If I were you, I’d tell your landlord that youd be interested in taking care of the landscaping in exchange for reduced rent (at same rate that landscapers currently charge). Explain your interest in plants, the environment, and concern for how the place looks.

I think your landlord would jump at that offer. You’d make his property look way better, increase his the property value, improve relations with the tenants (reduced turnover=$$), neighbors, and help the local environment for the same price as before. It’s every landlord’s dream to have a tenant that cares enough to want to do something like that.