Garden Myths - Learn the truth about gardening

The following articles were authored by Robert Pavlis

Rhubarb Myths

Rhubarb is a great vegetable that is one of the easiest things to grow. I deadhead the flower stem, mulch with wood chips and that is the only care the plant has gotten in 10 years. It produces every year. But gardeners need to make things more complicated and numerous rhubarb myths have developed.

Forced rhubarb is especially sweet - is this a rhubarb myth?

Forced rhubarb is especially sweet – is this a rhubarb myth?

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Will Oxalic Acid in Rhubarb Leaves Harm You?

Rhubarb is a favorite vegetable of gardeners in temperate climates since it is so easy to grow. We eat the stems, and know that you should never eat the leaves since they are poisonous due to high levels of oxalic acid.

I’ve known this fact since I was a kid so you can imagine my surprise when I learned a few weeks ago that this is all a big myth. Lets dig into the truth.

Will the Oxalic Acid in Rhubarb Leaves Harm You?

Will the Oxalic Acid in Rhubarb Leaves Harm You?

Is Oxalic Acid Poisonous?

Oxalic acid is a natural chemical produced by many plants. It is a nephrotoxin (a poison that affects the kidneys) and a corrosive acid. The LD 50 (median lethal dose) for humans is estimated to be 385 mg/kg. A 65 kg (143 lb) human would need to ingest 25 g to be lethal.

Clearly oxalic acid is lethal, but 25 grams is quite a bit.

Oxalic Acid in Rhubarb

The oxalic acid in rhubarb leaves is about 0.5 g/100 g. To reach the lethal dose of 25 grams, a 65 kg human would need to eat 5 kg of leaves. That is a pretty big salad!

What about the stalks? How much oxalic acid is found in the stalks? I found lots of references that said they contain much less oxalic acid, but only one reference gave a value of  0.4 to 0.5 g/100 g which is only a bit less than the leaves. It is odd that everyone says the value is lower but nobody reports a value.

Oxalic Acid in Vegetables

The reason for writing this post is that I came across a list showing the oxalic acid content of other vegetables and it was a real eye opener. Here are the values for some common vegetables (ref 2)

Carrot – 0.5 g/100 g

Chives – 1.48 g/100 g

Parsley – 1.70 g/100 g

radish – 0.5 g/100 g

Rhubarb leaves – 0.5 g/100 g

Spinach – 0.97 g/100 g

If rhubarb leaves are too toxic to eat because of the oxalic acid, why do we eat these other vegetables? Why are we not warned that carrots are as poisonous as rhubarb leaves, and that spinach is twice as poisonous? The reality is that oxalic acid is not as poisonous as people believe.

Some other foods that have high levels of oxalic acid include tea, coffee, draft beer, chocolate, berries and tofu.

Are Rhubarb Leaves Poisonous?

There are stories on the net about rhubarb leaves being eaten during the first world war and some people died from eating them. These stories may or may not be true. People may have died from something else and rhubarb was blamed. Or the leaves might have been sprayed with toxic pesticides which were in common use at time. Postmortems did not find oxalic crystals in the bodies suggesting that death was due to other reasons (ref 3 and 4).

Plants make thousands of chemicals and many are toxic. Rhubarb makes anthraquinone glycosides which have been proposed as the likely candidate for deaths. The reality is that there are almost no reported deaths due to eating rhubarb leaves and the science on what is the most poisonous thing in the leaves is inconclusive.

The bottom line is that the leaves may be poisonous, if you eat enough – so don’t do that! But oxalic acid is not the culprit.

References:

  1. The Poison Garden – Rhubarb; http://www.thepoisongarden.co.uk/atoz/rheum_x_hybridum.htm
  2. Wikipedia – Oxalic Acid; https://en.wikipedia.org/wiki/Oxalic_acid
  3. The Chemistry of Rhubarb; http://www.compoundchem.com/2015/04/16/rhubarb/
  4. Toxicants Occurring naturally in Foods; https://books.google.ca/books?id=lIsrAAAAYAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

 

Should cucumbers, squash, muskmelons and watermelons be grown near each other?

The reason cucumbers, squash, muskmelons and watermelons should not be grown near each other is that they are all cucurbits and may cross-pollinate to produce weird franken-gourds. This myth does have some truth in it, but it is not good gardening advice.

Cucumbers, squash, muskmelons and watermelons should not be grown near each other

Cucumbers, squash, muskmelons and watermelons should not be grown near each other

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Are Native Bees Dying?

Native bees are apparently in trouble. They are dying by the millions. We all need to plant more flowers to try and save the bees. Turns out that much of this is based on false information. We don’t actually know the status of most native bees.

In this post I will look at how this myth got started and discuss some real facts about native bees.

Are native bees dying? Bumblebee on flower

Are native bees dying? Bumblebee on flower

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Are Honey Bees Dying – Are We Losing Our Food Supply?

I have seen hundreds of reports about the honeybees dying. If we don’t do something soon we will loose 75% of our food supply. Chemicals are killing them by the millions. But are they really dying? Do we have a catastrophe on our hands or do we have a bunch of fear mongering authors who do not understand science?

From a myth busting point of view this is an interesting story. The truth depends very much on how you ask the question.

Are honey bees dying?

Are honey bees dying?

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