Companion plants to control weeds
From Triple Performance
The companion plant serves as soil cover during the cropping period, which prevents the emergence of weeds. This practice benefits by limiting the use of pesticides and synthetic fertilizers.
Principle
The companion plant, which is a cover crop, is grown in between commercial crops, it is not harvested to maximize its effects. Cover crops contribute to weed control by limiting the germination and development of weeds, either through their aggressive competition for light, water, and nutrients, or by emitting molecules toxic to other plants (allelopathy).
Competition
Cover crops, dead (mulch) or living, exert a pressure on weed development when they dominate. They compete with weeds for light, nutrients, and water, creating a smothering effect, but they can also compete with the crop. To benefit from this effect on weeds, well-established cover crops with rapid growth and high biomass are needed. The cover crop species must therefore be carefully chosen and managed to outcompete weeds while limiting competition with the crop and yield loss.
Allelopathic effects
Some plants have allelopathic effects, meaning they can release organic substances (phenolic compounds, nitrogenous compounds, terpenoids, terpenes…), which manifest as inhibition or stimulation of growth of nearby plants, or harm pests and diseases. These effects can be direct or indirect:
- True allelopathy: release of allelopathic compounds directly active.
- Functional allelopathy: release of compounds becoming active after transformation by a microorganism.
The release of these molecules can occur:
- At the root level: rhizodeposition.
- In the atmosphere via emission of volatile compounds and leachates from aerial parts of the plant.
- By decomposition of buried or fallen plant debris at the soil surface.
The synthesis of these compounds is influenced by genetics, environmental factors, the stage and processes of the plant. They never act alone as they are associated with competition for resources. Differences in allelopathic potential exist between varieties of the same species, so it is necessary to select the variety to plant based on the highest levels of secondary metabolites. Currently, few allelopathic varieties exist because this trait, which impacts yield, has probably been counter-selected in breeding. Indeed, there is a significant energy cost for the plant to produce these allelopathic compounds.
Among the crops that can be used for their allelopathy are mainly rye, but also tall fescue, wheat, Kentucky bluegrass, sorghum, forage radish and buckwheat.
Some allelopathic substances are specific to a group or plant family:
- Crucifers (glucosinolates: root exudates) can inhibit the growth of legumes. They reduce growth of foxgloves, amaranths, rumex, and vine.
- Most cereals by hydroxamic acids like DIBOA (present in root exudates) inhibit the development of annual dicotyledons.
- Invasive plants: The success of invasive plants is often linked to the release by roots of phytotoxic compounds to neighboring plants.
Effects on weeds[2]
- Rye and oat help control spring weeds.
- Rye: on white goosefoot, solanaceous plants, plantain, cleavers, panic, rumex.
- Oat: on germination of broomrape (in the inter-row of a legume); reducing effect on thistle, chenopods, wild oat, rumex and on dicotyledons in general.
- Strigose oat and buckwheat emit coumarin via their roots which inhibits root growth of weeds.
- Buckwheat would contribute to amaranth suppression via its root exudates, and quackgrass growth thanks to rutin accumulating in the soil. See detailed article: Buckwheat allelopathy.
- Sorghum (sorgoleone and cyanogenic glycosides): inhibits germination and growth of crabgrass, on solanaceous plants, amaranths, ragweed, panic, Abutilon, cock's-foot panic, and in the maize inter-row can help control nutsedge.
- Soft wheat would have an allelopathic effect on Ipomoea lacunosa, Indian goosegrass, and Palmer amaranth.
- Fenugreek (like oat) sown in legume seed intercrops (fava beans, peas) reduces, by exuding allelopathic molecules, the germination of broomrape crenata.
- Spanish clover (Desmodium uncinatum) in the maize inter-row can be effective against Striga hermonthica.
- Alfalfa (saponins): on thistle, chenopods, wild oat, rumex, and dicotyledons in general.
- Soybean helps limit weeds in maize.
