Horticulture is one of those fields where you don't necessarily need a degree, or at least a 4 year degree, to get a job with scheming court eunuch influence at a city department. Then every radical idea can be framed as sustainability and cost/labour-saving, with a very receptive audience because your coworkers observe those problems every day. I used that role to start a municipal native plant nursery from municipal pollinator gardens used as community seedbanks, while pushing for redevelopment where public land can do more socioecologically. Within a couple years the city went from $100k/year in non-native annuals to having an infinite supply of effectively free native plants that are rapidly changing all of its planting philosophy.

Landscape architecture is much more formal, requiring at least a 4 year degree and usually a masters, but it's much larger in scale and more consciously interdisciplinary with a strong focus on the humanities. It's the field where you can be equal parts historian, sociologist, modernist artist, architect, environmental engineer, and natural scientist while making decent money. The most radical David Harvey and Henri Lefebvre books are just daily conversation and the systems you practically work with. I'm going that route now to develop entire swaths of urban greenspace instead of changing it bit-by-bit. With the full MLA you can have a huge amount of influence in a department like parks planning.