I found a booklet online—The War Garden Victorious by Charles Lathrop Pack. From Chapter I:
The war garden was a war-time necessity.
This was true because war conditions made it essential that food should be raised where it had not been produced in peace times, with labor not engaged in agricultural work and not taken from any other industry, and in places where it made no demand upon the railroads already overwhelmed with transportation burdens.
The knowledge that the world faced a deficit in food, that there existed an emergency which could be met only by the raising of more food, was apparent to every well-informed and thinking man and woman during the early months of 1917.
The author, wishing, as every patriot wished, to do a war work which was actually necessary, which was essentially practical, and which would most certainly aid in making the war successful, conceived the idea in March, 1917, of inspiring the people of the United States to plant war gardens in order to increase the supply of food without the use of land already cultivated, of labor already engaged in agricultural work, of time devoted to other necessary occupations, and of transportation facilities which were already inadequate to the demands made upon them.
In March, therefore, some weeks before the United States entered the war, he organized for this work a commission known as the National War Garden Commission.
What were the causes which led to the world’s lack of food and the need of a largely increased production by the United States to prevent world starvation? The same question could be posed in present time. What's happening now with our food supply? Most cities do not have adequate stores of food to meet the needs of even a short-term catastrophe. I do not believe shortages will necessarily be felt due to catastrophe, but rather, from economics and weather change.
It's time to be watchful of these things, and to plot a course for our own action in the face of such changes.