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I have a heavy infestation of bark scale on 4 mature rhododendrons and 3 pieris japonica. I applied dormant oil to the rhododendrons yesterday and would like to do the same to the pieris. However, the pieris is already in full flower - is it too late to apply a dormant oil? I also have superior oil, would that be a better choice if the trees are already flowering? I live outside of New York City, zone 6b.

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Check the instructions on your dormant oil - if its a newer formulation, it should have instructions regarding dilution rates for use on non dormant plants - a weaker dilution, in other words. Otherwise, your superior oil, intended for use on greenleaved plants, might be better, but again, check the instructions.

Best to check your Pieris during winter, while its dormant, next year, to see whether the scale has cleared - dormant oil applied at the right time in the right concentration may be more effective, depending on the type of scale.

Further reading about horticultural oils here, if you're interested

http://www.colostate.edu/Dept/CoopExt/4dmg/PHC/hortoil.htm

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  • I find NEEM to be effective for scale and whitefly - I spray anytime I see trouble I cannot control with a thumb & forefinger and/or jet of water from a garden hose. – Jim Young Mar 16 '16 at 17:20

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