but I always thought the idea was that it isn’t being actively maintained

That would be logical, but unfortunately it is not so. On the art of cities/towns you have greenery absolutely everywhere, roads, roofs, houses, walls, tons of flower pots and other bigger and smaller containers. It again shows how idealist vision this is and how they know shit about greenery in city spaces, all this would very quickly just wither and die without constant care and the amount of plants needing that care means the labour would been simply crazy, especially that on the art if anyone is even doing that is always by hand. Even moderate amount of city greenery we have in some cities today need quite a lot of care. Alternatively they could make it less maintained by planting some crazy invasive species which would overgrown everything and look like Angkor Wat before rediscovery but that's not possible under most climates and not only monoculture weed is not close to the idea, but again, the erosion of cityspace would be huge.
Also there is a thing that solarpunk is nearly always vision of at least harshly understood degrowth if not outright primitivism and you have the full vision closer to postapo disaster than to any socialism.