Hello. a few weeks ago l planted 4 pestemons, they're now trying to flower being half open but the flowers have a white powdery look on them and are begining to droop, any advise on what to treat them with would help please.
Are your plants very young plugs? What you describe sounds like Powdery Mildew, but without photos, it's hard to say. Do you see any white specks on the leaves? Drooping could be your plants trying to adapt to the new environment, but also could be a sign your plants could be short of water or suffering a bit from heat or sun.
Make sure they are planted in well, mulched around the base with good compost or similar. Planting anything out in summer time will always be tougher, so make sure your plants are watered regularly in the first month, especially in dry spells. Water in the evening or early morning directed only to the roots and not leaves.
edited June 2018
I have what looks like talcum powder on one of my penstemons. It's only on the flowers and doesn't seem to be having any adverse effect.
Might this be mildew. It's in a shadier spot than the others which are unaffected, although they're all different varieties.
In London. Keen but lazy.
Yes they were young plants but not plugs, they were healthy when l planted them only got the white powdery look when the flowers started to open. I have watered them every day in the evening while it's been hot weather, no other plants near them are effected. I think l will try a spray for Mildew as it sounds like this could be it. Thank you.