Allelopathy — plants inhibit or promote each other through chemical compounds. Scientifically proven biochemistry! 🌿
❌ Strongly allelopathic plants
Walnut (Juglans)
Produces juglone — affects up to 18-36 m radius! Sensitive: tomatoes, peppers, potatoes, raspberries. Do NOT compost walnut leaves!
Sunflower
Inhibits tomatoes, potatoes, beans. 1-2 year residual effect in soil.
Fennel
Inhibits almost everything: tomatoes, peppers, beans. Always grow separately!
✅ Positive allelopathy
- 🌼 Marigold (Calendula): root exudates repel nematodes (60-70% weed reduction — ÖMKi research)
- 🌿 Basil: demonstrably promotes tomato growth
- 🧄 Garlic: allicin repels aphids, thrips, fungal diseases
Quick reference
| Plant A | Effect | Plant B |
|---|---|---|
| Walnut | ❌ STRONGLY INHIBITS | Tomato, potato, raspberry |
| Fennel | ❌ Inhibits nearly everything | Tomato, pepper, bean |
| Marigold | ✅ Protects | Tomato, pepper |
| Basil | ✅ Promotes | Tomato |
Summary
Allelopathy = free natural plant protection! Avoid toxic neighbours, exploit positive partnerships.
❓ Frequently Asked Questions
Why is walnut’s toxic effect so long-lasting and far-reaching (18-36 meter radius)? The compound juglone continuously enters the soil not just from roots but from falling leaves, husks, and rain-washed branches, and stays stable there for a long time — a continuously replenished “toxic zone” sustained by the tree’s root spread and leaf fall for decades.
Why shouldn’t walnut leaves be composted, when almost all other foliage can be? The juglone molecule resists the microbial breakdown that happens during composting — the temperature and duration of composting aren’t enough to break it down, so the finished compost can still damage sensitive plants when spread in the garden.
Why does yield stay reduced for 1-2 years where sunflowers grew previously? The sesquiterpene lactones released by sunflowers don’t break down immediately in the soil, but gradually over months to years — meanwhile inhibiting root development and germination of sensitive plants until their soil concentration drops low enough.
Why does calendula reduce nematode numbers in the soil? Calendula roots secrete compounds directly toxic to nematodes or that disrupt their reproductive cycle — this isn’t repellence but an actual, measurable killing effect, confirmed by ÖMKi research.