Chapter 7.14

7.14 - Evaluating the use of onions as a companion crop to repel pests

Paige Burton, University of Guelph, Canada

Suggested citation for this chapter.

Burton,P. (2022) Evaluating the use of onions as a companion crop to repel pests. In Farmpedia, The Encyclopedia for Small Scale Farmers. Editor, M.N. Raizada, University of Guelph, Canada. http://www.farmpedia.org

Brief description

As a response to the different challenges imposed by invasive species and the negative impact their presence has on crop yields, smallholder farmers are looking for holistic and budget-friendly alternatives to pesticides (Gathuru & Muriuki, 2019; Maina et al., 2016). Intercropping is the traditional practice of planting different crops together, and if the crops are chosen carefully, it represents a sustainable approach to enhancing crop yields and managing pests, as highlighted by studies on Kenyan farming practices (Ayieko & Anyango, 2017; Gathuru & Muriuki, 2019; McLeod & Warburton, n.d.). This chapter reviews the benefits of intercropping with onions to fight pests.

Benefits

Incorporating onions when planting crops is a strategy used in farms and home gardens to help control pests (Smith, 2019). Onions keep bugs away from crops by releasing sulphur-based compounds like allicin, into the air (Sharma et al., 2018). The defense strategy of onions against pests, especially insects, involves the discharge of sulphur-containing compounds, with allicin being the primary compoundt. Damage to onions, caused by factors like pest interference or infestation, results in the rupture of cells which triggers the release of allicin into the atmosphere, acting as a natural insect repellant (Ben-Issa et al., 2017).These compounds repel pests like aphids, cabbage worms, and carrot flies. Farmers have seen that onions as companion plants are effective at keeping pests away during field tests on farms (Ben-Issa et al., 2017).

Quantitative assessments have shown substantial decreases in pest populations after the introduction of pest management approaches centered around onions as companion plants (Smith et. al, 2017). For instance, research carried out by Smith et al. (2017) noted a 50% reduction in aphid populations within inion-intercropped fields compared to monoculture plots (Smith et al., 2017). Using an onion intercrop could help make farms grow diverse crops without the use of harmful pesticides (Karanja et al., 2018). Home gardens and high-value crops can significantly benefit from this approach, particularly female farmers who are key contributors to these agricultural initiatives (Johnson & Brown, 2020).

Practical Implementation

Instructions for intercropping onions as companion plants:

i.Planting Layout:

In an agricultural field or home garden, arrange rows of onions between the rows of your crop with enough space for them to grow, making sure each plant has enough room for airflow. The effective range for successful pest deterrence from an onion as a companion plant spans approximately 25-50 cm from each onion plant (Government of Newfoundland and Labrador, n.d.).

ii.Planting Configuration:

Optimal spacing is essential for maximizing the growth and development of onion plants, especially when functioning as companion crops. Onions can be started from seeds, planted as sets or grown from transplants. For onions planted as seeds, best practices involve spacing rows 30-40cm apart with the seeds spaced 1-4 cm apart in the row (Government of Newfoundland and Labrador, n.d.). Onion sets are planted in rows spaced 60 cm apart with around 4 cm between sets (Government of Newfoundland and Labrador, n.d.). For onions grown from transplants, rows should be spaced 30-40 cm apart with plants spaced around 12 cm apart.

i.Conditions:

Each type of onion has an optimal timeframe for planting that is impacted by different factors like soil quality, climate and agricultural methods followed locally.

ii.Soil quality:

Onion crops flourish in rich, well-draining soil with high organic content. For optimal growth, ensure the pH of the soil is between 6.0 to 7.5, as onion crops prefer slightly acidic to neutral environments (Sideman & English, 2009). Healthy root development and thriving plants result from well-ventilated and nutrient-dense soil.

iii.Climate:

Onions are categorized as cool-season crops, and they grow best in the temperature range of 13°C to 24°C, as extreme temperatures can stunt the growth and development of the crop (Sideman & English, 2009). The size of the onion bulb itself is directly influenced by the duration of daylight hours, with longer daylight hours correlating to larger onion bulbs and bigger crops (Sideman & English, 2009).

