Hi all,
GeoGardenClub is an app specific for managing gardens that supports the management of volunteer tasks but doesn't have features for tracking the number of hours individual volunteers spend in the garden that would be sufficient for grant reporting. It is great for the situation where you have many tasks and volunteers - volunteers can review tasks still needing to be done, and check them off when complete so other volunteers don't also do them. It allows for less micromanaging and increased volunteer autonomy.
For what it sounds like you need, I've also used a combination of Better Impact (volunteer management software) and Form Assembly (any form software would work) to manage volunteers for a food recovery program. Better Impact handles the scheduling, provides information about shifts, and includes the link to the Form Assembly form. Volunteers track time, mileage, and pounds of food recovered (can change this to a garden specific question) for our grant reporting needs. Better Impact is pricey. Maybe you can just use a free form software for tracking time spent if that is what your grants need?
Jenna