Plant Doctor Archive

Growing macadamias

Question I HAVE two 10-year-old macadamia trees, one of which is grafted. This season the grafted one produced a crop of wrinkly shelled nuts which were shrivelled and inedible. What is wrong with my grafted tree which has been a reliable and prolific cropper until this year? What is the fertiliser for these trees as I have been given contradictory advice. (Fertiliser kills them — this from a nursery — to “I use 50 tons of chicken manure on my 50 acres of macademias”.) The trees are growing in coastal clay/sand with reasonable drainage. I cannot get chicken manure at present and have a large bag of sheep pellets.

Answer MACADAMIAS are not usually attacked by pests and diseases (except rats and possums!), so your wrinkly and shrivelled nuts are a bit of a puzzle. It could possibly be damage from the green vegetable bug, which will pierce the nut and make the kernel discoloured and bitter. Insecticides are probably your best option here. Another possibility is that your trees could have the wrong balance of nutrients.

Macadamias grow best in free-draining soils full of rich organic matter, but will cope with anything from clays to sandy loams, as long as they have adequate drainage. As for the nutrients, a general fertiliser should do the trick, but be aware that some growers consider that high levels of nitrogen reduce the amount of nuts you will get (high nitrogen often will make plants grow leaves rather than flowers and fruit). There is some evidence that the trace elements boron and zinc can help nut formation and development. Both chicken manure and sheep pellets would provide a lot of nitrogen, so depending on the levels you have in your soil, you may want to avoid them.

I suggest you get soil and leaf samples from your trees tested for nutrient levels. Several companies offer this service in New Zealand.

You can get more information from the New Zealand Macadamia Society, https://www.facebook.com/groups/645085699216814/