You might also check out town or county road crews. If they trim branches or cut trees away from power lines, they pay to dump their wood chips in someone's dump.
If someone in your congregation is in town or county government, a word from them might motivate their road crews to use your parking lot as a free wood-chip-dumping-site for a week or so, and get you all the chips you can use.
If you don't have enough volunteers for weeding or planting next year, Boy Scout or Girl Scout troops might still pursue merit badges, and many of those have a "community project" requirement. Let a local Scout leader know they "may" use your beds to practice on, and you might get some free work done.
I never had a priest give me gardening work as penance, so that might be impractical. But maybe you could popularize "a new kind of confession" and set the rules to include yard work as penance. Just kidding!