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A neem leaf wash works as it dries. Mist or wipe the leaves, leave the plant somewhere out of strong light, and the sheen shows within an hour or two as the water goes and the thin film of oil settles on the leaf. There is nothing to wait for beyond that.
First wash
On a plant that has never been washed, the first pass lifts the loose dust but may leave a faint grey film where tap-water minerals have built up. A second wash a week later usually clears it. Wiping with a soft cloth while the leaf is still damp speeds this up on broad-leaved plants.
How long it lasts
The sheen fades as new dust settles, which in an ordinary room takes a couple of weeks. A wash once a fortnight keeps a houseplant looking freshly cleaned; more often than that is not needed and just wastes the mix.
Timing
- Work in the morning or evening rather than in midday sun, and let the leaves dry before the plant goes back into full light.
- Do a single leaf first on a plant you have not washed before and look at it the next day.
- Make the wash fresh each time; a diluted mix does not keep beyond a day or two.
The recipe and rates are on our leaf-shine spray page.