I just downloaded the Plant Factory and Plant Catalog (its Free) its amazing. its a software to generate vegetation like speedtree. You have to download the plant catalog to used the premade asset the size a bit large. Some of the premade asset have HD(HIgh),LD(Low) and RT(Realtime) that you can used based on your need they even have preset like season.
You can even use it for commercial use based on the FAQ on their web.
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