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Can I spray following a frost?

By: Clark Brenzil, Provincial Specialist Weed Control

Recent cool weather is a reminder of the potential of poor performance of herbicide applications following a frost. Cold weather reduces the efficacy of herbicides significantly and, in some cases, also results in crop injury. This makes it the perfect time to get into some of the mechanics of cold on herbicide activity and guidelines around spraying after a frost. 

We’re typically trying to control winter annual weeds, biennial weeds or the occasional perennial weed early in the spring, but annuals could already be emerged as a result of our warm winter and spring.

Foliar systemic herbicides are common in pre-seeding or pre-emergent burnoff, as well as in-crop treatments. The foliar systemic herbicides for burnoff include glyphosate (Group 9), Group 2 herbicides and Group 4 herbicides. Groups 2 and 4 are also used in in-crop and glyphosate post-emergent in glyphosate tolerant canola. This could also apply to Group 1 herbicides and, to some degree, Group 10 glufosinate (Liberty) in Liberty Link canola. Foliar uptake is the only route of entry into the plant tissues for glyphosate, glufosinate and Group 1 herbicides. Groups 2 and 4 have varying degrees of soil activity, depending on the product.

Foliar herbicides need to overcome several barriers to get to the site of activity in the plant. First is the waxy cuticle layer on the surface of the leaf, then the cell membrane. Because the site of action for many herbicides is mainly in the chloroplast, they also need to cross a second membrane. For foliar contact herbicides, the journey ends here.

That is sufficient for small annual seedlings, but larger plants pose a tougher challenge. With systemic herbicides, just getting into the cells of the leaf is not sufficient to kill the plant. The herbicide also needs to circulate to vital areas in the plant to be lethal. In this case, the herbicide usually rides along with the sugars being produced by the plant leaves and is shuttled to the actively growing areas in the plant, such as meristems and roots.

The most important factor that makes a foliar herbicide effective is receptive foliage (leaves). Frosts of -4 C or more usually cause significant leaf damage to annual plants, making them poor targets for herbicide applications. However, hardening off may allow an annual plant to recover completely from frosts as low as -5 C or -6 C. Winter annual weeds will tolerate a frost of -10 C or more and continue growing when conditions improve, with little tissue damage.

Symptoms of frost damage to leaves presents as a water-soaked appearance shortly after the frost. This is followed by a darkened or blackened appearance within a day or so, and then necrosis (brown, dry tissues) after a few days. If a frost damages weed leaf tissues, you will have to wait until new leaf tissue is produced before applying herbicide.

An actively growing plant is the key to getting good control, whatever herbicide you use. Because systemic herbicides are moving with the sugars in the plant, sunshine is an important factor in their thorough distribution. During cool, cloudy days, the sugar factory in the plant is not running at peak efficiency. Therefore, the pipeline of material from the leaves to the roots and growing parts of the plant via the phloem is reduced. The deeper the cold, the slower the movement in the pipeline. In weeds that have experienced cold temperatures, active growth may not begin again for a few days following the return of good growing conditions.

Under cold conditions, herbicides become more vulnerable to inactivation in the plant. Glyphosate, for example, has an affinity for organic material. As a result, it may be attracted to the inert structures (cellulose) in the plant, as well as other capture mechanisms. Group 2 herbicides can also degrade very rapidly in the plant through various biological and chemical processes. This makes these herbicides vulnerable to poor performance under cold conditions; the travel time from “source to sink” in the plant is extended, allowing further attack by degradation processes.

While the herbicide is battling these obstacles, it is also under assault from the weed’s metabolic and chemical processes that inactivate the herbicide; the herbicide is in a race from the time you apply it until it hits the site of activity. 

As a result, herbicides can get hung up in the leaves, meaning insufficient amounts will arrive at the meristems to kill the entire plant, and a superficial kill of the outer surfaces of the plant will result, but not the plant as a whole. This is fine for small annual weeds, but insufficient for larger annual and winter annual, biennial and perennial weeds.

Group 4 herbicides (2,4-D and MCPA, as well as dicamba and fluroxypyr) will likely be more tolerant of cold conditions, but they still rely on an actively growing plant to achieve control. Group 4s generally have a lower affinity for organic material inside the plant and can, therefore, tolerate more “down time” in the plant before becoming inactive. They also have a modest level of soil activity, which allows ongoing uptake of the herbicide for several days after application. This gives them a “second chance,” so to speak. Group 4 herbicides are simply versions of the natural auxin compounds in the plant that stimulate tissue growth. While the plant controls the natural auxin levels, using enzymes to degrade it, the synthetic versions are more difficult for the plant to degrade and lead to uncontrolled growth. Under the influence of Group 4 herbicides, the susceptible plant produces large amounts of undifferentiated tissues that eventually choke off transport pathways and cause death.

What this means is that the best thing you can do is wait to apply your herbicide until temperature conditions are suitable for ongoing plant growth. Normally this would be when nighttime temperatures are 5 C or greater and daytime highs are at least 10 C for the two days prior to application. Even with daytime temperatures between 10 and 15 C, bright sunshine is a must for proper performance. After a frost, it takes a couple of days of warmer temperatures for the plant mechanics to get back to functioning normally.

These temperature rules are an absolute for glyphosate and the Group 2 herbicides. With winter annuals, however, phenoxy herbicides may be more effective than these other options when nighttime temperatures dip to just above freezing. When spraying winter annuals in the fall, for example, applications of 2,4-D can be made, as long as the nozzles don’t freeze and application will still result in good control. Longer day length in the spring and a warmer microclimate at soil level due to greater solar radiation also help to mitigate the effects of cold nights, but daily low temperatures below the freezing point definitely make the application of all herbicides off limits.

The bottom line: if the plant (crop or weed) is not growing actively, there is a greater risk that your herbicide application is not going to meet your expectations for control or tolerance. It is always a good idea to contact the herbicide manufacturer to check on their temperature limits for product performance warranty purposes. If neither crop nor weed is growing, you can afford to wait a few days and delay the herbicide application until conditions improve.

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