I include a weekly gardener at all of my rentals. They keep the place looking good and they tell me if they see anything out of the ordinary (like a unpermitted dog, or trash, or anything else). The places are always look rent ready, so if I get a 30 day notice that someone is moving I can tell any prospects to take a look at the property and it is looking good from the outside.
Your situation in particular might be different. If the tenant is keeping up the yards and doing a fairly decent job, then just follow the HOAs letters and have the tenant tighten up their work. Eventually the tenant will figure out how to keep the HOA happy and the HOA will get tired of sending letters. Don't let the harass you into taking a financial hit, or doing something you don't want to do. Now, of the resident isn't doing a good job with the yards then bringing in a gardener might be a good solution to everyone involved.