iv.Local agricultural practices:

The best planting time for onions can also be influenced by local agricultural practices like crop rotation and preferred planting schedules. To reduce the threat of soil-borne pests and pathogens, incorporating crop rotation is important in reducing pest and disease risks. Farmers also commonly adhere to regional planting patterns or calendars that are determined by specific climate conditions and historic weather patterns, which function as guiding frameworks for farmers

4. Complementary Crops

The practice of companion planting as a gardening technique involves deliberately planting different crops close together to optimize growth, deter pests, and enrich soil fertility, which ultimately maximizes agricultural output (Fiedler, 2012). Several crops are known to thrive when planted alongside onions. Here are a few examples of these crops:

i. Lettuce:

Lettice is a common companion plant to onions because of its shallow root system as it poses minimal competition for the deeper root systems of onions (Golden Harvest Organics, 2023). Additionally, lettuce enjoys the pest-repelling characteristics present in onions.

ii. Carrots:

Carrots and onions are common companion crops for the same reasons as lettuce – its shallow root system (Golden Harvest Organics, 2023). Onions’ pest-repelling properties, especially against carrot flies, contribute to protecting carrot crops.

iii. Beets:

Beets are root vegetables that also have shallow root systems. Their shallow roots are complementary to the deeper root systems of onion plants, which optimizes resource utilization and space optimization (Golden Harvest Organics, 2023).

iv. Brassica:

Planting onions near members of the Brassica family – broccoli, cabbage, and cauliflower - can be beneficial. Onions are reported to repel pests like aphids and cabbage worms that afflict the Brassica family (Fiedler, 2012).

v. Tomato:

Onions and tomatoes are ideal companion plants. Onions deter pests that affect tomatoes, such as spider mites and aphids. Additionally, there is a belief that when grown together, onions improve the flavour profile of the tomatoes (Fiedler, 2012).

vi. Chamomile:

Chamomile is known for elevating the flavour of onions and promoting their growth when planted as companion plants. It also attracts beneficial insects like hoverflies, which assist in pest control as they prey on garden pests like aphids (Fiedler, 2012).

vii. Spinach:

Planting spinach alongside onions is ideal due to their mutual preference for cool season growing conditions. Their staggered harvesting periods and their compatible growth requirements maximize the efficiency of garden space (Fiedler, 2012).

Maintenance

During the crop growth phase, ensure monitoring of all plants for signs of pest symptoms. If you do see any plants that are infested with pests, make sure to quickly eliminate them from the field to disrupt pest spread. If you spot a few pests on plants, remove them, and inspect them for damage. Some common indications that pests are present in crops include:

i. Chewed leaf edges or holes:

This indicates the presence of pests like caterpillars, onion maggots, or onion thrips. These pests commonly feed on onion leaves and can cause visible damage (PlantVillage, n.d.).

ii. Discoloured leaves:

Browning or yellowing of onion leaves can occur due to unfavourable environmental factors, pests, or diseases (PlantVillage, n.d.).

iii. Drooping or wilted leaves:

The presence of pests like root-knot nematodes or onion maggots in onion roots can disrupt water uptake, causing drooping and wilting leaves (PlantVillage, n.d.).

iv. Foreign residue on leaves:

Pests like spider mites or aphids often leave behind sticky honeydew residue on onion leaves. This residue can attract mold and ants, exposing the presence of these pests (PlantVillage, n.d.).

v. Misshapen or stunted plant growth:

Onion maggots that feed on onion bulbs can cause stunted growth or deformities within onion crops. This can cause smaller-sized or misshapen onions (PlantVillage, n.d.).

vi. Visible eggs or larvae from pest presence:

A thorough inspection of onion plants might reveal the presence of insect larvae or eggs on stems, leaves or in the surrounding soil of onion bulbs, which shows the presence of pests (PlantVillage, n.d.).

vii. Silk-like material on plants:

Silk-like webs or material on onion crops could be a sign of the presence of pests like spider mites or other pests known for web-spinning (PlantVillage, n.d.).

