Kelliher has been instrumental in setting up the Ornamentals Round Table and providing an outward facing presence for Defra within the sector.
She is moving internally to an inward facing role dealing with Brexit and labour issues. Her successor is Nick Turner, who also covers organics.
HTA horticulture head Raoul Curtis-Machin said: "It's a shame that we'll be losing Kathleen from the roundtable group meetings. She has been consistent in listening and pointing us in the direction of more help and information. But people move around and we look forward to meeting and working with Nick Turner. We also hope that Defra brings in more knowledge and expertise around landscapes and ornamental plants. I am sure we shall continue working with Kathleen in other areas too."
Meanwhile, the NFU has appointed Crop Protection Association chief executive Nick von Westerholz to lead its newly created Brexit Unit as director of EU Exit and International Trade.
The NFU is working on its response to Brexit following meetings and polls of members.
NFU Director General Terry Jones said: "We have made no secret of our determination to seize the opportunities offered by the forthcoming Brexit deals to ensure that British farming has a profitable and productive future and is able to seize and capitalise on new opportunities. Farming and food production is politically and strategically important for the UK; farming provides the raw ingredients for the UK’s food and drink sector worth £108 billion, supporting 3.9 million jobs for people nation-wide as well as delivering high quality, traceable food for a growing population.
"In the coming weeks and months ahead it is essential that food and farming is front and centre of any talks about the UK’s relationship with Europe and the rest of the world and I am confident the building blocks being put in place now will ensure the NFU is able to work with the two new Government departments, Exiting the EU and International Trade.
"I am delighted Nick von Westenholz is re-joining the NFU to lead the new and strategically important Brexit team to ensure the voice of farming is at the heart of policy and decision-making in these crucial talks.
In addition, the NFU is recruiting a new Head of External Affairs and two new External Affairs Managers to strengthen its London presence.
Meanwhile, Friends of the Earth is urging the government to commit to matching or exceeding EU protections for the environment.
A recent YouGov survey, commissioned by Friends of the Earth, revealed 83 per cent of Britons said the government should pass laws providing a higher (46 per cent) or the same (37 per cent) level of protection for wild areas and wildlife species than current EU laws. Only four per cent wanted lower protection.
See more in the next edition of Horticulture Week magazine.