I think you can do well with this business if you start with people in your neighborhood and ask them if you can have an opportunity to perform this service for them. Word of mouth will travel fast if you provide a good value. I think the key to doing very well here is to package your service. For example, try to find a price that works for mowing, weeding and fertilizing altogether.
I’ve tried a fair few things on this list and I’m a big fan of those side hustles that have the potential for ongoing passive income once you’re set up. For me, the most successful have been blogging and T shirt designs (I use Merch by Amazon but want to look into Teespring as you suggest). I’m currently working on an Etsy printable business, again for the passive income potential!