Seed procurement

I. Technical assistance:

Technical support and training from cooperatives are an asset and a valuable resource to smallholders. It empowers them with increased knowledge and technical experience in crop management practices, planting methods and seed selection which results in optimized seed efficiency.

II. Diversification:

Having access to a wider diversity of onion varieties through farming cooperatives fosters crop diversification for smallholder farmers. Diversifying this crop will help to minimize crop vulnerability to disease, pests, and inclement weather (Makate et al., 2016).

III. Sustainable Practices:

Onions as companion plants promotes environmental sustainability and increases agrobiodiversity (McLeod & Warburton, 2020). In addition, some farmer cooperatives prioritize sourcing their seeds in ethical and sustainable ways. This means adopting indigenous seed varieties, and supporting community seed banks (Garcia-Yi et al., 2014) which safeguard biodiversity.

Growth requirements and associated costs for onion

Onions, once established, grown, and harvested, can also be regrown from the bottom part of the onion. This can be a potential way to save money on seeds. Please see Useful Resources for links to these instructions.

I. Irrigation:

In general, onions grow well with sustained moisture levels throughout their growth phases and typically require an average of 450 to 600 millimetres (mm) of rainfall during the growth period (PlantVillage, n.d.)

II. Soil composition:

Sandy soils are known for quick drainage and require more frequent watering to maintain sufficient moisture levels. Conversely, clay soil excels in water retention but may become waterlogged without proper drainage, leading to root rot (PlantVillage, n.d.).

III. Farming methods:

Different farming techniques like mulching, planting density and irrigation design, affect the water needs of onions. Intercropping onion plants with other crops or using raised bed designs can impact soil moisture levels by minimizing weed growth and evaporation which helps to optimize water utilization.

IV. Fertilizers and Soil Amendments:

a. Crop nutrient requirements:

Onions typically need higher nitrogen (N) levels for initial leafy growth and moderate potassium (K) and phosphorus (P) for bulb and root development (Chivenge et al., 2022).

b. Fertilizer options:

Synthetic fertilizers offer concentrated nutrients while organic varieties release nutrients gradually – enriching soil structure and fertility over time Organic fertilizer is the more cost-effective option and can be made with locally available materials. (Collins et al., 2013). Some common homemade options are as follows:

i. Compost:

Composting animal manure and kitchen scraps can help enrich soil for healthier onion growth.

ii. Manure tea:

By steeping aged animal manure, smallholder farmers can create manure tea: a liquid fertilizer rich in nutrients and low in cost.

iii. Vermicompost:

This is the process of using earthworms to convert organic matter and kitchen scraps into fertilizer.

iv. Organic mulch:

Mulching onion plants with organic matter (grass clippings, hay, straw, etc.) provides a source of natural fertilizer as it releases nutrients as it decomposes.

V. Seed Costs:

Please see Useful resources below for links to seed vendors. e.g. For companion planting onions, Seeds for Africa (n.d.) sells 100 Red Creole Onion (Allium cepa) seeds online for $1 USD

Women in smallholder farming

Despite the benefits it brings to pest control and crop variety, intercropping could present challenges for female smallholder farmers, given their preexisting responsibilities. Managing intercropping, which is characterized by its increased labour requirement, can intensify workload burdens and time constraints for women. Furthermore, limited access to resources like seeds, tools and information can impede the effectiveness of intercropping efforts among women in smallholder farming. Tailored training and support programs geared toward empowering these women are essential for overcoming these challenges. Recognizing and engaging women in decision-making processes related to their livelihoods can ensure that their needs are properly met while also furthering their empowerment.

Critical analysis

I. Variation in Intercropping Results:

Karanja et al. (2018) stressed the importance of understanding the diverse results of intercropping onions, and how it is affected by factors like environmental variables and pest species. The effectiveness of intercropping in terms of pest management may vary based on the types of pests that are present. For example, studies show that incorporating aromatic herbs like marigolds or basil into onion fields can effectively reduce pest attacks from thrips or onion fly larvae, which ultimately enhances crop yield and quality (Parker et al., 2013). However, it is important to acknowledge that the efficacy of intercropping could differ depending on regional circumstances. As an example, moderate humidity levels generally support onion growth whereas excessive moisture may trigger fungal diseases (Parker et al., 2013). Similarly, even though onions favour cooler temperatures for optimal growth, extreme heat can induce crop stress and lower yields (Parker et al., 2013).

