Folklore, checked

19 things gardeners are widely told, each checked against the evidence and answered in plain words. Widely believed, rarely trialled — and none of them gates a plan here. They answer the question you were going to ask.

• A layer of gravel or pot shards at the bottom of a container improves drainage. Contested
• Adding sugar or molasses to the soil makes tomatoes and other fruit sweeter. Refuted
• Basil improves the flavour of adjacent tomatoes. Contested
• Burying banana peels feeds plants potassium (roses in particular). Contested
• Coffee grounds acidify soil. Contested
• Companion planting charts encode real relationships. Refuted
• Compost tea (or manure tea) greatly boosts growth and suppresses plant disease. Contested
• Crushed eggshells deter slugs. Contested
• Crushed eggshells in the planting hole prevent blossom end rot. Refuted
• Dish soap makes a safe, effective homemade insecticide, the same as insecticidal soap. Refuted
• Epsom salts prevent blossom end rot. Refuted
• Household vinegar is an effective weed killer. Contested
• Marigolds repel most garden pests and protect the whole garden. Contested
• Mixing sand into clay soil improves its drainage. Refuted
• Planting by lunar phase improves germination and yield. Contested
• Sprinkling human hair, soap shavings, or urine around the garden reliably keeps deer and rabbits out. Contested
• Talking to your plants, or playing them music, makes them grow better. Contested
• Watering plants in the midday sun scorches the leaves (water droplets focus sunlight like lenses). Refuted
• You must rotate every crop to new ground every single year. Contested