Philip Sopher wrote about the history of beer gardens:

• Duke Albrecht V of Bavaria, in modern day Germany, decreed in 1533 that beer could only be brewed between September and April.
• This was in part because beer brewed during colder months tasted better.
• This encouraged breweries to create underground cellars to store beers for the summer.
• To keep the beers cool they planted trees above the cellars.
• Soon they placed a layer of gravel and some tables and the areas became popular drinking spots.
• So popular in fact, that the owners of inns and taverns wanted to ban breweries from directly selling beer to customers as they were taking away too much business.
• In 1812 the government compromised and said that breweries could continue to sell beer, but they couldn’t serve food.
• This didn’t prevent patrons from bringing their own food, so beer gardens went on to become a nice place to have picnics.

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Source: The Atlantic