Effective weed control brings better onion stands, higher yields

Onion growers should have and follow a weed control plan for season-long weed control. Research has demonstrated that a few weeds in an onion field, especially during the first few weeks of growth, may reduce stands and yields. With a number of preemergence and postemergence herbicides labeled for onion, growers should be able to maintain control of most weeds throughout the growing season. Preventing new weed germination and killing emerged weeds in the cotyledon stage is critical to success.

A delayed preemergence application (10 to 20 days after seeding, before onion emergence) allows many weeds to emerge, which are then killed with bromoxynil, which is added to the pendimethalin. The primary weeds early in the season are ladysthumb, common purslane and redroot pigweed. The delayed preemergence pendimethalin plus bromoxynil should be applied just before onions emerge. Emerged onion plants may be killed by bromoxynil. A few loops emerged from the soil surface is the latest safe stage to apply this treatment.

Additional weeds that may germinate early include common chickweed, common lambsquarters, marsh yellowcress, shepherdspurse and Virginia pepperweed. Common purslane is not killed by bromoxynil but can be controlled later with GoalTender. Ladysthumb and common lambsquarters that exceed 2 to 3 leaves often survive Goaltender application, so early kill with bromoxynil is important for season-long weed control. Delayed preemergence application of Prowl H2O also gives the inter-planted barley more time to become established.

The next preemergence application of pendimethalin should be made at the onion two leaf stage (2 LS). The delayed preemergence treatment maintains preemergence herbicide activity for one to two more weeks during the critical early growth stages of onion. At the full 1 LS (second leaf showing), oxyfluorfen (GoalTender) may be applied at up to 4 fl oz (0.125 lb ai) over the top of the onions for broadleaf weed control. Goal 2XL should not be used at the one leaf stage because it can injure the onions. This application is critical to maintain control of difficult weeds such as common lambsquarters, ladysthumb, marsh yellowcress and Virginia pepperweed.

In a cool, damp spring, common chickweed may germinate over several weeks and remain active throughout the season. Oxyfluorfen does not control common chickweed, so Nortron may be added to the preemergence or postemergence tank mix for control. Chickweed normally dies out in a warm, dry year. If it persists, it can be controlled with a postemergence application of Starane Ultra or Chateau. If the barley has not been killed yet, add a postemergence graminicide, such as Fusilade, Poast, or SelectMax, at the onion 1-2 LS.

At the onion 2 LS, apply pendimethalin or s-metolachlor (Dual Magnum) or dimethenamid-P (Outlook). At the 3 LS, flumioxazin (Chateau) at 1-2 fl oz/acre (0.032-0.064 lb ai) may be applied to improve ladysthumb and pigweed control, and to help suppress spotted spurge. The Chateau label allows tank mixing only with Prowl H2O. Subsequent treatments of oxyfluorfen, pendimethalin, s-metolachlor, flumioxazin, and the graminicides may be applied as labeled. The objective should be to maintain the field weed-free throughout the season. Starane Ultra is labeled for use at the onion 2-6 leaf stage. It controls most broadleaves.  It should be applied during warm, dry conditions to avoid onion injury. Application to weak onions during adverse weather may result in stand and yield reduction.

Weed control in onions on mineral soil is more difficult because of greater potential for crop injury from the herbicides. Most of the preemergence herbicides labeled for onion are labeled for use at the onion two leaf stage on mineral soil. Onions are very sensitive to weed competition during the first few weeks of onion growth. The Satellite Hydrocap (pendimethalin) label allows application preemergence to onions on mineral soil at 1.5-3 pt (0.71 – 1.42 lb ai/acre). There is a maximum of 3.2 pt/acre per year for Satellite Hydrocap. The label warns not to apply before the loop stage if heavy rains or excess irrigation are expected. Growers on mineral soil should use the lowest rate of Satellite Hydrocap and work up to a rate that is safe on their soil. After the 2 leaf stage, onions on mineral soil are tolerant of pendimethalin but rates above 1.5 lb ai may cause slower onion growth. Onions on mineral soil generally may be treated the same as muck onions with postemergence herbicides. Chateau may cause stunting if applied above 1 ounce per acre on mineral soil. Start with 0.5 oz (0.016 lb ai/acre) and work up to a rate that is safe on your soil type.

With a wide choice of herbicides and timings, growers should be able to maintain onion fields relatively weed-free under normal growing conditions. Hand weeding may be required if weeds escape herbicide application, but any weeding activity in onion fields may displace onions and reduce yields. Wet weather may interfere with application plans, so always anticipate problems and work ahead a few days when possible.

MSU Extension weed control recommendations may be found in Extension Bulletin E-433, Weed control guide for vegetable crops.

– Bernard Zandstra, Michigan State University Department of Horticulture

Common purslane. Photo: Steve Gower