Weed control is critical early in the season since spring weeds germinate rapidly and grow vigorously relative to the slower growing onion plants. If weeds are not adequately managed during this early period, they become difficult to manage as time progresses and will out-compete the onion crop. Onion fields and borders should be maintained weed-free for the first 10 to 12 weeks so that weed pressure will not significantly impact plant growth and ultimately reduce onion yields.
Weeds can be controlled with cultivation and herbicides or a combination of the two approaches. Hand weeding crews may be needed to control those weeds that escape cultivation or herbicide applications. Onion seedlings are very sensitive to herbicides and few herbicides are registered as pre-emergents. A contact herbicide (RoundUp, Gramoxone) can be applied before onion seedlings emerge to help manage weeds until seedlings have two or more true leaves and are more tolerant to herbicides.
In organic systems, mulches (such as straw, cardboard, etc.) can provide good weed control in and between rows if applied in a thick mat before weeds emerge (Fig. 7.3). There are also OMRI-approved organic herbicides that can assist in weed management in these operations. These organic herbicides are primarily contact herbicides and must be applied to the green tissue of the weeds. Care must be taken when using these contact herbicides that the chemical does not get on the onion seedlings. Most organic herbicides have limited residual activity so weed control involves a combination of approaches like tillage, hoeing, and mulches, in addition to the herbicides.
Herbicide labels often change, so make sure to always consult the label to determine if onion is listed on the label, what precautions are required, and what rates and application methods are allowed. It is critical to read and understand the label.
Unmanaged weeds may outcompete the onion crop.
Important Considerations for Herbicide Use
- Carefully read and follow all label directions.
- Use herbicides only on crops for which they are approved and recommended on the label.
- Use the recommended amount of product and apply it as stated. (Too much material may damage the crop and make it unsafe for consumption.)
- Apply herbicides only at times specified on the label and observe the recommended intervals of the time of planting and the time between treatments.
- Follow re-entry intervals (REI) and pre-harvest intervals (PHI).
- Don’t spray in high wind conditions.
- It is a violation of the law to use herbicides other than as directed on the label. The EPA has the authority to seize any agricultural commodity that carries a pesticide residue in excess of the established tolerance levels. In addition, if residues of unlabeled chemicals are detected on fresh produce, they could be traced back to your farm.
Finally, herbicides are just one tool available for weed control and their use should supplement other good weed-management practices.
Herbicides for weed control are applied in the following ways:
- Pre-plant incorporated: incorporated into the soil prior to seeding or transplanting onions
- Pre-emergence: applied to the soil after planting but before onions or weeds emerge
- Post-transplant: applied to the soil after crop is transplanted either before weeds have emerged or after clean cultivation
- Post-emergence: applied to weeds after both weeds and onions have emerged
- Directed post-emergence: applied as a directed or shielded spray post-emergence on small weeds in rows of taller crops or in row middles. When using a post-emergence herbicide, the entire weed must be covered for maximum control.
Figure 7.3 Straw mulch between rows of onion helps suppress weeds