From the Old Farmer’s Almanac daily Q&A page:

What exactly was a Victory Garden during World War II?

To
help the war effort, citizens were asked by U.S. Secretary of
Agriculture Claude R. Wickard to plant vegetables wherever they could
find a plot of land to do so. By 1945, the year the war ended, an
estimated 20 million victory gardens had sprouted in sidewalk
boulevards, town squares, and odd parcels of land in the cities and out
in the country. These gardens were producing 40 percent of the
vegetables grown in the United States at that time. The term "victory
garden" originated in a book of that title from England, published in
1603. During World War I, U.S. patriots planted what became known as
"liberty gardens" as well.

If I was better at history, I’d know what cause that the English were worried about being victorious for, in 1603.  Also, I wonder why it is that small plots of veg were rare in 1603, such that they needed a special name – was the Enclosure movement already afoot, removing the commons from the fairly comfortable subsistence farming of the peasant and giving it to sheep?  More to research.