[Honestly, Hijikata's poetry skills are... not the greatest in the world. If he had more self-awareness about that he'd realise that he's better at prose, but poetry has a long and honourable tradition among samurai, so he's stubborn about it, even if he just managed to repeat a word in even such a short poem.]

Don't force it. Haiku are meant to capture the feeling of the moment. Maybe think of your focus subject first? The air or the plants, the clouds or the mountains...