If you use small clay pebbles for your houseplants, then you can bask in the knowledge that you are doing them a big favour. What’s more, you don’t need to worry if those little balls have developed a whitish tinge. It is not mould. Those balls cannot go mouldy as they are not made from organic material, says houseplant expert Thomas Helk. The white deposits are just harmless salt residues from the nutrient solution that’s added to the irrigation water used in this plant care method. It is not damaging to your health, either, which is why you often see indoor plants in these hydroponic systems…