From The New York Times Garden Book: ...clear glass encourages the growth of algae... The bottle garden should be cleaned inside and out once a month. Algae

From The New York Times Garden Book: ...clear glass encourages the growth of algae... The bottle garden should be cleaned inside and out once a month. Algae growth tends to be rapid in bottle gardens... This monthly scrubbing and cleaning out is the key to the whole success story. Source: http://books.google.com/books?id=rSiFAAAAQBAJ&lpg=PT522&pg=PT522#v=onepage&q&f=false http://books.google.com/books?id=rSiFAAAAQBAJ&lpg=PT522&pg=P... and: https://news.ycombinator.com/item?id=7858411 https://news.ycombinator.com/item?id=7858411