They discussed collaborating on education programmes, the Award of Garden Merit, membership promotion and influencing policy. HTA director-general David Gwyther met with RHS acting director general Gordon Seabright and marketing director Dan Wolfe.
He said: "We're pleased, following our encouragement, that the RHS is getting more actively involved in policy influencing. It has been a little quiet on that front. It will be able to work with us on the key issues that face growers and retailers - things like research and development, plant health challenges and ill considered controls on peat or water access that do need to be informed by reputable organisations like the HTA and the RHS."
Gwyther said the RHS's reputation would ensure it was listened to by politicians and welcomed its help as "extremely good news".
The charity joined the All-Party Parliamentary Gardening & Horticulture Group (APPGHG) at its AGM in January but denied that it planned to lobby politicians.
"We are in the process of joining the APPGHG because the RHS wants to have a good relationship with everyone who has an interest in horticulture and gardening.
"We have identified any number of areas where we can work collaboratively."
Seabright said that science and education would be priorities this year and that co-operation with the HTA was vital on both.
RHS director of science and learning Simon Thornton-Wood added that it was time for collaboration "against the tide" of diminishing research budgets.
"We're going to be stepping up our activity in relation to delivering good science and we see ourselves as filling a gap created in the withdrawal of funding. The RHS could and should be informing that policy discussion more.
"None of these policy debates are simple but people in public debate do have an inclination to simplify things. What the RHS can do is help to make sure that those debates are pitched at a more informed level."