Borgmann Aquaponics Hydroponics
Polycultures are the practical heart of permaculture. Instead of separating individual species in rows, plants are combined in ways that protect, complement, and jointly utilize the soil. The scientific foundation ranges from allelopathy and rhizosphere ecology to tritrophic food webs. This article explains the mechanisms, evaluates the accompanying friend-foe matrix, and derives concrete planting plans from it.
1. Ecological Foundations
1.1 Allelopathy
Allelopathy refers to the influence of one plant on another through biochemical substances released into the soil, the air, or through tissue decomposition. These substances can inhibit or promote the growth of neighboring plants. Garlic, for example, releases sulfur-containing compounds that suppress soil-borne fungal pathogens, thereby protecting roses and strawberries in close proximity.[1] Fennel, on the other hand, produces terpenes that inhibit the germination and growth of numerous vegetable species — which is why it is listed almost exclusively as a bad neighbor in common friend-foe tables.[2]
1.2 Rhizosphere and Nutrient Cycles
The rhizosphere, the immediate soil area around plant roots, is a highly active habitat. Legumes such as beans, peas, and broad beans form symbioses with nitrogen-fixing bacteria of the genus Rhizobium and enrich the soil with plant-available nitrogen. Neighboring plants, particularly nitrogen-hungry cabbage species and maize, benefit directly from this.[3] Deep-rooted plants like parsnips and carrots access minerals from deeper soil horizons that are not available to shallow-rooted plants like lettuce; after the deep-rooted plants die, these minerals become available to all neighbors through the decomposition of the root mass.[4]
1.3 Tritrophic Interactions and Push & Pull
The concept of tritrophic interaction describes relationships between plant, pest, and beneficial insect as a triangular system. When infested by pests, plants emit volatile organic compounds (so-called herbivore-induced plant volatiles, HIPVs) that attract parasitic wasps and other beneficial insects.[5] The push-and-pull system built upon this combines repellent plants (push) that keep pests away from the crop with attractive plants (pull) that lure pests into a trap or concentrate beneficial insects. Article 6 of this series covers Push & Pull in detail; the polyculture planning in this article takes the principle into account when selecting companion plants.[6]
1.4 Light, Space, and Water Competition
In addition to biochemical interactions, physical factors play a crucial role. The classic North American Three Sisters planting — maize, bean, squash — illustrates spatial arrangement exemplarily: maize grows tall and provides a climbing support for the bean; the bean fixes nitrogen; the squash spreads close to the ground, shades the soil, reduces evaporation, and suppresses weeds.[7] Each of the three species occupies a different ecological niche without displacing the others.
2. The Friend-Foe Matrix: Method and Evaluation
The accompanying matrix covers 67 vegetable, herb, and flower plants in a symmetric pairing matrix. The value 1 stands for a recommended combination (push-pull effects, nutrient synergies, mutual pest deterrence), the value 0 for an unsuitable or inhibiting combination. The data corresponds to the state of research on companion planting as documented in, among others, Riotte (1998) and Pfeiffer (2010).[8][9]
Three categories can be derived from the matrix: plants with a high proportion of positive neighbors (broad compatibility), plants with a high proportion of negative neighbors (problematic neighbors), and neutral plants with balanced behavior.
