A quarter teaspoon per quart of water, sprayed on leaves. What soap spray does on plants, what it does not do, and the plants that will not tolerate it.
Gardeners have used plain soap and water on plants for a very long time, and it remains one of the simplest things you can mix at home. It also gets over-promised online, so here is what it does, at what strength, and where the honest limits are.
Garden spray: ¼ teaspoon Pure-Castile Soap per 1 quart of water.
That is far weaker than any household cleaning mix, and the weakness is deliberate. Soap strips the waxy cuticle that leaves rely on to hold water. Too strong and you scorch the plant you were trying to help: leaf edges brown, tender growth wilts.
Use soft or distilled water where you can. Minerals in hard water react with soap and leave a visible film on leaves.
Soap solution works on soft-bodied insects such as aphids, spider mites, whitefly and mealybugs, and it works on contact. It disrupts the outer layer those insects depend on to retain moisture. Once the spray dries, it has no further effect: nothing residual is left behind protecting the plant.
Two consequences follow from that:
It does nothing to beetles, caterpillars, or anything with a hard shell. It is not a fungicide. And it is a soap-and-water spray, not a registered pesticide. If you have an infestation that is damaging a plant you care about, a product formulated and approved for the job is the right answer.
Spray a few leaves and wait 24 hours. Plants vary enormously in tolerance.
Known to be sensitive: ferns, succulents and anything with a waxy or powdery bloom, sweet peas, nasturtiums, and most seedlings. Waxy-leaved and hairy-leaved plants generally take it worse than smooth ones.
Early morning or evening, never in direct sun. Soap film plus strong sunlight burns foliage. Cool and overcast is ideal.
Not on a plant that is already stressed. Drought-stressed or newly transplanted plants have little margin: water first, spray days later.
Rinse after a few hours if you are treating something delicate, or if the plant is indoors and will not get rained on.
It appears in a lot of homemade garden recipes and it is wrong twice over. Vinegar is acidic and castile soap is alkaline, so mixing them in one bottle breaks the soap down into a useless curd. And vinegar strong enough to matter is a herbicide. It damages the plant you are spraying, which is the opposite of the goal.
The same quarter-teaspoon spray is used on vegetable plants. Rinse produce with clean water before eating, which the label recommends anyway. There is a separate fruit and vegetable wash recipe using the same soap.
The soap is unscented, contains no essential oils, synthetic dyes or foaming agents, and is biodegradable, which is the reason it ends up in gardens in the first place.
Pure-Castile Soap, 64 fl oz → | Fruit and vegetable wash →
Yohan & Alex
Co-founders, Whole Naturals
For external use only. Keep out of reach of children. Discontinue use if irritation, redness or discomfort occurs.