- White mustard, forage radish and spring vetch would suppress white goosefoot, scentless chamomile, chickweed and have a more marked effect when mixed.
- Roots of sunflower emit compounds that inhibit weed germination within several centimeters radius and some cultivars have demonstrated effectiveness against weed problems in subsequent wheat crops.
Effects on pests[2]
- Black nightshade, via its root exudates, stimulates hatching of cyst nematodes of potatoes but does not allow them to complete their cycle.
- White mustard and forage radish have nematicidal effects via their root exudates on the nematode cyst of beet.
Effects on diseases[2]
- Brassicaceae (glucosinolates): affect a soil-borne pathogenic fungus, Aphanomyces euteiches, responsible for root rot of peas.
- Brown mustard, white mustard, and forage radish act on crown rust of cereals.
- Varieties of brown mustard or radish rich in glucosinolates allow, through biofumigation, to sanitize the soil and reduce pressure of Rhizoctonia solani causing brown rot in beet crops, and on potatoes or maize.
- Brown mustard, by biofumigation, reduces abundance and incidence of the pathogen Ralstonia solanacearum on solanaceous plants like tobacco.
- Mixtures of white and brown mustards would suppress Verticillium dahliae, vectors of verticillium wilt of potato.
Companion plants by family
| Family | Names | Root and aerial system | Seeding density/ha pure crop | Seed cost €/ha | Planting cost €/ha
(seeds + mechanization*) |
Weed smothering | Drought sensitivity | Slug sensitivity | Soil and atmospheric N Trapping | Frost sensitivity | Glyphosate or herbicide sensitivity | Difficulty of destruction |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Grasses | Rough oat | Fibrous / erect | 35/50 kg | 50 | 100 | medium, allelopathy | low sensitivity | low | medium + | medium | high | Medium |
| Spring oat | Fibrous / erect | 70/100 kg | 18 | 77 | medium, allelopathy | low sensitivity | very sensitive | medium | medium | high | Medium | |
| Moha | Fibrous | 25 / 30 kg | 18 | 96 | high | low sensitivity | sensitive | low | very frost sensitive | high | Low | |
| Winter rye | Fibrous / erect | 80 kg | 18 | 106 | medium, allelopathy | good | very sensitive | high | low frost sensitivity | high | Difficult | |
| Sorghum | Fibrous / erect | 15 / 25 kg | 50 | 100 | high, allelopathy | resistant | medium | high | very frost sensitive | high | Low
Allelopathy (sorgoleone) | |
| Legumes | Spring fava bean | Taproot / erect | 180 kg | 48 | 86 | low | sensitive | low | high | medium | low | Medium |
| Vetch | Fibrous,taproot / creeping | 35/60 kg | 70 | 110 | excellent | low sensitivity | medium | high | frost sensitive | good | Medium | |
| Forage lentil | taproot | 30 kg | 60 | 121 | excellent | low sensitivity | low | low / medium | medium | low | Medium | |
| Bird's-foot trefoil | semi-erect and semi-creeping | 10 / 15 Kg | 70 | 190 | low | low sensitivity | medium | high | non frost sensitive | medium | Difficult | |
| Alfalfa | taproot / erect | 20 / 25 kg | 45 | 74 | Allelopathy
medium |
low sensitivity | medium | high | very low frost sensitivity | low | Medium | |
| Sweet clover | taproot, very vigorous | 10 / 15 Kg | 120 | / | allelopathy | low sensitivity | no data | high | non frost sensitive | / | Medium / low | |
| Forage pea | Fibrous,taproot / creeping | 60/80 kg | 85 | 110 | medium | medium | medium | medium | sensitive | low | Medium | |
| White clover dwarf | superficial | 6/10 kg | 18 | 81 | good | very sensitive | sensitive | high | very low frost sensitivity | low | Medium | |
| Alexandrian clover | fibrous/taproot | 10/15 kg | 48 | 140 | low | sensitive | very sensitive | high | medium | low | Easy | |
