I do this all the time and I use the same batch all month long and replace them after a month. Chop off some scallions and stick them root-down in a Mason jar above your sink. When water starts to evaporate, add more or replace water. They grow fast!
I adore this idea. I'm going to become a guerrilla gardener!! :-) How to make seed balls. Seed balls are great for reclaiming derelict areas with thin or poor soil, making them ideal tools for guerrilla gardening. Especially useful for tossing into empty or neglected lots. Best thrown just before a rainy spell to ensure germination