If armed with seeds or plants, he can manipulate the growth of plants within its natural lifespan/capabilities (ex: he could make a rose bud bloom, or rapidly age it to withering - he could also rapidly grow a bush from the seeds.) He can manipulate the shape of vine growth to some extent, but is limited by the natural length of the plant (ex: using a climbing rose for a lengthy rose whip rather than a hybrid rose. Or, less nefariously, growing the climbing rose so it climbs up a trellis. I will feverishly research plants for this.)

Once it's fully grown he can no longer manipulate its shape, nor can he just, like, make them move. He can only age it into a desired shape while it still has room for growth.

Is he going to attack someone with roses? Entirely possible.