2.1 Plants with Particularly Broad Compatibility
The following species show a particularly high number of positive neighbor values in the matrix and are suitable as structuring guiding plants in polyculture systems:
| Plant | Good Neighbors (selection) | Bad Neighbors (selection) | Main Effect |
|---|---|---|---|
| Calendula | Tomato, bean, squash, cucumber, lettuce, carrot, asparagus | — | Root exudates inhibit nematodes; beneficial insect magnet |
| Nasturtium | Tomato, cucumber, squash, maize, cabbage species | — | Aphid trap plant (pull); attracts hoverflies |
| Borage | Tomato, strawberry, squash, cabbage species, bean | — | Pollinator magnet; said to deter tomato hornworm |
| Basil | Tomato, pepper, cucumber, leek | Sage | Volatile oils act repellent against aphids and whitefly |
| Dill | Cucumber, lettuce, carrot, onion, leek, cabbage | Tomato, fennel, carrot (late) | Attracts parasitic wasps; umbelliferous flowers as beneficial insect habitat |
| Garlic | Tomato, strawberry, rose, carrot, lettuce | Bean, pea, cabbage | Antifungal sulfur compounds; aphid deterrence |
| Marigold (Tagetes) | Tomato, bean, cucumber | Bean (some varieties) | Nematode suppression through root exudates; whitefly deterrence |
| Strawberry | Garlic, borage, spinach, onion, lettuce, leek | Cabbage, fennel | Benefits from neighbors' fungal protection; groundcover-compatible |
| Carrot | Onion, leek, garlic, tomato, lettuce, dill | Fennel, dill (late) | Onion/leek scent confuses carrot fly; classic combination |
| Tomato | Basil, borage, calendula, marigold, nasturtium, parsley, carrot | Fennel, cabbage species, maize (close) | Benefits from almost all herb companions; strong push-pull anchor |
2.2 Problematic Neighbors
Some species show conspicuously many zero values in the matrix and should be cultivated spatially isolated or in their own beds:
| Plant | Problem | Recommendation |
|---|---|---|
| Fennel | Terpenes inhibit germination and growth of many vegetable species; hardly any positive neighbors in the matrix | Own bed away from the main garden; tolerate dill as only companion |
| Cabbage (all species) | Strong nitrogen consumers; suppress sensitive neighbors; many allelopathic interactions | Own bed area; bean and dill as companions; rotate cabbage species among themselves |
| Peppermint | Strong rhizome spread physically displaces neighbors; terpene release inhibits some species | Container cultivation or rhizome barriers; positive as border plant against ants |
| Mustard | Glucosinolate breakdown products can inhibit germination; unproblematic as green manure after incorporation | Use as green manure, not as permanent companion |
3. Classic Polyculture Systems
The best-known polyculture system originates from indigenous agriculture in North America and combines maize, bean, and squash. Maize forms the climbing support, the bean fixes nitrogen, the squash shades and mulches the soil. The system works in USDA zones 5 to 10 and is suitable for areas from 4 m².[7]
Planting distance: Maize 40 cm, bean in a circle around each maize stalk, squash 80 cm between the maize groups.
The classic Mediterranean combination for Zone 1. Basil is widely believed to improve the flavor of tomato and acts repellent against aphids. Calendula keeps nematodes away and attracts hoverflies, whose larvae eat aphids.[8]
Arrangement: One calendula per tomato plant; basil in a row between the tomato stakes.
One of the scientifically best-studied combinations. The scent of leek confuses the carrot fly (Psila rosae); the scent of carrots inhibits the leek moth. The mutual olfactory camouflage demonstrably reduces pest pressure on both crops.[5]
Arrangement: Alternating rows at 20 cm spacing; harvest leek in autumn, leave carrots until frost.
Cabbage white butterflies and flea beetles are the most common pests on brassicas. Dill attracts parasitic wasps that parasitize cabbage white caterpillars. Nasturtium at the bed edge draws aphids away from the cabbage plants as a trap crop.[6]
Arrangement: Nasturtium as a closed border; dill in groups between the cabbage plants; no more than one dill plant per m².
Cucumbers are sensitive to powdery mildew and spider mites. Borage as a companion attracts pollinators and is said to deter spider mites. Dill attracts beneficial insects and creates a microclimate that increases humidity around the cucumber plants.[9]
Arrangement: Borage and dill once per two cucumber plants; do not plant too densely, as dill competes for water when flowering.
Garlic protects strawberries from gray mold (Botrytis cinerea) through antifungal sulfur compounds. Spinach as a groundcover understory keeps the soil moist and suppresses weeds without competing with the shallow strawberry roots.[1]
Arrangement: Garlic cloves 15 cm next to the strawberry plants; spinach spread evenly as a mulch substitute.
