Sirius watched the weed go sailing through the air. It looked like she had some sort of personal vendetta against weeds. Best keep his distance. He might have let her be and walked off had she not asked for his opinion. Sirius flicked some ash from the cigarette onto the pavement. He didn't want to find out what this bird would do if he got any on her or her plants.
"Dunno. How about some flowers? Maybe some daffodils or pansies?" he suggested, hitching his shoulders in a shrug. He didn't really care what she put there. Plants were none of his concern unless they had special mind-altering properties.
But it might be a little too transparent if he told her to plant gillyweed, yeah? So best to stick with flowers.