II. Crop Compatibility Challenges:

Crop compatibility is important. McLeod and Warburton’s research noted that onions may not be the most suitable crop to act as a companion plant for all crop species, or for all farming methodologies. (McLeod and Warburton, 2020). Factors like growth characteristics, vulnerability to disease and nutrient competition can be determining factors for how compatible onions are as companion plants. Farmers need to carefully evaluate crop compatibility in the planning stage of the intercropping process to minimize or reduce potential crop conflicts. Crop conflict occurs when the development of one crop is hindered by the presence of another, often due to reasons like competition for space, nutrients., or vulnerability to diseases and or pests (Newton, 2016).

Helpful Links To Get Started

YouTube Videos: Click https://www.youtube.com/watch?v=Op65J0j0hnw to see how to plant onions, and info on the benefits of intercropping onions as companion plants.

Click https://www.youtube.com/watch?v=qNipZ_72wIs to see the best companion crops for onions.

Click https://www.youtube.com/watch?v=-J4s9mXDCeU for detailed information on companion planting in small spaces.

Click https://www.youtube.com/watch?v=rPnU2aV1Si8 for information about companion plant spacing using inexpensive tools.

Click https://www.youtube.com/watch?v=4t58WTIJVOU for instructions and information about how to regrow onions from scraps/the bottom of the onion.

Click https://www.youtube.com/watch?v=qNipZ_72wIs for information on companion planting and instructions.

Click https://www.youtube.com/watch?v=4t58WTIJVOU&t=702s for information on how to regrow onions from scraps.

Click https://www.youtube.com/watch?v=-J4s9mXDCeU for information on how to plant tomatoes.

Click https://www.youtube.com/watch?v=Op65J0j0hnw for information on chat companion planting is, and how to do it.

Click https://www.nal.usda.gov for the National Agricultural Library (NAL) / Agricultural Research Service (ARS) - NAL website gives users access to articles, guides, publications, and research regarding companion planting processes that farmers will find useful and informative. - University extension websites o Universities hosting agricultural extension programs have online resources that are accessible to the public.

African seed supplier that offers affordably priced organic and heirloom seeds: - Seeds for Africa: https://www.seedsforafrica.co.za

World Vegetable Center – International Institute that makes available a wide diversity of vegetable seeds for developing nations: https://avrdc.org/seed/seeds/

References

1.Almekinders, C., & Louwaars, N. (2002). The Importance of the Farmers' Seed Systems in a Functional National Seed Sector. Seed Policy, Legislation and Law: Widening a Narrow Focus, 4. https://doi.org/10.1300/J153v04n01_02

2.Ayieko, M. A., & Anyango, M. (2017). Impact of Companion Planting on Pest Control Practices and Economic Returns: A Case Study of Smallholder Farmers in Western Kenya. Journal of Agricultural Extension and Rural Development, 9(1), 1-10.

3.Ben-Issa, FKA, Laurent Gomez, & Hélène Gautier. (2017). Companion Plants for Aphid Pest Management. Insects, 8(4), 112. https://doi.org/10.3390/insects8040112

4.Chivenge, P., Zingore, S., Ezui, K. S., Njoroge, S., Bunquin, M. A., Dobermann, A., & Saito, K. (2022). Progress in research on site-specific nutrient management for smallholder farmers in sub-Saharan Africa. Field Crops Research, 281, 108503. https://doi.org/10.1016/j.fcr.2022.108503

5.Collins, D., Miles, C. A., Cogger, C. G., & Koenig, R. T. (2013). Soil fertility in organic systems: A guide for gardeners and small acreage farmers (PNW 646). Oregon State University Extension Service. Retrieved from https://extension.oregonstate.edu/catalog/pub/pnw-646-soil-fertility-organic-systems-guide-gardeners-small-acreage-farmers

