After a frosty winter and a dismal spring, garden centres were bracing themselves for a bad year. But now retailers are reporting record-breaking plant sales, as a result of the hottest May weather in a century and a newfound passion for gardening among millennial Instagram users.

“At the end of April we had a huge [sales] deficit and were frighteningly behind,” said Colin Dale, head of plants at the family-owned Notcutts garden centre chain. “May was a miracle: it has been so good it has taken us over budget for the year, which seems almost impossible.”

Notcutts has also benefited from an Instagram-led trend for succulents and cacti which is drawing millennials, and even teenagers, to garden centres for the first time.

Dale said the vogue for these houseplants had boosted sales by about 50%, and the group is now using social media to try to convert these newcomers into full-blown gardeners. Major chains including Wyevale Garden Centres and Dobbies have also reported a sales surge in May as the heatwave unleashed pent-up demand among gardeners frustrated by the cold spring.

“After experiencing a record month of trading in May, we’re continuing to see a strong appetite for gardening as Brits finally invest in their outdoor spaces for the summer months,” said David Mitchell, buying manager for horticulture at Wyevale.

“Gardeners are taking advantage of the later spring to stock up on replacement plants and enjoy the late flowering of roses. Sales of outdoor furniture and accessories are also continuing to thrive as Brits increasingly want their gardens to be sophisticated extensions of their homes.”

The Garden Centre Association, which represents 180 outlets around the country, said its members had seen sales down by 20% in the first four months of 2018, making it one of the worst starts to the year in recent memory. Iain Wylie, its chief executive, said there was a dramatic turnaround in May, kicked off by the warm bank holiday at the beginning of the month, with a 20% growth in sales. “Speaking to garden centre operators in mid-April, it was all doom and gloom, but the May bank holiday was without exception a record-breaking weekend, and fortunately that continued,” said Wylie.

Some gardeners prefer to put their feet up while a robot does the work. Photograph: Bosch

However high street sales trends suggest some Britons are not looking to put too much effort into their gardens. Sales of robot mowers are up 75% on last year at John Lewis as cheaper models, starting at about £500, become available.

The department store has also seen a spike in demand for quirky garden accessories such as tiki-style cocktail bars and outdoor rugs and cushions.

With many high street chains complaining of falling sales as consumers cut spending on clothing and big ticket items such as furniture, the recent good weather has ridden to the rescue of gardening and DIY stores.

Sales of fencing and other materials needed for garden projects, which typically sell well in the early spring are, however, unlikely to be made up.

“People wanted to get out in their gardens, and there was nothing there as they hadn’t had an opportunity to plant spring bedding,” Wylie said.

Summer bedding and cottage garden plants, particularly lupins and geums, had seen “massive demand” since the Chelsea Flower Show, he added.

Wylie said sales would now be driven by weather, but most expected the season to lengthen further into June as gardeners play catch-up. However, he added: “There is always an end to the season, which comes at the point when people go on holiday or want to enjoy the fruits of their labours rather than slaving away in the garden.”