The campers help pick and grow the strawberries. The satyrs play magical tunes on their reed pipes to increase the strawberry produce. Dionysus and his children, Pollux (and Castor, formerly), use their powers to grow the strawberries as well.
"Delphi Strawberry Service" is printed on the vans CHB uses for quests, or strawberry deliveries. The camp fences have a sign that says: "Pick Your Own Strawberries Please".
When Percy Jackson leaves the beach at Montauk and flees to Camp-Half Blood with his mother and Grover Underwood, he reads signs saying "Pick Your Own Strawberries" on white picket fences. Annabeth later tells him the signs are present to make mortals see Camp Half-Blood as a strawberry field. Although the name "Delphi Strawberry Service" isn't mentioned, Chiron explains to Percy that they grow strawberries for export to restaurants in New York and Mount Olympus. The camp director, Mr. D (Dionysus), god of wine, can make fruit-bearing plants grow with no effort. Although his power works best for wine grapes, Mr. D is forbidden to drink wine or grow grapes.