While the poem is addressed to the flowers of Heidelberg, it was actually written in Berlin in 1886. Rizal was studying in Berlin at the time and was deeply impressed by the city's botanical gardens. He wrote the poem as a tribute to the beauty and serenity he found amidst the flowers.
So, the poem itself wasn't physically created in Heidelberg, but it was inspired by the beauty of the flowers he encountered during his time in the city.