Fall weed control

Fall can be an excellent time for weed control. Perennials move resources down into their growing points and roots at this time of year, making fall a particularly effective time to spray these weeds.

Fall is also a good time to spray winter annuals, and possibly annuals in circumstances described below. Ideal timing is often different for each weed group – early fall for perennials, late fall for winter annuals and annuals.

Controlling fall weed growth also limits weed uptake of soil moisture and nutrients, although by October these uptakes will be minimal. 

Use effective herbicide tank mixes to prevent selection for or to manage herbicide-resistant populations. Note fall herbicide options are greater for fields planned for cereals next year.

Sections in this article:

• Pre-harvest
• Perennial weeds
• Winter annual weeds
• Annual weeds
• Frost effect
• Product choice

Pre-harvest

Pre-harvest herbicide application can dry down weeds to make straight combining easier. The timing is often too late to stop weeds from producing mature seeds.

Pre-harvest is a good time to spray moderate to heavy infestations of annual or grassy weeds. The timing can work for perennials as well because these weeds need four to six weeks of regrowth after harvest to provide enough leaf area to take in adequate herbicide for their large roots and growing points. Fall applications may need three times the active ingredient to provide the same control as pre-harvest.

Even so, set reasonable control expectations for pre-harvest herbicide on any big weeds. Kochia, for example, becomes challenging to control with herbicides after the weed reaches 15cm in height. By harvest, kochia can be shrub-sized.

In field trials, Agriculture and Agri-Food Canada (AAFC) research scientist Charles Geddes typically uses glyphosate plus saflufenacil to help dry down the kochia and allow AAFC staff to straight combine the plots. His recent study found that pre-harvest glyphosate plus saflufenacil decreased viable kochia seed production by 28 per cent, on average, when applied 10 days before harvest. That still leaves a lot of viable seed.

Pre-harvest spray results improve with warm sunny days and high water rates to improve coverage. Another option is to swath areas with a lot of large weeds, like kochia, and dry them down that way. Canola Encyclopedia section on pre-harvest aids.

Perennial weeds

Perennial weeds can continuing growing for years. With the onset of lower temperatures and shortened days in the fall, they start moving sugars to below-ground tissues to enhance winter survival. Spraying perennials in fall takes advantage of this downward flow, providing better control than at any other time of year.

Perennials to target in the fall: Canada thistle, dandelion, quackgrass, foxtail barley

The best spray timing is from mid September to early October, before a killing frost. Perennials need time after harvest cutting to accumulate new leaf tissue to absorb herbicides. Four weeks is a minimum recommendation and six weeks is ideal. Even when waiting this long, leaf surface area is still just a fraction of what it was prior to harvest. Therefore fall glyphosate rates will need to be higher (possibly two to three times higher) than the pre-harvest rate to get the same concentration of glyphosate into the plant root. 

Warmer temperatures and bright sunshine improve herbicide activity. Apply glyphosate and other systemic herbicides during the heat of day when perennial weeds are actively growing and putting energy into their roots.

The trade off with waiting too long for leaf regrowth is losing healthy leaf tissue to frost. Without healthy leaf tissue, the herbicide can’t translocate to the weed’s crown and storage roots – the targets for control. (More on frost below.)

Winter annual weeds

Winter annuals germinate in late summer or fall, survive the winter, and get a head start on crops and annual weeds in the spring. Late summer moisture will bring on more winter annual emergence.

Winter annuals to target in the fall: narrow-leaved hawk’s-beard, stork’s-bill, annual sow thistle (common and spiny), cleavers, downy brome 

October until freeze up is usually the best time to control winter annuals. Apply herbicide when winter annuals have emerged and are at the right stage for control. Many of the post-harvest product labels have weed staging listed, and winter annuals can hit those stages fairly quickly.

If waiting until spring, control may not be as good as in the fall. In spring, winter annuals have less leaf area to target early on, have a more established root system, and are moving energy out of (instead of into) root systems. If pre-seed burnoff is delayed due to field moisture, winter annuals can quickly outgrow the stage for adequate control.

With winter annuals, the goal is to break winter dormancy (which you can do with a Group 4 herbicide) or kill new buds being laid down for next year (which you can do with glyphosate). 

