The Office for National Statistics said: "Anecdotal evidence from retailers suggests that good weather contributed to growth [in April].
"The stronger-than-expected rise in sales pushed the value of the pound above $1.30 to its highest level since September last year. Because of recent rises in inflation, the amount spent in shops and online was 6.2% higher in the three months to April compared with a year ago - the biggest rise in 15 years."
In many garden centres, after a record April, sales in the first part of May were around a quarter down on May 2016, when a window of good weather brought in the short peak season.
Longacres' Michael Ainley said: "I’ve heard everyone saying April was record-breaking but May has ruined most of it. It’s been a lively couple of months."
He said Longacres new acquisition at Bybrook Barn in Kent was still growing fast but its Bagshot and Shepperton centres, after a "record-breaking April, have been struggling to make May match May 2016".
He said it was "just the weather" that was causing the problem, with footfall down and sales reduced. While everything "flew" in April, plants have suffered in May with bedding having a later start and "just starting to kick in" with online sales developing.
Coolings business development manager Neil Jackson said so far 2017 had been "our best year ever" but sales had "lost some ground in the past couple of weeks".
He added: "With good weather we’re rammed and when it’s iffy we’re quieter."
Jackson said summer bedding started in April with frost warnings, then there was a frost, and was selling "strongly" now.
RHS Chelsea Flower Show begins on 23 May and the BBC will show coverage from 21 May at 5pm on BBC2 with a total of 15 programmes across the show on BBC1 and BBC2. Elements include Carol Klein on new plant trends, and Griff Rhys Jones on plants. Mary Berry will cover edible flowers and presenters including Monty Don, Rachel de Thame, Sophie Raworth, James Wong, Ellie Harrison, Joe Swift and Nicki Chapman will discuss show aspects including show gardens, new plants and medals.
The Met Office weather forecasts for 23 May to 1 June said there would be no more frost, that temperatures would be warner, but there would be showers: "This period is likely to start on a changeable note with sunshine and showers for many, some of which could be heavy and thundery. However, it should begin to turn more settled for a time from the west, gradually bringing drier and brighter weather across the UK, although there remains some uncertainty over how quickly this change will take place.
"Thereafter, confidence remains low but it looks like the end of May will see a return to more unsettled weather with showers or longer spells of rain and stronger winds and perhaps coastal gales. Temperatures are likely to be close to or slightly warmer than average especially in any settled and sunnier spells, though with some cooler nights."
Meanwhile, the monthly IHS Markit UK household finance index report, which uses Ipsos data collected from May 10-14, said household finances remained under the "greatest pressure since mid-2014".
Households report one of the fastest rises in living costs since early-2014, and the degree of pressure on financial wellbeing was among the sharpest seen for two-and-a-half years.
At 42.4, down fractionally from 42.5 in April, the index remained well below the post-crisis peak seen in January 2015 (46.2). Looking at the average reading in the three months to May, the decline in household finances was faster than seen at any time since autumn 2014.
Worsening household finances reflected a further steep rise in living costs during May. Although inflation perceptions moderated to a five-month low, the index remained higher than the average since March 2014. Higher living costs resulted in one of the sharpest falls in cash available to spend for two-and-a-half years in May. Survey respondents also indicated that their need for extra unsecured borrowing continued to rebound from the lows seen in 2016.