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How to Grow, Harvest and Care for Rhubarb (and why it has a reputation as a Killer!)

Written by Nicola Wallis

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Posted on 26 July 2021

Killer Rhubarb?

There are many myths and rumours surrounding rhubarb that has it pegged as a killer.  The main reason for its dangerous reputation is believed to date to World War I, when rhubarb leaves were officially recommended on the home front as an alternative food. This was very bad advice as the leaves are poisonous and should not be eaten.

We don't recommend it but the threat isn't actually that great from eating the leaves.  A lethal dose of oxalic acid is somewhere between 15 and 30 grams.  So, you’d have to eat several pounds of rhubarb leaves at a sitting to reach a toxic oxalic acid level.  We can only assume that in WW1 they ate a lot of them as at least one death was reported and I'm guessing the civil servant who issued this advice was sacked!

Rhubarb as a Medicine

Rhubarb stalks have long been used as a medicine - first by the Chinese and, later by the Greeks and Romans who all used the dried roots as a laxative.

This medicinal use lead it to be illegal to grow rhubarb in Russia.  This is because in the past Russia supplied and controlled the export of the dried root, controlling its price.  It banned the export of the seeds to block the plant being grown elsewhere.  This lead the price to rocket and at one point be more valuable than gold.       

What is Rhubarb?

Rhubarb is a rhizomatous perennial whose leafstalks (regularly referred to as ‘sticks’ or 'stems') are grown as a vegetable but used mainly as a dessert and is delicious in desserts and jams. I am particularly partial to rhubarb crumble and custard!

Rhubarb grows in two crops, the first is known as forced rhubarb and the second is called main crop rhubarb

Forced Rhubarb

Forced rhubarb grows early in the year under pots or 'forcing jars' and as the name suggests, the practice of forcing rhubarb involves getting it to grow earlier and faster than it would normally. Growing under pots means the rhubarb stems grow tall in search of light. In turn, lack of light also reduces the oxalic acid in the stems. 

Oxalic acid is what gives rhubarb it’s sour taste and is also found in Swiss chard, spinach, beets, peanuts, chocolate, and tea. The reduced oxalic acid in forced rhubarb means it is sweeter, has a finer texture, is more tender and delicately flavoured than the maincrop variety.  Many gardeners have one plant that they force and one they leave for later crops.  

Main Crop Rhubarb

Maincrop rhubarb, is what you see in  most gardens.  It is hardy and grows outdoors in almost any garden soil and arrives in spring and crops over a long period.

When to Harvest Newly Planted Rhubarb

Resist the temptation to harvest any newly planted rhubarb for the first year as this will prevent weakening of the crowns and reduce the plant’s vigour. The following year pick just a few stems and after that, the plant should be well established and can be harvested normally. 

Harvesting Established Rhubarb

After two to three years rhubarb plants should have well developed root systems that enable them to support strong growth with healthy foliage and flowers. You can then begin to harvest rhubarb from March or April onwards for early cultivars, and late April or May onwards for maincrop varieties with the optimum period being from mid-May until July. It is possible to take rhubarb stalks earlier, but these must be limited in order not to kill the plant.

When should I stop harvesting?

In general, it is best to stop harvesting by June or at least taking only a few stalks after then. The stems do remain edible and tasty through the summer, yet the reason it is best to stop is so that you don’t over harvest and weaken the plant.

It is worth noting that checking stem colour is not reliable for an indication of ripeness as there can be discrepancies of red or green tones depending on the rhubarb variety.



How to Pick Rhubarb

To Pull or Slice with a Knife?

Although slicing won’t kill the plant, technically, pulling is preferred as the plant recovers quicker. When stalks are sliced with a knife the part withers away and that’s pretty much that!

  • Grasp the stalk near the bottom.
  • Lean it to the side and in one motion gently twist and pull the stalk up. (The twisting and pulling motion should be gentle you’re not trying to wrestle with the plant!)
  • The stalk will pop, separate from the rhubarb plant at the root and come cleanly away.
  • Leave at least 1/3 of the stalks on the plant in spring time to ensure it continues to grow and thrive throughout the summer. Never take all of the stems from the plant as some are needed for the plant to feed from.
  • After you cut the stalks from the plant, cut the leaves from the stalk and throw them in the compost bin. To reiterate - the leaves of the rhubarb plant are poisonous and should never be eaten.

Oxalic Acid

Oxalic acid is a natural substance found in many plants, including leafy greens, fruits, vegetables, nuts, seeds, and cocoa. Rhubarb leaves are considered inedible due to their high concentration of oxalic acid.  There is an old wives tale that rhubarb should not be harvested in August due to a concentration of oxalic acid building up in the stems too as the season progresses.

This is a myth.  Build-up is mostly in the leaves which are not eaten and the amount in the stalks is not sufficient to have a toxic effect. so you can carry on harvesting, taking care not to over harvest and weaken the plant. 

The leaves can be composted and will decompose perfectly well as the amount of oxalic acid is low and the molecule doesn't survive well outside of the plant cells and eating plants grown in rhubarb-leaf compost is also perfectly safe.

For full details of How to Add a Rhubarb Crown to your plot check out this blog too.  

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