October 27, 2026

Is using flour in the garden worth trying on brassicas

Flour dusted on brassica leaves is a cheap experiment against soft-bodied pests, and we know of no trial data showing it works. Gardeners dust plain flour on cabbage, kale, and broccoli to put off cabbage caterpillars and similar chewers. If you try it, dust in a dry window. Rain rinses the coating off, and a dusting that lasts one day tells you nothing about whether it helps.

What flour is supposed to do

The claimed mechanism is physical. Fine flour clings to damp leaf surfaces and to the soft bodies of caterpillars and aphids. Some gardeners say it clogs their mouthparts or dries them out. Others say a dusted leaf is less appealing to chew. We do not know which of these holds. We have found no controlled numbers for any of them.

That makes flour a low-cost test. Run it the way you'd run any untested remedy, with a comparison plant you leave bare.

What flour can harm

• Thick coats. Flour that covers a leaf blocks light from reaching the leaf tissue, so the plant makes less sugar while the coat sits there. A light haze is enough.
• Wet paste. Flour that gets soaked turns to paste and dries as a crust. The crust holds moisture against the leaf, which gives mold a place to grow.
• Self-rising flour. It carries salt and leavening. Salt draws water out of leaf cells, so use plain flour only.

Pests flour cannot reach

Flour sits on leaves, so it does nothing for pests that feed below the soil line. In our own garden this season the worst losses came from voles. They took five of our eight celeriac plants in the ground bed, stripped every tuber from one potato plant, and severed a watermelon vine at its base. A leaf coating never touches an animal eating roots underground.

Our hardware cloth on one raised bed covered only the sides. A vole still took a cucumber there, because it came up from below. If your damage starts at the roots or the soil line, put your effort into barriers under the bed.

When rain washes flour off

Flour sticks best to leaves with a trace of dew on them. Rain soaks the starch film, and runoff carries it off the leaf. A heavy shower clears most of it. Overhead watering does the same thing, so water dusted beds at the soil line. Our guide on the best time to water your garden covers when to water so leaves dry fast.

Rain decides how often you'll redust. In our garden July brought 10.89 in of rain and September brought 1.35 in. A dusting in July would have needed redoing after nearly every storm. The same dusting in September would have stayed on for long stretches.

Picking a dry window

The Today screen in Siltwell shows free watering guidance for the garden as a whole. It checks the forecast for today and tomorrow. It shows hold if the two-day forecast adds up to 5 mm of rain or more. A hold is also your cue to wait on dusting. When it says covered or water, less than 5 mm is forecast for those two days, and a dusting has a chance to stay put.

For a longer window, the free dry-spell alert fires when under 5 mm of rain is forecast over the next 6 days. The watering guidance appears only when your garden has active plantings, so log your brassicas first.

The guidance reads a forecast for your whole garden. It cannot foresee heavy dew, a sprinkler running on the next bed, or a shower that lands on your yard when the forecast said dry. If your brassicas share a bed with crops that want frequent overhead water, the dust won't last long. Our guide to planting companions by shared watering covers grouping plants by water need.

How to tell if it worked

Dust half a row and leave the other half bare, on the same day, in a dry window. After a week, count chewed leaves and look for caterpillars on the undersides in each half. If the dusted half looks no better, move your time to hand-picking and row cover.