QUESTION: The water- and pebble-filled trays under my houseplants invariably fill with green scum. Is there a way to inhibit this nuisance?

ANSWER: Light on stale water equals algae. Light on stale water plus fertilizer equals algae bulked up on fast food. Start fresh by washing the tray and the pebbles with hot soapy water. Rinse well. After that, change the water weekly.

You can buy a little extra time by adding a tablespoon of peroxide to each quart of pebble water. But don't depend exclusively on better living through chemistry: Cleanliness is next to scumlessness.