Wednesday 28 February 2018

Garden myth or truth?

Healthy Yorkshire food
In writing about gardening myths I sometimes find a grain of truth within them. So too today.

Canadian sleuth Robert Pavlis hunts down gardening myths with great zeal. His blog is solely about gardening myths and his mind is endlessly fertile in seeking and finding fantasy in most unlikely places. His research is precise and he freely provides his own evidence base. 
He does home based experiments that expose silly notions. His particular targets are snake oil salesman who brazenly repeat the most outrageous product claims which he fearlessly exposes. I do get the impression that over the pond the public are more gullible than here.

Robert’s discoveries frequently surprise me.
A few weeks ago he published his personal top ten myths of the year. I have picked out two to which I have in the past been an unthinking believer and shamelessly admit that my post today plunders his thunder.

Myth *1. Eating rhubarb leaves will kill you …. and it’s the oxalic acid ‘what did it’ 

Not very appetising
Actually two myths in one. You would have to eat oodles of rhubarb leaves to come to harm… and if you do die  it isn’t the oxalic acid!

Most vegetables contain natural plant toxins. They are their natural defences to pest and disease. Indeed many scientists wonder if the toxins are the reason why vegetables are good for us! The scientific principle is known as hormesis and in a nutshell this hypothesis states ‘a little of what is bad for you does you good’.

It would take 25 grams of oxalic acid to kill us. That is a massive amount. Robert calculates you would need to eat a serving of 5kg of rhubarb leaves. I think he might be wrong in as much as most laboratory measures are ‘dry weight’ in which cases it would take very much more.

I believe Robert about the oxalic acid but am not minded to try the leaves
Where it gets interesting is that the oxalic acid in rhubarb leaves is  .5 gram/100gm whereas the ‘leaf stalks’ you eat contain  .45 gram/100gm. Almost identical - no real difference at all!

Weight for weight same oxalic acid content
Even more revealing is that oxalic acid is very common in many other vegetables. Carrots and radish, weight for weight, contain exactly the same oxalic acid as rhubarb leaves. Spinach contains twice as much, chives three times so and parsley a whopping great four.

Three times the oxalic acid
There is no way I will forgo my healthy parsley and egg sandwiches or parsley sauce with my fish pie.

Robert suggests that if small quantities of rhubarb leaves really do harm you the culprit might be anthraquinone glycosides. Even so there are today almost no (or even any) reported deaths of anyone eating rhubarb leaves. With their known laxative properties and no doubt horrible taste there are better ways to go.

Guess what I am thinking?
A medical drug extracted from crushed rhubarb roots was once highly prized and very expensive a little more than a century ago and has been fought over from ancient times. In the seventeenth century so called rhacoma root was three times the price of opium.

Myth *2  Adding peat acidifies your soil

Well of course it does but is the fall in pH significant or long lasting? The answer is a qualified no. 

Granulated sphagnum peat
Although I have long considered adding peat to improve a soil as extravagant and arguably environmentally irresponsible I do fear that on public platforms I might have recommended peat to acidify soil. It’s amazing what rubbish gardening experts utter when the listening public expects you to know.
( I do not regard it environmentally irresponsible to use peat as a compost ingredient although regular readers will know I do not use peat in compost myself)

The thrust of Robert’s argument Is that if your soil contains calcareous matter as alkaline soils invariably do then the capacity of the soil minerals to raise pH change is huge compared to peat’s puny capacity to lower it.  Scientists call this resistance ‘buffering capacity’.

Put the argument another way. If you are using a completely peat compost you need very little lime to make your compost as alkaline as pH 8. and to maintain it as such for a fairly long time. 
Recall that the mineral content of calcareous soil is lime. Recall too that some peats such as ‘Somerset peat’ form in alkaline conditions and are naturally alkaline.

Soils whose mineral content is mainly silt and/or sand will have less of a buffering capacity that resists soil acidification by adding peat - but often such soils will already be acid

The above argument overlooks that the fact that adding peat might improve the physical nature of your soil and that incorporating peat might improve the growth of acid lovers such as rhododendrons. But please don’t expect miracles on chalk or limestone soils.

These peat blocks were once very popular for making peat beds
Where peat excels is to grow acid loving plants in raised beds of pure peat and other acid ingredients or better, use it in tubs. It needs to be raised otherwise the alkaline water leaching from soil will soon make it alkaline.
(One of Robert Pavlis’s correspondents wrote that in the USA  they grow their precious blueberries on a buried peat bale and that works very well for several years}.


I wrote about how to grow rhubarb and lapsed into Yorkshire dialect

We visited a rhubarb farm

I spoke up for peat compost

I could not resist using this picture on this particular snowy morning


  1. It can be a good thing to question what one has been led to believe for a long time, no matter in what area of life.
    I like rhubarb but have never tried to eat the leaves, simply because I was always told that they are not edible.

    1. I think my personal take on oxalic toxicity and concentration is that I can without fear or reservation enjoy as much rhubarb as I want and similarly enjoy other veg high in this chemical but retain a little caution about eating green rhubarb leaves - something I do not wish to do!

  2. I can’t imagine why anyone would want to eat rhubarb leaves.

    1. Nor do I. I don't think today's post has enhanced anyone's life but I do find these things interesting. Apparently rhubarb leaves were recommended in a government leaflet in WW1 - and quietly withdrawn!
      I was pleased with my original post about how to grow rhubarb - too many gardeners just treat it so badly!

    2. As far as I understand it, the warnings against eating rhubarb leaves came during one of the world wars when at least one person died after eating them, though given that's all the information I've come across, you wonder whether the leaves were innocent bystanders. I've always thought the idea of their being that dangerous something that needed to be taken with a huge pinch of salt. The thing that makes me laugh is that some people are so adamant that they can't add rhubarb leaves to their compost heap because they'll poison the bug life in there.

    3. Robert Pavlis suggests in his original article that deaths eating rhubarb in the war were anecdotal and not supported by the evidence.
      As to leaves in compost it just shows how false notions develop. It's plausible that rhubarb leaves are slow to decay in a compost heap and even that some organisms don't like it but to suggest poisoning buglife generally, that's silly

  3. Thanks for the mention in the report. Several of your pictures show forced rhubarb. We might have that here is Canada, but I have never seen it. Must be another one of those British things!

  4. Hi Robert
    It's always a relief to get a positive response from one of my 'sources'.
    If you had followed the rhubarb links you would see that many of the pictures were taken at a rhubarb production farm.
    I would venture that most of rhubarb sales over here are of forced rhubarb and almost all in Spring or early Summer.
    I would describe it as more of a Yorkshire thing.


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