In the pathway: Understanding companion planting
Allelopathy
The chemical influence of one plant on its neighbours, positive or negative.
Explanation
Some plants release compounds (essential oils, juglone from walnut) that help or inhibit their neighbours. Knowing these relationships is the basis of companion planting, but many effects attributed to allelopathy in popular gardening are in fact competition for light, water and nutrients.
Refined definition
Allelopathy is the biochemical effect of one plant on another through the release of compounds (essential oils, root exudates, volatile compounds or substances leached by rain). It differs from simple competition (water, light, nutrients). It can be positive (growth stimulation) or negative (inhibition of germination or development). The term was coined by Hans Molisch in 1937.
Key points
Three cases worth knowing
Walnut (juglone): slows some sensitive plants (tomato, potato, pepper, aubergine, cabbage, alfalfa, blueberry), according to lists drawn mostly from observations. The effect comes mainly from the American black walnut (Juglans nigra), under its canopy and within its root zone, which extends on average 15 to 18 m from the trunk of a large tree (Morton Arboretum); the common walnut (J. regia) rarely bothers its neighbours (Purdue Extension). Fennel: reputed to hinder its neighbours. In the laboratory, its essential oil slows the germination of two weeds (Gharibvandi et al., 2022); in the vegetable garden, its effect on neighbouring crops has been little studied, and tradition grows it apart, as a precaution. Sunflower: allelopathic compounds studied mainly for weed management; its extracts reduced wheat growth in pots (Anjum and Bajwa, 2010).
A few documented positive effects
Clovers and legumes → nitrogen fixed through the Rhizobium symbiosis, released mainly as their residues decompose (a soil input, not an allelopathic effect). French marigolds (Tagetes patula) → root exudates (α-terthienyl) active against Meloidogyne nematodes, as a dense planting before the crop (studies Gommers 1972, Ploeg & Maris 1999; UF/IFAS). Aromatics (basil, thyme, rosemary) → volatile compounds reputed to repel some pests, but results vary in the field.
The gap between laboratory and garden
Many effects demonstrated on concentrated extracts in the laboratory become marginal in the garden, where the mass of plant material, the soil, the weather and pest pressure play an enormous role. Treat allelopathy as a helping hand, not a guarantee. Rotation, living soil and observation remain the pillars of a healthy vegetable garden.
Watch out for
- • Do not confuse allelopathy (chemical effect) with competition (water, light, nutrients), many claims that “plant X kills plant Y” are in fact simple competition.
- • Many allelopathies in popular gardening manuals have never been scientifically verified. Distinguish documented effects (Macías, Willis) from oral-tradition claims.
Sources
- Molisch, H. (1937). Der Einfluss einer Pflanze auf die andere: Allelopathie. Iéna : Gustav Fischer.
- Gharibvandi, A. et al. (2022). Weed management by allelopathic activity of Foeniculum vulgare essential oil. Plant Biosystems, 156(6), 1298-1306.
- Anjum, T. & Bajwa, R. (2010). Sunflower phytochemicals adversely affect wheat yield. Natural Product Research, 24(9), 825-837.
- Gommers, F.J. (1972). Increase of the nematicidal activity of α-terthienyl and related compounds by light. Nematologica, 18(4), 458-462.
- Ploeg, A.T. & Maris, P.C. (1999). Effect of temperature on suppression of Meloidogyne incognita by Tagetes cultivars. Journal of Nematology, 31(4S), 709-714.
- Krueger, R., Dover, K.E., McSorley, R. & Wang, K.-H. : Marigolds (Tagetes spp.) for Nematode Management, fiche ENY-056 (UF/IFAS Extension, 2007, révisée en 2016)