Recited from memory by: The Thinker
The author writes:
In strip 3339 you may have noticed that a giant oak tree suddenly appeared right in the middle of the main courtyard at the front entrance to Buckingham Palace. I just want to re-iterate that we had already revealed the identity of the responsible party in strip 3443!
Once again, who else but Henry Rigel, the head gardener at Braseby Manor, the green-fingered miracle worker who could cultivate fields of wheat on the surface of the Moon with only a single seed, one full watering can, and a trowel. Oh wait, he actually did, strip 2486 in fact.
So, please, stop speculating! The question was answered ages ago! Thanks!