Onion Weed Management: How to Control Weeds in an Onion Field

Importance of Weed Control in Onion Cultivation

Effective weed management is crucial when growing onions and other Allium species. Onion plants compete poorly with weeds, especially during the first developmental stages, due to the relatively narrow, upright leaves. If left unchecked, weeds deprive onion plants of essential resources like sunlight, space, water, and nutrients, potentially hosting harmful pathogens such as Pantoea ananatis, which causes center rot in onions, and pests like cutworms and wireworms. High weed populations can significantly affect the yield of bulbs as well as their quality. All onion growers must have a suitable integrated weed management program, which may differ significantly between countries, law framework, means of production, industry at which the product targets, etc.

Common Weeds Found in Onion Fields

Various weed species commonly compete with onions, affecting crop growth and quality. Key weed species include:

Common Chickweed (Stellaria media)
Ladysthumb (Persicaria maculosa)
Purslane (Portulaca oleracea)
Redroot Pigweed (Amaranthus retroflexus)
Marsh Yellowcress (Rorippa palustris)
Shepherd’s Purse (Capsella bursa-pastoris)
Wireweed (Polygonum aviculare)
Clovers (Trifolium spp.)
Capeweed (Arctotheca calendula)
Vetch (Vicia sativa)
Fumitory (Fumaria parviflora)
Heliotrope (Heliotropium europaeum)
Variegated Thistle (Silybum marianum)
Loosestrife (Lythrum hyssopifolia)
Virginia Pepperweed (Lepidium virginicum)

Proper identification and targeted management of these weeds are vital to minimize competition and protect onion crop yields.

Effective Weed Control Techniques for Onion Fields

Some onion farmers cover the part of the soil between the plant rows with a black mat. They also cover the space between the young plants within the row. The black mat prevents the development of weeds while increasing the soil temperature. Alternatively, some smallholder farmers may apply mulch (5-7.5 cm or 2-3 in). Soil solarization during the warmest months of the year may reduce the viability of weed seeds. Crop rotation, which is competitive with weed crops, like cereal rye (Secale cereale) and Indian and white mustard (Brassica juncea and Sinapis alba), can also be helpful.

Farmers often perform one pre-planting plowing/tillage to prepare the seedbed and eliminate perennial weeds. The stale or false seedbed technique could also help control annual weeds 2-4 weeks before planting the crop. Fumigation and chemical weeding using herbicides (for pre- and post-emergence) can also reduce weed seeds' viability, especially in fields with large problems. But you must never use any chemical before discussing it with a local licensed agronomist. This is an efficient alternative, or complementary measure, for conventional onion farmers. OMRI-approved herbicides are available for organic growers and can be used as a supplementary measure within an integrated weed management plan. To avoid herbicide resistance, it’s essential to use these substances selectively and avoid over-reliance. Before using any chemical product, growers are strongly advised to consult a licensed agronomist.

To avoid any yield losses (due to weeds), the onion field should be kept weed-free for the first 10-12 weeks of plant growth. After the emergence of the crop or the transplanting, it is possible to perform shallow cultivation to control the weeds growing between the rows and hand hoeing for the ones growing between onion plants. In all cases, farmers should be careful not to harm the bulbs and the shallow root system of the onion plants. However, cultivation practices may be challenging due to the high plant density in onion (and garlic). More specifically, onions are often planted with 4-10 lines on planting beds up to 2 meters (80 inches) wide when furrow irrigation is applied. As you can understand, this reduces the access of the farmer or/and the machine. To reduce the weed pressure on his/her crop, an onion farmer is advised to invest in pre-planting weed management measures mentioned above.

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