By Wendyhaw
London,
United Kingdom 
Can anyone tell me what the powdery residue is on some of my plants, is it harmful and how do I get rid of it?
16 Jun, 2013
Answers
Mildew sometimes attacks plants when they are dry at the roots & cold or wet on the leaves.
17 Jun, 2013
Many thanks for your replies.
17 Jun, 2013
Its mildew. One of the regulars on here suggests spraying with one part milk to ten parts water (I think it was full cream milk!) I think your first plant in the picture is an Acanthus - mine looks exactly the same, every year unfortunately.
16 Jun, 2013