| Crimson clover | fibrous, taproot / erect | 12/15 kg | 48 | 140 | medium | medium | medium | high | low frost sensitivity | low | Difficult | |
| Purple clover | Fibrous,taproot / erect | 15 / 20 kg | 18 | 81 | excellent | sensitive | sensitive | high | very low frost sensitivity | medium | Difficult | |
| Common vetch | Superficial / creeping | 45 kg | 70 | 146 | excellent | very sensitive | sensitive | very high | low frost sensitivity | low | Difficult | |
| Purple vetch | Superficial / creeping | 45 kg | 70 | 146 | excellent | good | low | high | low frost sensitivity | good | Medium | |
| Hairy vetch | More resistant than common vetch; Seeds can germinate over several years; very cold resistant; mechanical destruction difficult. | |||||||||||
| Crucifers (not before rapeseed) | Camelina | taproot / erect | 3/5 kg | 18 | 61 | excellent, allelopathy | low sensitivity | medium | low | low frost sensitivity | low | Easy |
| White mustard | taproot / erect | 8 / 10 kg | 17 | 61 | allelopathy | sensitive | medium | high | medium | medium | Easy | |
| Brown mustard | taproot / erect | 3 / 4 kg | 17 | 61 | allelopathy | sensitive | medium | high | medium | medium | Easy | |
| Abyssinian mustard | taproot / erect | 6 kg | 30 | 70 | allelopathy | sensitive | medium | high | low frost sensitivity | good | Difficult | |
| Forage rape | taproot | 6/10 kg | 18 | 67 | excellent | low/ good | sensitive | high | low frost sensitivity | low | Difficult | |
| Chinese radish | taproot, / erect | 5 / 8 kg | 33 | 81 | Allelopathy
excellent |
Resistant | sensitive | high | sensitive | low | Easy | |
| Asteraceae (excellent before rapeseed) | Nyjer | taproot / erect < 2 meters | 8/12 kg | 18 | 96 | low | good | very sensitive | low / medium | very frost sensitive | medium | Easy |
| Sunflower | fibrous,taproot / erect | 20 / 25 kg | 18 | 96 | low | good | very sensitive | strong if sown early | very frost sensitive | medium | Easy | |
| Hydrophilaceous | Phacelia | Fibrous / erect | 6/10 kg | 48 | 150 | excellent if sown early August | medium | medium | medium if sown early August | low frost sensitivity | medium | Easy |
| Papilionaceous | Fenugreek | taproot / erect | 10 / 15 kg | 70 | 121 | low | medium | medium | high | low frost sensitivity | low | Medium |
| Polygonaceae | Buckwheat | superficial | 45 / 55 kg | 70 | 131 | medium, allelopathy | medium | sensitive | high if sown early | very frost sensitive | low | Easy |
| Linaceae | Flax | taproot / erect | 20 / 30 kg | 18 | 131 | low | medium | low | low | medium | high | Easy |
Crucifers / Brassicaceae
Mustard, Camelina, Radish, Forage rape.
Advantages
- They have a good germination capacity even in dry conditions.
- They show good initial vigor.
- They ensure biomass production over a short duration.
- They have a good effect on soil structure.
Disadvantages / limits
- Pay attention to seeding density. To prevent them from taking all available space, sow at most 15 to 20 seeds/m² or about 1.5 kg/ha.
- At these early sowing dates, watch for flea beetle pressure early in the cycle especially in rapeseed systems.
Examples
- Camelina:
- Good smothering effect on weeds.
- Rapid seed set - nectar source.
- Mustard:
- Returns nitrogen over several years.
- Keeps soil moist in spring.
- Forage rape:
- Rarely flowers before winter.
- Forage radish (mid-July sowing):
- Allelopathic effects.
- Good biomass production.
- Often difficult to destroy.
Grasses / Poaceae
Oat, rye, triticale, moha, sorghum, ryegrass.
Advantages
- Good structuring effect.
- High biomass production (limits weeds + high carbon/sugar content, stimulating microbial activity).
