The True and Fair Foundation's report, released this week (PDF), claimed one in five charities spend less than half their income on charitable activities, and singled out the RHS as one of 17 of the UK's largest charities which it claimed spend less than 65p on charitable activities for every £1 raised.
The report made headlines in national newspapers but was criticised for misrepresenting how charities work.
Biggs explained: "The report for instance doesn't take into account the funds we pay into our reserves to spend on the major charitable projects we have planned for the future, as part of our recently announced £160 million investment in the future of horticulture, including a new community urban garden and significantly increasing our horticultural science work.
"The report also doesn't take into account that we have to invest in our commercial work to be able to raise the necessary funds to support our charitable work. The RHS receives no government funding and has to be commercially savvy to enable us to raise significant funds to manage and invest in a vast portfolio of charitable projects, including running Europe's biggest community gardening campaign, working with over 20,000 schools, conducting scientific research and safeguarding collections charting 500 years of gardening history. Today we engage with and support more communities at grassroots level and spend more on charitable activities than ever before.
"We are extremely grateful to our members, donors and everyone who supports us and monitor our financial position closely, always looking at ways to increase our reach and enrich people's lives through plants."