Lessons should generally support the primary (or Main Lesson) curriculum of the grade, and adjust for the developmental stage of the student. Lessons will change with the seasons and many will be directed by the site and the capacities of the school garden.
For an overview of Waldorf Curriculum in the garden and beyond, the following resources are recommended:

Krause, Rudolf. 1992. Gardening Classes at the Waldorf Schools. Kimberton, PA: Bio-Dynamic Farming and Gardening Association.

Stockmeyer, Karl E.A. 1969. Rudolf Steiner’s Curriculum for Waldorf Schools. Kidbrooke Park, Forest Row, East

At the Waldorf School of the Peninsula garden classes are taught for one period a week to grades 1-8. The classes are split so that there is no more than 16 students in the garden at any given time. There are two gardens for the school’s two campuses: a smaller garden for the rural-suburban lower school and a larger garden for the urban-suburban middle/high school campus. There is one full time garden teacher with occasional guest teachers who teach special topics and guide projects, particularly in the upper grades. Parents often volunteer to assist and occasionally lead projects in the lower grades as well.