The following article came to my attention:
It describes how an artist will be doing tours of companies that have a letterbox office in The Netherlands. The remarkable part is that these companies are deemed "real" (i.e. not a letterbox company) by the Dutch tax agency as they all have a houseplant in the office.
The article cites part of the book
Het Euro Evangelie by Arno Wellens:
De belastingdienst bedacht een regel die (buitenlandse) bedrijven die in Nederland geregistreerd zijn, verplicht om een kamerplant op kantoor hebben. Dat staat in het boek Het Euro Evangelie van financieel journalist Arno Wellens. Omdat er af en toe iemand op kantoor moet zijn om de plant water te geven, vormen de planten het 'bewijs' dat bedrijven die op papier in Nederland zijn gevestigd, hier ook metterdaad kantoor houden.
The tax agency came up with a rule requiring (foreign) companies, registered in The Netherlands, to have a houseplant in the office. This is written in the book Het Euro Evangelie by financial journalist Arno Wellens. As someone needs to be in the office to water the plants now and then, the plants are "proof" that the registered companies actually do business in their Dutch office.
My understanding is that once it is determined that a company actually does business in The Netherlands it is no longer considered a shady letterbox company and it can enjoy various tax perks (please correct me if I'm wrong).
The OneWorld article cites an article in De Groene Amsterdammer, which refers to 925.nl. The latter refers to questions asked in The House of Representatives, but the questions seem only to be related to letterbox companies. I cannot find where they mention the houseplant issue.