Insecticidal soap is a spray of potassium salts of fatty acids. It kills soft-bodied insects and mites that it wets directly, by disrupting the cell membranes of the outer body and removing protective waxes. Once dry it has no residual activity, and it is relatively low in toxicity to people and mammals.
In practice
Growers use soap in veg against aphids, spider mites, whitefly nymphs, thrips larvae and mealybug crawlers. It only works where it touches, so leaf undersides must be soaked, and repeat sprays every few days catch hatching eggs. Hard water reduces efficacy; soft or RO water helpsbeing reviewed. It can scorch leaves of some cultivars, in heat or under strong light, so test on a few leaves first. Dish detergents are not the same product: they are formulated differently and can damage leaves. Keep soap off flowers. Check what is registered for cannabis locally.
Don't confuse with
- Horticultural oil: smothers pests; can be combined, but phytotoxicity risk rises.
- Surfactant or wetting agent: improves spreading, not a pesticide by itself.
- Dish soap: a cleaning product, not a registered insecticide.
Tetranychus urticae, the commonest cannabis mite indoors: a 0.5 mm sap-feeder that stipples leaves pale and spins webbing, breeding fastest in hot, dry rooms.Nutrient toxicity is damage from an excess of an element in the plant, either directly or by blocking uptake of others; boron, manganese, copper and chloride are common culprits.Small soft-bodied sap-sucking insects of the family Aphididae that cluster on stems and leaf undersides, excrete sticky honeydew and multiply rapidly by live birth.Small white-winged sap-sucking insects of the family Aleyrodidae, mainly Trialeurodes vaporariorum and Bemisia tabaci, that fly up when disturbed and coat leaves in honeydew.General hardness (GH) is the calcium and magnesium dissolved in water, expressed as mg/L CaCO3 or German degrees (dGH); hard water supplies some Ca and Mg.Photon efficacy is how many micromoles of PAR photons a fixture produces per joule of electricity (µmol/J); higher means more light for the same power bill and heat.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.A refined petroleum or plant oil sprayed to smother soft-bodied pests and eggs and suppress powdery mildew on contact; used only on foliage, never on flowers.An oil pressed from seeds of the neem tree (Azadirachta indica) that smothers soft-bodied pests and, in crude forms, contains azadirachtin, an insect growth disruptor.Contact products act only where the spray touches the pest; systemic products are absorbed and move inside the plant, which raises residue concerns in cannabis.
