How to Control Summer Weeds

Jess Sampson

Dec 19, 2022

In a year where we have seen well above average rainfall and widespread flooding across the country, it is important to manage your summer weeds. On the east coast, we expect to see a significant weed seed carry over into 2023 and beyond. 

Uncontrolled weeds can have many ongoing effects, including contributing to the soil seed bank. This can cause ongoing problems with crop rotation, weed management, nutrient availability, and increased costs. Effectively managing summer weeds can reduce ongoing weed numbers and run down the seed bank. 

Controlling summer weed growth can help prepare for next year’s crop by:

  • Retaining plant available nitrogen - nitrogen stores are not being used up by weedy biomass.

  • Increasing plant available water.

  • Reducing weed diseases and nematodes. 

  • Reduced levels of rust inoculum.

  • Pest reduction (aphids, etc.). 

  • Assist in crop establishment from reduced competition.

Double Knock

In tough seasons like this one, a double knock strategy is most effective in summer weed control. A double knock method has many benefits, including assisting with reducing glyphosate resistance risks, reducing the weed seed bank and aiding in the control of hard to kill weeds, such as windmill grass, feathertop rhodes and fleabane. For best results in a double knock, the first spray should be a systemic herbicide (eg: glyphosate, atrazine, 2,4-D Amine) to maximise translocation, followed by a contact herbicide (eg: paraquat, glufosinate, saflufenacil) shortly after.

Other methods such as heavy grazing or burning could also be used as a “second knock.”

Tips for controlling Summer Weeds:

  • Keep water rates high (above 70L/Ha).

  • Use AMS when using hard water.

  • Add a surfactant to increase uptake and translocation on tough weeds.

  • Spray actively growing weeds that are not stressed.

  • Spray weeds at the correct size, according to label specifications. 

  • If you have livestock, spray grazing can be really effective.

  • Summer grasses may need a higher rate of Glyphosate to control - keep an eye on ARG resistance and double knock or mix sprays as needed.

Follow the correct mixing order when filling your tank.

Harvest Weed Seed Control 

The best time to start controlling your weeds is at harvest. During harvest, weeds that have grown in the crop and still contain weed seeds in the head are picked up by the harvester and are then spat back out and spread across the paddock in straw or chaff. Destroying these seeds, or “relocating” them to an easy to control area at harvest time is a great way to get a head start on managing your weed burden. A great place to look for information on this is WeedSmart Australia - Industry Voice On Weed Control.

Identifying your target weeds will greatly assist in getting a good kill. Some common summer weeds to look out for may include:

  • Afghan melon (Citrullus lanatus)

  • Paddy melon (Cucumis myriocarpus)

  • Caltrop (Tribulus terrestris)

  • Feathertop Rhodes grass (Chloris virgata)

  • Flaxleaf fleabane (Conyza bonariensis)

  • Windmill grass (Chloris truncata).

  • Sowthistle (Sonchus oleraceus)

  • Serrated Tussock (Nassella trichotoma)

  • Spear Grass

If you are unsure or after some more advice always consult your agronomist or reach out to one of our agronomists: Anthony Stibbard or Jess Sampson.

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Jess Sampson

Dec 19, 2022

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