather brown and dry looking. Have the frosts been too much for the tree, or might it start to perk up once the frosts disappear? Is there something I can do to protect the tree until then, or is it too late?

Silk tree is the common name for Albizia julibrissin, which is deciduous. Grevillea robusta is commonly known as silky oak and is evergreen. I think this is the tree you have. Silky oaks are hardy once established. The best time to plant to get trees and shrubs established in areas where hard frosts are experienced is spring.

Wait until the risk of frost has passed and trim back the damaged foliage — do not do this before then, as new growth will be cut back with any late frosts. You will have to wait until temperatures warm up to see how severely the frost has affected your tree. If the tree has been irreparably damaged, it could be a couple of weeks or months before you know.

There are a few things you can do to protect your tree in the future. Avoid applying fertiliser in autumn as this encourages soft new growth. Fertilise plants in late summer so new growth gets a chance to harden off. Apply a thick layer of mulch in late summer before frosts appear; this will help insulate the roots — pea or barley straw is ideal. Use frost cloth, which is available from hardware stores and garden centres, which can be left on the tree or removed each day.

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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