"Gardening" was standard RAF slang for sowing mines in rivers, ports and oceans from low heights, possibly because each sea area around the European coasts was given a code-name of flowers or vegetables.
[4][5] The technique is claimed to have been most effective against messages produced by the German Navy's Enigma machines.
This crib-based decryption is usually not considered a chosen-plaintext attack, even though plain text effectively chosen by the British was injected into the ciphertext, because the choice was very limited and the cryptanalysts did not care what the crib was so long as they knew it.
Most chosen-plaintext cryptanalysis requires very specific patterns (e.g. long repetitions of "AAA...", "BBB...", "CCC...", etc.)
Suspecting it was Midway island, they arranged for the garrison there to report in the clear about a breakdown of their desalination plant.