Rhipsalis, the new efig office plant of the year, is one example, with giant houseplants Monstera, Strelitzia nicolai and other 1970s favourites such as ficus and ferns selling best at prices of up to £1,200 but more generally £250-£400. Holt said the bigger the better, up to 7ft, and trends have moved on from succulents.
"We reached a wider audience than normal thanks to the show, articles in the Evening Standard and local papers, and being on BBC Radio London," Holt explained. "I've always grown Rhipsalis for 20-odd years and it was never popular, but in the last two years everyone wants them. They're such great houseplants because they're so easy to look after.
"Houseplants have become so trendy because of one thing - Instagram. It has to be. People see nice pictures of a houseplant in a macrame basket and when they see it, they want it. But it's very much a London thing. I'm not so sure it's reached too far out of London.
"It's great for me because I always wanted to sell this amount and finally, for the first time in my long history in the houseplant industry, it has moved like this and been trendy. It's not happened like this - not since the 1970s."
Holt pointed out that all the plants come from Denmark and Holland and are not easy to get. "You have to talk to the suppliers," he said. "If you want something unusual you have to know what you're talking about or how can you possibly sell it to the consumer?"
He added that Brexit's effect on sterling is making prices rise, with Monstera up from EUR65 to EUR78 in a fortnight and asparagus ferns now retailing at £7.99 against £4.99 last year. "Fortunately, that's not putting people off," he said.
British growers cannot compete with Dutch prices. "We don't have the expertise anymore," he said, and there are better grower-retailer networks in Holland and Denmark.
Ian Drummond, outgoing chairman of interior landscaping industry trade association efig, said houseplants are trendy but garden centres are failing to exploit the opportunity.
Drummond, who is also a director at Indoor Garden Design, has been working backstage at Glastonbury and London Fashion Week installing houseplant displays. While events such as this would previously have dressed areas with cut flowers, they now use houseplants, he said.
"I'd love to work with garden centres to improve boring ranges and boring displays of houseplants. It's rare for the younger generation to have access to outdoor space so houseplants are a good way for them to garden."
Ikea has led sales of houseplants, while shops such as Habitat and Urban Outfitters use them in displays. Drummond said florists are stepping up with new ranges but garden centres are lagging behind. Hanging plants and ferns are big, he added, while on the commercial side he has never been so busy and people are regularly showing him photos of their houseplants grown at home.
The top three in the running for efig office plant of the year were Rhipsalis, Zamioculcas and Euphorbia tirucalli. The longlisted plants were Pachira, Peperomia (see Plant Focus, p28) and stag's head.
Latest Garden Centre Association figures show houseplant sales were up 29 per cent in February and five per cent in May. Drummond has a new book, At Home with Plants, published by Mitchell Beazley in April 2017.
Top 10 Dutch houseplant companies:
Ter Laak Orchids: 13 hectares in the Netherlands, 1.2 hectares in Guatemala, turnover: €28.0 million. Uses Decorum label.
So Natural (phalaenopsis) 20 hectares, turnover: €50 million
Emsflower (pot plants, bedding plants and perennial plants) 81.3 hectares of glass and 3 hectares of container fields in Germany and the Netherlands, turnover: € 43.9 million
Bunnik Plants (houseplants) 25 hectares, turnover: € 66,6 million