We have already seen modular plant pots that create vertical home gardens, and a plant wall that can purify indoor air. Now, a German company is taking things a step further with an innovation they hope will be an answer to creating low-cost, low-maintenance outdoor urban greenscapes. The company has developed a freestanding plant filter, the CityTree, that can clean the surrounding air of particulate matter while offsetting 240 tons of CO2 equivalents per year. The 12-foot-high unit combines a vertically-installed moss culture with vascular plants for a green wall that can ingest particulate matter, nitrogen dioxide and ozone, and has the same cleansing effect as 275 urban trees but at 5 percent of the cost and just 1 percent of the space. Thanks to solar panels and a fully automated provision of water and nutrients using a built-in tank, the CityTree requires only a few hours of maintenance each year, and has a production footprint of just four tons of CO2.