Do I have to use mixed neem oil immediately, or does it keep?

When neem oil is mixed with water it begins to break down rapidly, so you want to use it immediately. UT Horticulture Specialist Celeste Scott says once mixed it remains effective for a maximum of 24 hours.
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Horticultural Oils

Related Resources:
Neem Oil General Fact Sheet
Insect Control: Horticultural Oils