4. Crop Rotation and Polyculture as a System
Polycultures cannot replace crop rotation but can usefully complement it. The basic principle of crop rotation — namely, not placing botanically related species on the same area for several years in a row — applies in permaculture just as it does in conventional cultivation. The crucial difference: while conventional crop rotation is structured temporally (Species A in year 1, Species B in year 2), permaculture planning integrates temporal and spatial rotation simultaneously.[4]
A simple scheme for Zone 2 with four beds: Bed 1 takes brassicas in the first year, Bed 2 legumes, Bed 3 root vegetables, Bed 4 nightshades and squash. In the second year, each group moves one bed further. Within each bed, the friend-foe matrix determines which companion plants are placed.
| Bed | Year 1 | Year 2 | Year 3 | Year 4 |
|---|---|---|---|---|
| A | Cabbage + Dill + Nasturtium | Legumes + Borage | Root vegetables + Onion | Tomato + Basil + Calendula |
| B | Legumes + Borage | Root vegetables + Onion | Tomato + Basil + Calendula | Cabbage + Dill + Nasturtium |
| C | Root vegetables + Onion | Tomato + Basil + Calendula | Cabbage + Dill + Nasturtium | Legumes + Borage |
| D | Tomato + Basil + Calendula | Cabbage + Dill + Nasturtium | Legumes + Borage | Root vegetables + Onion |
5. Limitations of the Concept
The scientific evidence for individual companion planting combinations is inconsistent. Many recommendations are based on empirical knowledge and were collected under specific soil, climate, and variety conditions that are not readily transferable. A systematic review by Zehnder et al. (2007) found that for a considerable portion of the combinations mentioned in gardening literature, no reproducible field trials exist.[10]
This does not mean rejecting the concept, but rather an invitation to one's own observation. Those who create polycultures should document growing conditions, combinations, and results. Even negative results are valuable data — for one's own garden and, if shared, for the growing citizen science database on companion planting.
- Record planting date, variety, combination partners, and bed position.
- Note pest infestation and harvest results compared to monoculture.
- Compare at least two seasons, as weather influences can override individual years.
- Photos at fixed times (planting, flowering, harvest) ensure comparability.
6. Outlook: Article 4 and the Plant Selection Tool
Article 4 covers soil building as a prerequisite for functioning polycultures: compost, raised beds, and mulch create the soil conditions under which the synergies described here can fully take effect. Article 7 provides an interactive tool based on the evaluated friend-foe matrix that filters combination suggestions by location, USDA zone, and objectives.
References and Sources
- Becker-Dillingen, J. (1956). Handbuch des gesamten Gemüsebaues. 6th Ed. Paul Parey, Berlin.
- Putnam, A. R. & Tang, C. S. (Eds.) (1986). The Science of Allelopathy. Wiley, New York.
- Peoples, M. B. et al. (1995). Biological nitrogen fixation: An efficient source of nitrogen for sustainable agricultural production? Plant and Soil, 174(1), 3–28.
- Gliessman, S. R. (2007). Agroecology: The Ecology of Sustainable Food Systems. 2nd Ed. CRC Press, Boca Raton. Chapter 13.
- Finch, S. & Collier, R. H. (2000). Host-plant selection by insects — a theory based on appropriate/inappropriate landings. Entomologia Experimentalis et Applicata, 96(2), 91–102.
- Cook, S. M. et al. (2007). Companion cropping can affect host selection by the pollen beetle. Ecological Entomology, 32(5), 546–551.
- Mt. Pleasant, J. (2006). The science behind the Three Sisters mound system. Histories of Maize (Eds. Staller et al.). Academic Press, Burlington. pp. 529–537.
- Riotte, L. (1998). Carrots Love Tomatoes: Secrets of Companion Planting for Successful Gardening. Storey Publishing, North Adams.
- Pfeiffer, E. (2010). Mischkulturen im Biogarten. Ulmer Verlag, Stuttgart.
- Zehnder, G. et al. (2007). Arthropod pest management in organic crops. Annual Review of Entomology, 52, 57–80.
- Image: Album Vilmorin. Le Jardin Potager / The Vegetable Garden, no. 13 (1862)
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