Harness rhubarb's poison

01:40, Nov 22 2012

A gardener got in touch last week wanting to know whether it would be safe to spray fruit trees with rhubarb leaf spray while the fruit is just starting. They are trying to keep codling moth at bay this year.

I wonder about how toxic rhubarb leaves are as I have at times found holes in my rhubarb leaves and other gardeners have reported similar at times.

So a search of the internet revealed the leaves contain oxalic acid and the estimated lethal dose of oxalic acid is about 375mg/kg. Rhubarb leaves contain on average 0.5 per cent oxalic acid, which means to cause death, a 70kg person would have to eat about 5kg of leaves. However, sickness occurs at much lower doses. It is usually fairly rapid in onset and unpleasant enough (abdominal pain, vomiting, etc) to keep the average person from continuing to eat the leaves. The oxalic acid may also have some effect on formation of kidney stones if enough is consumed.

The stems of rhubarb - which we eat - also contain much smaller amounts of oxalic acid so are safe to consume. It would appear that cutting up rhubarb leaves and boiling to remove the oxalic acid, and then spraying plants with the resulting liquid, will help control some insect pests. The concentrate will be more harmful to your own health if not handled carefully.

Another question I often get asked is, is it OK to compost rhubarb leaves? Because they are usually mixed with other material in a home garden situation I see no problems. If, on the other hand, you were to compost a ton of leaves there would likely be a greater concern.

Gardeners spend a lot of time outdoors, which is very healthy for you, but sunlight can cause problems in regards to too much, or not enough. The time of the day we spend outside in our gardens will make a difference. Remember, only "mad dogs and Englishmen go out in the midday sun".


Gardening times are best before 10am or after 4pm and even then you can get burnt.

Gardeners with potatoes and tomato plants in their gardens should be applying Neem Tree Granules to the soil to protect against psyllid attack damage and spraying the plants with Neem Tree Oil occasionally for the same.

I have noticed in my own gardens a few of the seasonal garden pests appearing, so a preventive spray of Neem Tree Oil is well worthwhile.

Problems? Phone me on 0800 466 464 or email wallyjr@gardenews.co.nz.

The Southland Times