Overall houseplant sales have quadrupled in the last four years to make up 10% of overall turnover.
Centre turnover is up 14% this year, with a two-year old cafe helping. The cafe has a themed cacti menu including cactus-leaf tortilla and cacti mocktails.
Chris Pugh's Cacti in Derbyshire is supplier of many of the cacti and succulents.
A series of events has promoted the festival, including workshops. The main issues brought up are watering and light required.
Senior planteria assistant Vicky Jameson says sourcing unusual cacti and houseplants makes the difference over retailers such as supermarkets and Ikea.
He says indoor plants have "exploded" thanks to online picture-sharing, more flats being built without gardens, people understanding the health and well-being benefits of plants, and students and office workers wanting plants in their work spaces.
Other garden centres outside London can exploit the trend further by giving houseplants more space to "assault" customers as they enter and committing to a wide range of varieties and sizes. Online sales of houseplants have been high, but garden centres can take a greater share if they have the range.
Hulatt said the category almost died in garden centres a few years ago as other retailers took the market but now he sees no end to the trend.
Hulatt believes he has a younger clientele now, aged 30-45, partly thanks to indoor plants.
Airplants, cacti, bromeliads, sanseveria and unusual plants are all collectible. Fittonia is a star performer, as are monstera. Common plants such as Kentia palm and yucca are less desirable. Margins of more than 60% are achievable and sales are now evening out year-round rather than having peaks in January, December and Mother's Day, said Hulatt.
The festival is set to run through August.