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The wash itself is simple to make; the questions we are asked are about timing. When in the day, what time of year, and how often.
Time of day
Morning or evening. Mist when the plant is out of strong light and the leaves have time to dry before the sun is on them, which indoors means before the room warms up or after it has cooled down. Do not mist in strong midday sun; water on the leaf and bright light do not mix well. Outdoors, pick a still morning so the spray lands where you point it.
Time of year
Indoors, any time. Houseplants collect dust all year round and a wash every fortnight or so keeps the leaves clear. The one seasonal wrinkle is the neem oil itself, which sets solid below about 20 °C. In a cold kitchen or a garage the pure oil and the water-soluble concentrate will both be solid in the bottle; stand it in a bowl of warm water, cap on, for ten minutes and it pours again. Use warm water for the mix and it stays milky in the sprayer.
Outdoors, the same wash goes on roses and other ornamentals from spring to autumn. Leave soft new growth for a week or two before its first wash, wait until a freshly planted shrub has settled in, and give it a miss when there is a frost forecast, because a wet leaf on a cold night does no one any favours.
How often
Once a fortnight for ordinary household dust; every month or two for succulents, which gather it slowly; a little more often for a plant beside a road or a busy kitchen. If you are unsure, wait until the leaves look dull rather than washing to a calendar.
The rule that does not change
Mix fresh every time and use it within 48 hours. A diluted neem wash does not keep, so make what you need for the session and tip the rest away. Recipes for the wash, with Ourons water-soluble neem or with pure neem oil and liquid soap, are on our leaf-shine spray page.