- The most suitable grasses are Brazilian oat, forage moha, and sorghum.
Disadvantages / limits
- Not all grasses are adapted to early sowing.
Other families
Examples
- Brazilian oat (sowing August to November):
- Presence of allelopathic effects that help reduce weed presence.
- Significant nematicidal effect (especially in no-till conditions).
- Good structuring effect.
- High biomass production.
- Better initial vigor than black oat or white oat, especially in dry conditions.
- Beware of nitrogen hunger after developed Brazilian oats.
- Moha:
- Cleaning and smothering crop against weeds.
- Very good resistance to heat and drought.
- Good structuring effect (less than sorghum).
- High biomass production and rapid seed set (sow early July).
- Good destruction by frost.
- High attractiveness to slugs.
- High nitrogen demand.
- Triticale (October sowing):
- Rye, spelt (October sowing):
- Smothering species against weeds.
- Low requirements.
- Allelopathic effects.
- Increased density to compensate for average development speed.
- High attractiveness to slugs.
- Forage sorghum (sowing May to July):
- Very good biomass production.
- Good resistance to heat and drought.
- Good structuring effect.
- Allelopathic effects.
- Good destruction by frost.
- Medium attractiveness to slugs.
- Poor soil cover.
Legumes / Fabaceae
Fava bean, pea, vetch, clover, vetch, fenugreek, lentil, alfalfa.
Advantages
- They provide nitrogen to the soil.
- They stimulate microbial activity.
- They do well in associations.
- They have varied root systems which is interesting to combine.
- Good association with rapeseed because legumes establish more slowly than rapeseed (rapeseed is sensitive to competition until stage 4F).
Disadvantages / limits
- Demanding for sowing quality (good seedbed preparation) and require moisture for good germination.
- The large amount of nitrogen released by their residues after cover crop destruction stimulates weed emergence, especially when legumes are used as green manure.
Examples
- Fenugreek :
- Good "companion" plant for oilseed rape.
- Rapid emergence.
- The curry smell disturbs pest insects but attracts hares and deer.
- Spring faba bean :
- A staple of summer cover crops: a true nitrogen factory.
- Due to the size of its seed, the plant requires moisture to germinate. Sowing depth: 2-3 cm.
- Very good when associated with oilseed rape and winter cereals (Diana variety, 90 kg/ha).
- Resistant to Aphanomyces.
- Vetch :
- Good production of biomass and high nitrogen production.
- Toxic to animals.
- Very good "companion" plant for oilseed rape.
- Low attractiveness to slugs.
- Adapted to calcareous clay soil.
- Seed cost among the highest.
- Bird's-foot trefoil :
- The cover crop least sensitive to herbicides.
- Alfalfa :
- Successive mowings allow to clean the plots (including perennials). Be careful to control it well in spring.
- No sensitivity to diseases.
- Good performance in drying conditions.
- Can be associated with a grass for a balanced fodder.
- Its deep rooting limits competition with crops (even ensures water rise by capillarity along its root).
- Limited outlets.
- Grows poorly in acid soils; inoculum at sowing mandatory if pH<6.5.
- Delicate harvest.
- Also, when establishing the following crop, care must be taken to limit the biomass of alfalfa to prevent it from sheltering voles.
- Sweet clover :
- Repellent for voles and field mice, very aggressive in the 2nd year.
- Forage pea :
- Good smothering effect on weeds.
- Good growth even in dry conditions.
- Good association with triticale.
- Rapid start.
- If destruction with tines: opening discs.
- Density to be adapted for the risk of lodging.
- Alexandrian clover
- Good initial vigor.
- Spreading habit: possible mineralization after destruction.
- Quite sensitive to herbicides.
- Very good cover between two straw crops.
- Medium to high attractiveness to slugs.
- White clover dwarf:
- Needs to be "calmed" in spring, can become a weed.
- Difficult to destroy.
- Crimson clover :
- Slow establishment in autumn, to be destroyed early (dries the soil).