A special note on cleavers

Cleavers have become more of a winter annual, perhaps due to no-till and direct seeding. Fall is a good time to manage them because by spring, many cleavers are big enough to escape control with usual pre-seed burnoff herbicide rates. Look for cleavers this fall in fields planned for canola next year. Grading tolerances are fairly low for cleavers, and the weed seems to be getting worse in some fields.

Annual weeds

Annual weeds grow for one season. If they germinate in late summer or fall, they don’t usually survive the winter.

Annuals to watch in the fall: volunteer canola, kochia

If weeds in a field are mainly annuals and significant further seed set is unlikely before freeze up, leaving them untreated to die through the winter is probably the most economical choice.

Kochia

Research out of southern Alberta suggests that kochia regrowing after a mid-August (or later) harvest will not produce seed before a killing frost in most years. Using glyphosate to control those annual weeds in fall may simply add selection pressure for the development of glyphosate resistance.

Volunteer canola

Leaving canola seeds undisturbed so they germinate in the fall or get eaten by birds and insects is a good way to reduce the volunteer seedbank. Canola seeds that remain on the soil surface when the snow flies will deteriorate over the winter. If you want to do something, Charles Geddes, in his U of M PhD, showed that light harrowing promotes fall emergence of volunteer canola. This emerged canola winterkills and helps to deplete the seedbank going into next spring. 

The clubroot issue. It can take as little as three weeks after emergence for actively growing canola volunteers to form galls with viable spores. Letting volunteer canola and other clubroot-hosting weeds (stinkweed, shepherd’s purse, flixweed and wild mustard) grow for more than three weeks after harvest could increase the clubroot spore load in a field. This goes for all fields — not just this year’s canola fields. Note that clubroot growth requires warm soils. The risk scenario described in this paragraph is, therefore, much lower with a late harvest and cool soils.

Frost effect

Killing frost for weeds is hard to define since so many factors influence it. Different plants have different inherent tolerance to frost and weather preceding the frost can affect tolerance. A Canada thistle plant with little exposure to cooler temperatures may be susceptible to a frost of -3 to -4°C, whereas a plant that has gradually acclimatized to cooler conditions and progressively deeper frosts may survive a frost of -10°C or more. This is why it is critical to continually check the condition of leaf tissues before filling the sprayer.

If weeds are green and leaf tissue is still relatively pliable after a frost, growers may still have an opportunity to control perennial or winter annual weeds with glyphosate or another systemic herbicide. Control is possible on warm sunny days after a frost if at least 60 per cent of the leaf area is healthy. If most of the weed leaf area is dead, herbicide uptake will be minimal and waiting is recommended.

For glyphosate to be effective after a frost, weeds need at least 48 hours with nights above 4°C and daytime highs of at least 13°C.

Herbicide does not work better after a frost

Frost does not enhance the movement of sugar, and with it herbicide, to the roots of perennial weeds. However, while frost doesn’t improve control, it may not necessarily reduce control if enough healthy tissue remains to allow sufficient uptake of the herbicide.

Snow

Snow that accompanies frost is likely more of a help than a hindrance with respect to overall weed condition. The snow layer is likely to insulate the weed leaf material from the colder conditions that follow it. If good spraying conditions return after an early fall snow, spraying could resume earlier than it would with frost alone. Re-assess leaf condition a couple days after the snow has melted.

Product choice

Talk to retailers or agronomists about best fall-applied products and rates for specific target weeds and sizes. Be sure to specify what crop will go on that field next year. Fields planned for canola will have fewer options than fields planned for cereals.

Fall-applied herbicides for fields planned for canola

Carryover risk

Dry conditions could reduce the list even further. Residual products registered for use the fall before seeding canola can cause herbicide carryover damage if drought conditions prevent timely breakdown. Several products with high carryover risk are broken down almost exclusively through hydrolysis — the process of water splitting the herbicide molecule in two. Hydrolytic breakdown of herbicides decreases in drought, and adsorption of herbicide to soil particles is increased, both of which will increase carryover.

More…

• Prairie Weed Monitoring Network
• Herbicide resistant weeds in the prairie region
• Canola Encyclopedia – Weed management
• Canola Watch Fundamentals – Including more on weed control
• Canola Digest – Nasty weeds get an “R” rating