Thank you, firewns and Siebe!
There is another sutta which describes how planting of Bo trees was recommended by the Buddha himself:
When the Buddha was absent from Jetavanarama devotees naturally missed him, so Ananda, the Buddha’s attendant, asked the Buddha what in his absence might be used to pay homage to him.
– The Buddha answered that bodily relics, things reminiscent of him and things that he had used, in particular the great Bodhi Tree under which he had attained Enlightenment.
– Ven. Ananda then had a seed of the Bodhi Tree brought to Jetavanarama and planted so that it would be, as the Buddha himself said, as if the Buddha were constantly present at Jetavanarama.
If anyone knows which sutta it is, please provide the name.