- Mobilizes a lot of nitrogen in spring and releases it late.
- Purple clover (Sowing in July or under a cereal cover (April))
- Nitrogen fixation.
- Successive mowings allow to clean the plots (including perennials).
- No sensitivity to diseases.
- Species smothering to weeds.
- Limited outlets for livestock feed.
- Sensitive to drought and heat.
- Vetch :
- Strong competition against weeds.
- Moderately sensitive to herbicides.
- Good association with rye, spelt, triticale and oats.
- Strong frost resistance.
- Very favorable to biological activity.
- Little sensitive to frost (to be reserved for situations with significant winter frost).
Examples
- Flax :
- Good structuring effect on the topsoil.
- Interesting effect against insects (flea beetles).
- To be destroyed before lignification.
- Pearl millet (sowing June to mid-August) :
- Low water requirement.
- Quickly covers the soil.
- Nyjer (Sowing July-August) :
- The earlier the sowing, the better the weed inhibition.
- Good destruction by frost.
- Good association with oilseed rape.
- Dislikes calcareous soils.
- Sensitivity to slugs.
- Beware of the risk of sclerotinia.
- Phacelia :
- Buckwheat :
- Good vigor in dry conditions.
- Provides flowers in autumn and beneficial insect effect.
- To avoid before maize: improves phosphorus availability.
- Typical companion plant dose: 2kg/ha.
- Low attractiveness to slugs.
- Beware, at high density, competition with the crop is strong with a significant slowdown in oilseed rape growth.
Economic impact
- Planting cost : on average (see table), the sowing cost of a cover crop (seed and machinery) is around €100/ha.
- Impact on yield : Various experiments have shown possible yield gains with legume-based associations (0 to 400 kg/ha). This increase is mainly observed in shallower soils (calcareous clay type), where stimulation of oilseed rape by the cover crop compensates for the lack of soil fertility. In deeper soils (such as loam), yield gains are not systematic (available nitrogen). On the other hand, associations with non-legumes often lead to significant and non-negligible yield losses (up to 1,000 kg/ha).
Environmental and agronomic impact
Advantages
- Alternative to the use of residual herbicides.
- Can be used as cover crop, intercrop or green mulch.
- Complements well no-till.
- Reduces the weed seed bank in the long term.
- Soil quality: Increases organic matter (depending on biomass produced) and improves soil structure. Protects the topsoil against erosion and fertility loss.
- Fertility: Mobilizes fertilizing elements at the surface.
- Floral diversity : Favors natural enemies and pollinators.
- Pests : Companion plants, by increasing plant diversity and nitrogen concentration, help limit the impact of autumn pests (fewer larvae per plant). To achieve these goals, other elements of the technical itinerary must be mobilized (earlier sowing date, nitrogen placement at sowing, organic matter input, etc.)
- Beneficial effects on the agro-ecosystem : optimization of natural resource use (solar radiation, water, soil nutrients), reduction of water runoff, nutrient leaching, soil erosion and weed suppression.
Disadvantages / risks
- Incompatible with hoeing and flaming during their growing season.
- Do not produce a marketable harvest.
- Risk of producing a host fauna for certain pests (slugs, aphids…)
- Depressive effect on the yield of the following crop if destruction is too late (high C/N ratio).
- Risk of inhibiting germination or growth of the main crop.
- May carry over a disease or pest between two crops.
- May require an additional machinery pass.
Sources
- Activating the agronomic assets of cover crops, CA Drôme, 2017
- Soil cover in intercrop: Turning a regulatory constraint into an agronomic asset, Chambers of Agriculture of Lorraine (2016) and Meuse (2023)
- Alternative weed control in vegetable farming, Chamber of Agriculture of Ain, 2016
- Magellan Guide, 2016
- Establishing allelopathic crops in arable farming, GECO, 2021
- Summary of work of the soil conservation agriculture group since 2014, GVA, 2018
- Webinar, French Association of Plant Biotechnologies, 2021
