Harris said sales were boosted by the firm taking over publishing Kew's magazine, science bulletin, Curtis's Botanical Magazine and guidebook.
He added: "There is a lot of interest in academic horticulture and botanical books because the market has been starved for so long. Most publishers have moved out of the academic end of the market and that has left a gap. We've moved in and met that need.
Monographs such as forthcoming books on heathers, tulips and magnolias are filling niche markets, said Harris. "A lot of publishers say you can't make money from that type of book, but the right monograph and author make it profitable," he explained.
He added that a large proportion of sales were now from overseas, with University of Chicago Press as its distributor for North America. Digital books were also playing a larger role.
Trying to compete with the remainder books market and the likes of Dr Hessayon's books in garden centres was difficult, he conceded, pointing out that Kew did not remainder books.
Plant food manufacturer Growth Technology recently ordered 2,500 copies of Growing Windowsill Orchids to sell in garden centres.
New from Kew Publishing is The Last Great Plant Hunt: The Story of Kew's Millennium Seed Bank by Sue Seddon, Carolyn Fry and Gail Vines.