Temperature And Quackgrass Control With Roundup

Although quackgrass can reproduce by seed, it spreads primarily through a persistent and vigorous rhizome system. Rhizomes are an underground network that spread out from the host plant just below the soil surface. Along the rhizomes are buds, which set root and produce new plants.

To effectively control quackgrass, sufficient quantities of herbicide must be absorbed by the plant and transferred, or translocated, down into the rhizomes. Several postemergent herbicides can control quackgrass in broadleaf crops. The main advantage of using in-crop herbicides is that quackgrass may be suppressed while spraying for annual grass weeds and volunteer cereals.

However, Roundup (glyphosate) is still the most effective herbicide on quackgrass. Roundup applied in the fall or in the spring before planting may give more consistent control of quackgrass than any of the postemergent grass herbicides.

Several factors can influence the effectiveness of quackgrass control with Roundup. The stage of plant growth, the vigour of the plant, air temperature and frost each play an important role in quackgrass control.

Tillage can also be a factor. During a study at Agriculture and Agri-Food Canada's research station at Lacombe, Alberta, quackgrass was consistently controlled better with herbicides under conventional tillage than zero tillage. This does not imply that quackgrass cannot be controlled under zero tillage systems. However, it emphasizes that quackgrass control in zero tillage systems requires careful management of herbicide applications.

Other findings at Lacombe indicated that Roundup applied in the fall and again the following spring can reduce the viability of quackgrass rhizome buds under both conventional and zero tillage systems.

Quackgrass Control Checklist

Treat quackgrass at the 3 to 4 leaf stage for consistent control.

Quackgrass must be actively growing in order to achieve weed control success. An active plant draws Roundup into its system and down to the rhizomes.

The growth stage at the time of herbicide application is a very important factor affecting Roundup control of quackgrass. Apply Roundup at the three to four leaf stage for maximum and most consistent control. When applied at earlier or later growth stages, results are less consistent.

For optimum control, apply Roundup when temperatures are in the midteens (Celsius)

Temperature is one of the most important factors influencing herbicide translocation and quackgrass control. Studies have shown that Roundup control of quackgrass is better at 16 degrees C than at 32 degrees C. Research at the University of Guelph demonstrated that at higher temperatures, Roundup translocation increased to the shoots of the plant, at the expense of movement to the rhizomes. The result was decreased amounts of Roundup in the rhizomes and a corresponding decrease in control.

As temperature drops below 15 degrees C, effectiveness also diminishes. See the table presented below.

Frost will have an effect on quackgrass control with Roundup

The type of frost will directly influence the degree of quackgrass control by Roundup. Always check the air temperature at the height of the quackgrass plant to ensure an accurate reading. Temperature recorded at three feet above ground level can vary from the temperature recorded at soil level, and the temperature outside your kitchen window will likely vary considerably from the temperature out in the field.

Light frost will not reduce quackgrass control with Roundup

A light frost ( -2 to -3 degrees C) just before or just after Roundup application will not reduce quackgrass control, provided air temperatures return to the mid-teens. In fact, research shows that Roundup translocation to rhizomes after a light frost is actually increased.

During the fall or early spring, light frosts may occur on successive nights. Research recently conducted at the Brandon Research Station has shown that application of Roundup between successive light frosts will not reduce quackgrass control.

Heavy frost (-5 degrees C or lower) will reduce quackgrass control

Roundup movement to the rhizomes is reduced when quackgrass shoots are damaged by a heavy frost. Research conducted at the University of Guelph suggests that approximately three days are required after Roundup application and before a harsh frost in order for Roundup to be effective. If the time interval between application and a heavy frost is less than three days or the herbicide is sprayed too soon after frost, quackgrass control will be reduced.

If a heavy frost occurs, wait three days and determine if the quackgrass has recovered before applying Roundup. At least 60 percent of the plant must still be active.

If the season is late, and heavy frosts are a strong possibility, wait until spring. Applications too late in the season can be a waste of time, effort and money.

Effect of Air Temperature After Roundup Application on Quackgrass Control

Roundup rate1
Ammonium sulphate
(% v/v)
Shoot control2
(% of shoots killed)
Bud germination3
(% of buds killed)
5C l0C 15C 5C l0C 15C
0.25 0 11 23 17 7 4 4
0.50 0 26 29 52 18 13 44
1.0 0 51 71 70 82 78 83
0.25 4 46 58 79 54 48 81
0.5 4 52 69 89 97 89 100
1.0 4 60 75 91 100 100 100

1 Agral 90 at 0.5 percent v/v was added to all treatments. The Roundup formulation used contained no surfactant.
2 Percentage of top growth control 14 days after treatment.
3 Percentage of buds not producing regrowth after 14 days.

Source: Agriculture and Agri-Food Canada Brandon Research Station

Other Considerations

Spray Volume

  • Roundup is more effective at low spray volumes (5 gallons per acre) than at high volumes However, drift concerns increase at lower spray volumes.
  • Add recommended adjuvants only at low use rates (less than 0.5 L/ac) or high spray volumes (higher than 15 gallons per acre).

Water Quality

  • "Hard" water reduces Roundup effectiveness.
  • Ammonium sulphate can help overcome water quality problems.

Preharvest Roundup

  • Roundup applied preharvest gives acceptable control of quackgrass.

Roundup is a registered trademark of Monsanto Canada Inc.

For more information, contact your local ARD/MASC Service Centre