It will help if you put something in the bag that they will want to save. Calender maybe--or or course a gardening calender--what to do and when. Prune. Plant. Water. Harvest. Treat for weeds. And something only YOU can give them: a gardening calender localized for your particular city. Date of tulip bloom. Date of last and first frost, growing zone, average date of first mowing. Best date to plant tomatoes.
Also you could include a coupon for a free lawn mower blade sharpening, if they bring it to your office in March. If you can find a cooperating (but not competing business) a half off coupon might work.