I do something similar. I will tell you what I do and give you a few suggestions. I have an email list of most of my customers. Even if they aren't a regular customer and I have only serviced them once, I try to get them to get join my mailing list. I use a email service called mailchimp
. I ask for their birthday when they sign-up. I have it set that it will automatically send out a birthday email to them 1 day before their birthday with a 50% off coupon enclosed for a single mowing service. The coupon is valid for 1 year after their birthday so it is fair for everyone (winter birthdays). Much cheaper than a mailed card, it is done automatically, and isn't usually redeemed. I have been doing this for years and I think only 1 or 2 coupons have been redeemed. Gives your customer service point across at a smaller cost. I also get a benefit from it because I can send them monthly newsletters and keep the communication lines open.
My suggestions is to do this on a coupon basis and make this available for everyone, no matter what season their birthday falls in.