Marketing manager Stuart Lowen said: "We're all running businesses and we have to watch the costs - it's not a cheap marketing activity." The event is estimated to cost the company £30,000.
"We're committed to working with the parks and feel that the competition has been worthwhile but we feel perhaps we can use our marketing spend in a different way to support the parks."
Lowen said the company's marketing spend on parks would not disappear but that it had "a lot of ideas" about how it might be better targeted.
He paid tribute to the work of the RHS team, saying that the event was the "one horticultural show of the year where bedding plants are truly valued".
RHS head of shows development and competition architect Bob Sweet said the charity was in advanced talks with two possible sponsors for the event.
The potential backers include one horticultural company and one non-horticultural company, according to Sweet, who added that he planned to make an announcement by October.
"Over the years we have had a very good relationship with Ball Colegrave," he said. "Of course, times change and times are changing for Ball Colegrave in the supply of young plants for local authorities that are finding it even more difficult to plant bedding on the scale that they used to."
He continued: "From a show perspective at Tatton Park, we want to keep alive the skills and the ability to grow bedding in a creative way. We don't want those skills lost and we don't want bedding to completely die as a fashion."
Sweet said if the new sponsor is non-horticultural, then Ball Colegrave could supply the plants. But if it is horticultural, it could supply its own plants.
The RHS plans to extend the sponsorship deal to meet some of the entrants' costs for the event, such as transport.