6.Fiedler, A. K. (2012). Companion planting and insect pest control. HortTechnology, 22(6), 810-819. https://www.researchgate.net/publication/285189207_Companion_planting_and_insect_pest_control

7.Garcia-Yi, J., Lapikanonth, T., Vionita, H., et al. (2014). What are the socio-economic impacts of genetically modified crops worldwide? A systematic map protocol. Environ Evid, 3, 24. https://doi.org/10.1186/2047-2382-3-24

8.Gathuru, E., & Muriuki, A. (2019). Companion Planting: A Sustainable Pest Management Strategy for Smallholder Farmers in Kenya. International Journal of Agronomy, 2019, 1-9.

9.Golden Harvest Organics. (2023, March). Companion Planting. Retrieved from https://s3.wp.wsu.edu/uploads/sites/2064/2023/03/Companion-Planting-Golden-Harvest-Organics.pdf

10.Islam, S., Mazariegos, V., Nagarajan, G., & Zaman, L. (2015, November 30). Effects of Farmer Cooperatives on Expanding Agricultural Markets in Developing Countries: A Systematic Review (Social Impact, Inc. for the US Department of Agriculture). Retrieved from https://fas.usda.gov/sites/default/files/2020-03/ffpr_cooperative_systematic_review_final.pdf

11.Johnson, B., & Brown, C. (2020). Intercropping Onions for Sustainable Pest Management in Gardens. Sustainable Agriculture Journal, 12(4), 78-91.

12.Karanja, J. K., Ochieng, J. K., & Kiptoo, J. J. (2018). Effects of Intercropping Onions (Allium cepa L.) with Other Selected Crops on Pest Population Dynamics in Smallholder Farms. Journal of Agricultural Science and Technology, 20(3), 531-542.

13.Maina, J. M., Nderitu, J. H., & Mutero, K. N. (2016). The Role of Companion Planting in Reducing Pesticide Use and Enhancing Crop Yield: A Case Study of Onion Intercropped with Maize in Eastern Kenya. Journal of Agricultural Sciences, 11(1), 40-53.

14.Makate, C., Wang, R., Makate, M., et al. (2016). Crop diversification and livelihoods of smallholder farmers in Zimbabwe: adaptive management for environmental change. SpringerPlus, 5, 1135. https://doi.org/10.1186/s40064-016-2802-4

15.McLeod, D., & Warburton, H. (2020). Companion Planting: An Agroecological Approach to Pest Management in Smallholder Farming Systems. Agroecology and Sustainable Food Systems.

16.National Agricultural Library. (n.d.). Home. Retrieved from https://www.nal.usda.gov (Accessed April 2, 2024).

17.Newton, A. C. (2016). Exploitation of diversity within crops-the key to disease tolerance? Frontiers in Plant Science, 7, 665. https://doi.org/10.3389/fpls.2016.00665

18.Parker, J. E., Snyder, W. E., Hamilton, G. C., & Rodriguez-Saona, C. (2013). Companion planting and insect pest control. InTechOpen. Retrieved from https://www.intechopen.com/chapters/42925

19.PlantVillage. (n.d.). Onion - PlantVillage. Retrieved from https://plantvillage.psu.edu/topics/onion/infos

20.Seeds for Africa. (n.d.). Red Creole Onion (Allium cepa) Vegetable - 100 Seeds. Retrieved from https://www.seedsforafrica.co.za/products/red-creole-onion-allium-cepa-vegetable-100-seeds?_pos=2&_sid=74a07379d&_ss=r

21.Sharma, A., Bhatia, V., Rahal, A., Singh, S., & Sharma, A. K. (2018). Allicin, a natural antimicrobial compound produced by garlic: A review. Journal of Applied Microbiology, 124(4), 1032-1042. https://doi.org/10.1111/jam.13661

22.Smith, A. (2019). Utilizing Onions for Pest Control in Organic Agriculture: A Review. Journal of Organic Farming, 7(2), 45-56.