"Most of all we want to show that consumption can also work a different way," says Happy Tree founder Sebastian Schönfeld.
"I see this completely as also being a prompt to think about things in other areas of one's life too.
From renting out just 400 trees last Christmas in Cologne and Düsseldorf, the firm is aiming to rent out 3,000 in twelve different towns, including Hamburg, Bremen and Hannover.
Around three-quarters of the Nordmann trees survived the experience last year, Happy Tree said, although Schönfeld hopes to boost that to 90 or even 95 percent in the future.
Growing trees in pots or in the earth for temporary festive transplantation is a concept that's existed in the USA, UK and Switzerland for a long time.
But the sustainable alternative to erecting a dead tree in the corner of your living room and covering it in baubles – only to throw it out onto the street weeks later – represents a tiny fraction of the German market.
The Christmas Tree Growers' Federation warns that "in a pot, you can quickly destroy [a Nordmann]," said spokesman Hans-Georg Dressler.
Too-warm conditions and little water can represent a deadly combination for the living trees.
But 25-year-old tree nursery owner Kersten Scholz, who plans to offer 150 trees as "wandering pines" this year from his business in Bad Zwischenahn, Lower Saxony, says that he prepares the trees for the temperature shift by warming them indoors first.
There's also a trick to keeping the trees alive through multiple rentals: regularly replanting them even when they're not out and about.
"That's how they develop small roots that can grow out later," Scholz said – although not all trees will survive the process.
Tree nursery owner Kersten Scholz moves a Christmas tree in a pot. Photo: DPA
After the trees return from their Christmas excursions, they're gradually re-accustomed to the winter cold before being planted outside again for two years of recovery.
And the expense of all this special care is passed on to customers, with a tree costing between €44 and €79 from Scholz and €65 to €80 from Happy Tree.
Meanwhile, the German Nature Protection Federation advises people to grow their own tree in a pot, such as a native German spruce.
That way the roots don't get damaged and there's a natural limit to the size of the tree – as long as you have a few years' patience while it gets there.