Marshall, still confident about the season after what he called a "sensationalist" report by Gardening Which? that scored his centres lowly in a survey of garden retailers, said daffodils and primroses were his best sellers because gardeners wanted colour in their gardens.
He added: "We're not going to move into summer bedding. We're going to let the season take its course and recognise things are late. But I understand it's going to be a hot summer - a man in Abergavenny says so."
That man is Positive Weather Solutions (PWS) senior forecaster Jonathan Powell, who said temperatures this summer will be higher than those of 1976 and August could beat the record of 38.5 degsC set in Kent in 2003.
Powell added: "We're coming off the back of 18 months of correctly predicted long-range forecasts. This summer will encourage good gardening growing conditions with long sunny days broken up by a few showers. This summer's weather will give encouragement to gardeners who work hard this spring."
PWS correctly forecast the big freeze this winter and rain last summer, when the Met Office said it would be mild. The Met Office no longer issues seasonal forecasts after predicting a barbecue summer in 2009.