They are an unloved root vegetable traditionally grown for cattle feed, and when pulled from the ground they look like an ingredient destined for a witch’s cauldron.

But the humble mangelwurzel will be in pride of place in the Great Pavilion at this year’s Chelsea flower show (19-23 May), after becoming the subject of an online craze among young gardeners.

The vegetable stand in the pavilion has in the past showcased perfectly spherical tomatoes and straight-as-an-arrow carrots. But Kate Cotterill and Lucy Hutchings of She Grows Veg, who will be growing the vegetables on display at this year’s show, are more interested in showing new heritage varieties of vegetables than displaying symmetrical cabbages.

The duo, who own a seed company and aim to get the younger generation interested in growing their own food, have cultivated a giant, bright red mangelwurzel variety which has gone viral on Instagram, with many people growing it and marvelling at its size and vibrant colour.

Cotterill said that manglewurzels were now the best selling seed on their website.

“What we find is that when veg is really different, people get really excited about it,” she continued. “Usually they have been growing the same thing year on year so it’s exciting when it’s new.

“People that have allotments are very competitive about what they grow and when mangelwurzel comes out of the ground it likes so bizarre almost like a mandrake from Harry Potter. Loads of people have entered them into vegetable growing competitions and won prizes.”

The vegetables are very popular on social media. One mangelwurzel reel on the She Grows Veg Instagram account got 2.8m views and 58,000 likes.

Of the online craze, Cotterill added: “The mangelwurzel has inspired a nation to grow a different, delicious, highly productive and very easy to grow variety that is accessible for everyone.”

The vegetable is described as being halfway between sugar beet and beetroot and is easy to grow. Cotterill said it was “delicious mashed, roasted or pickled” and “sweeter and less earthy than a beetroot”. Its greens could also be eaten and it also made “fantastic” wine.

The mangelwurzel is commonly known as fodder beet, because of its use as livestock feed in farming for centuries. It had been unfashionable to eat and viewed as a peasant crop, because people would only eat it when other vegetables were in short supply. The vegetable’s spooky look also led to them being carved for Halloween, before pumpkins became popular.

Other vegetables on their stand will include unusually coloured crops. The bestselling colour for tomatoes, carrots and chillies this year is black. They will also be showing off pink chicory, purple cauliflower and unusual coloured radishes.

Cotterill said black vegetables had become their best sellers. “That deep, dramatic colour comes from anthocyanins, the same compound that makes blueberries a superfood. It’s a powerful antioxidant, which makes black veg genuinely good for you, and it also lends a beautiful, earthy depth of flavour.” Varieties on show will be an “intensely sweet” black cherry tomato and a chilli Machu Picchu which is “dark and smoky tasting”.

Another more traditional plant returning to the great pavilion is the wisteria, the climbing purple flower not seen in the tent for 15 years.

The charity Plant Heritage will be hosting six nurseries in the Great Pavilion, showcasing a series of national collections. One is the Brantwood Trust, which will feature their national collection of wisteria.

Simone Daye, head gardener at the Brantwood Trust, said: “I think the reason there’s been such a big gap is because of the unpredictability of wisteria. Looking at our wisteria in the last three years its never just settled on one week – there’s been up to a three-week difference in flowering period. Its flowering times are influenced by the fluctuations of winter and spring. Currently we’re desperately trying to hold it back which is extremely difficult to do so. It also has to be transported extremely delicately.

“From my knowledge several people have been tried in the past to bring it to RHS Chelsea and been unsuccessful – I’ve heard of it flowered and ready to go and then suddenly died off.”

She said she was “quietly confident” her wisteria would be a success because “we’ve got them in pots and are moving them around constantly in the weeks leading up to the show. They are being moved and monitored every day – out in the sun or into a cold barn depending on the weather.”

Recipe: Mangelwurzel gratin

Ingredients:

• 600g mangelwurzel, peeled and thinly sliced

• 200ml double cream

• 100ml milk

• 1 clove garlic, crushed

• 100g mature Cheddar, grated

• 1 tsp nutmeg

• Salt and pepper

Method:

1. Preheat oven to 190°C (fan 170°C).

2. In a saucepan, heat cream, milk, garlic, nutmeg, salt and pepper until just below boiling.

3. Layer sliced mangelwurzel in a baking dish. Pour cream mixture over.

4. Sprinkle cheese on top.

5. Bake for 40–45 minutes until bubbling and golden brown.