In some cities, the municipal utilities announce a date for each street when the Christmas trees are collected. In others, non-profit organisations, e.g. scouts, take care of the collection. In my region these dates are around Epiphany, i.e. 6th of January. In any case you can bring the tree to a local muncipal recycling centre (Wertstoffhof, Bringhof, …)

Some towns offer a pickup service on a certain day in january, others have a drop off area. It depends where you live at.

The local waste management companies (Entsorgungsbetriebe) should offer informationen about its handled in town/area.

There’s a special pickup date at the beginning of the year for christmas trees. On this day you usually see all the trees pile up on the side of your street. You probably missed it at this point already.

You can drop it off for free at the nearest recycling center with the garden stuff, I suppose. If that’s not feasible you can call them and tell them you missed the deadline. Maybe there’s something else you can do in your city / town.