The article in the German Wikipedia is rather complex. To translate it properly, I would have to get out my dictionary and plan to spend at least an hour.

However, a little study shows that in parts of Germany, esp lower Saxony and the Luneberger Heide (heath), people cut young birches and oaks (or oak branches, perhaps) on the Friday before Pentecost and make garlands and wreaths from them. At the proper time, a guest who passes under the garland enters the house and partakes of a little drink (alcoholic, which comes as no surprise) and either gives or gets a small coin. There is nothing in the article about May - it all has to do with Pentecost.

(For you non-church-goers, Pentecost comes 50 days after Easter and celebrates the coming of the Holy Spirit to the followers of Christ in the form of a rushing wind and flames. After it came, the followers could preach in all languages.)

DMcG, the trees you saw leaning against the doors may have been recent deliveries, waiting to be made into garlands and wreaths.

Although the use of birch and oak may seem to have pagan overtones, this custom developed after the more correctly-Biblical practice of building fires at the front door proved insalubrious.      :)