The Queen insisted she was up to a key part of her duties as she took hold of a shovel to plant a tree herself.
The 93-year-old, when asked whether she would perform the task on her own, said: "No, no, I'm still perfectly capable of planting a tree."
She made the remarks while on a visit to the National Institute of Agricultural Botany (NIAB) near Cambridge to celebrate its centenary.
She planted the tree to mark the occasion, moving some soil onto the plant's roots in an already-dug hole.
The Queen had earlier viewed an exhibition celebrating 100 years of crop research and toured a glasshouse.
One of the centre's longest-serving employees Teresa Stratton handed the Queen a posy of chrysanthemums, roses and wheat before the monarch left in a waiting Bentley.
Ms Stratton, 58, who has worked at NIAB for 41 years, said she told the Queen of other crops that NIAB is working on.
"We're the only place in the UK that does grape research and she was very interested," said Ms Stratton.
"She told us they were growing vines at Windsor and although she probably wouldn't drink the wine she was quite interested in it."
Chief executive Dr Tina Barsby said: "I mentioned that English wines were becoming more and more popular and better quality, and she said she doesn't drink wine but she hears they're very good."
Dr Barsby described NIAB's role as the "plant variety police".
The Queen first visited in 1969 to mark its 50th anniversary.
She will officially open the new site of the Royal Papworth Hospital in Cambridge later.