Crop Science · Knowledge

What Doesn't Work

Popular garden and crop remedies assessed against the published evidence — what fails, what works, and what is widely misunderstood. Sourced from peer-reviewed research and university extension trials.

Pages in this section

WORKS BUT MISUNDERSTOOD

Beer traps for slugs

Beer traps demonstrably attract and drown slugs. No trial has shown they reduce populations or crop damage, and they may draw slugs toward the plants

PARTLY TRUE

Baking soda versus potassium bicarbonate for mildew

The famous Cornell formula works, but the oil is the stronger half. And for powdery mildew specifically, potassium bicarbonate outperforms sodium bica

PARTLY TRUE

Coffee grounds as pest control and soil fix

Caffeine genuinely kills slugs. Spent coffee grounds do not contain enough of it to matter, and as a mulch they can do harm.

PARTLY TRUE

Compost tea for disease control

Non-aerated compost tea has some published support. Aerated compost tea, the version most enthusiastically promoted, failed in 16 of 18 university tri

PARTLY TRUE

Copper tape for slugs

Copper delays slugs but does not stop them, and in two field-realistic trials produced no reduction in damage. The barrier concept is sound; copper is

WORKS BUT MISUNDERSTOOD

Diatomaceous earth once it gets wet

DE works by abrasion and desiccation, which requires it to be dry. Slugs and most garden pests are active precisely when it is not.

DOESN'T WORK

Eggshells as a slug barrier

Tested directly against unprotected plants, crushed eggshell rings made no measurable difference to slug damage.

DOESN'T WORK

Epsom salts as a garden cure-all

Magnesium is needed in small amounts and usually already sufficient. Excess magnesium sulfate inhibits calcium uptake and can worsen blossom-end rot.

WORKS BUT MISUNDERSTOOD

Marigolds for nematode control

Marigolds genuinely suppress nematodes, but only as a dense cover crop grown for months. Interplanting a few does little, and the wrong cultivar can i

GENUINELY WORKS

Milk spray for powdery mildew

Five greenhouse experiments found cow's milk above 10% gave 10% disease severity against 56% for the water control — statistically comparable to a con

DOESN'T WORK

Ultrasonic and electronic pest repellers

Ten field studies of electronic mosquito repellers found no effect. Bed bugs were equally happy on both sides of four commercial devices.

WORKS BUT MISUNDERSTOOD

Vinegar as a weedkiller

Acetic acid is a genuine contact herbicide with no translocation and no soil residual. It reliably kills small seedlings and reliably fails on perenni

12 pages in this section.

Product claims, registrations and availability vary by jurisdiction. Always read and follow the applicable product label.