Flax TIPS: Fall Weed Control and Nitrogen Applications

November 1, 2016 0 Comments

Fall Weed Control

Fall weed control can be an important management tool for targeting certain weeds, especially for an uncompetitive crop like flax. Flax has limited in-crop herbicide options so making sure fall-applied herbicides are considered can go long way to setting up your flax for success.

There are a number of factors to consider:

1. Does the type or stage of weeds warrant application?

Perennial weeds e.g. Canada thistle, quackgrass, dandelion, foxtail barley

  • An effective way to control perennial weeds is in the fall when the plants start to move energy reserves (sugars) to the roots in preparation for over-wintering. We want the plants to move systemic herbicides along with sugars. To effectively exploit this in Canada thistle or dandelion, the plants should have been knocked back with a top growth removing herbicide application, then allowed to regrow to a rosette which is targeted again pre or post-harvest. On their own, pre-harvest applications of glyphosate can be very effective at controlling Canada thistle, but a multi-year approach is generally required.

Winter annual weeds e.g. Cleavers, shepherd’s purse, stinkweed, narrow-leaf hawksbeard

  • Fall can be a good time to target winter annual weeds as well, especially as a way of diversifying activities in spring. Late fall applications are more effective than early fall applications. An early spring burn-off is also a good option that can also capture any new emergence, but if wet conditions prevent field access, winter annuals may become more developed which may lead to reduced herbicide efficacy. Winter annual cleavers are becoming problematic. They can become difficult to control once they have reached the 2-3 whorl stage, and Group 2 and 4 resistant populations have been found. Therefore, if cleavers have germinated in the fall, control may be warranted.

Annual weeds e.g. Wild oats, green or yellow foxtail

  • For some tough-to-control weeds like wild oats or green foxtail, fall-applied soil residual herbicides can provide a good option for targeting emergence early in the growing season. Wild oats are some of the most troublesome weeds in flax, and with growing presence of herbicide resistance (Group 1), soil residual herbicides like Avadex (Group 8) or Triallate (Group 8 and 3) can reduce populations. A word of caution, Group 3 herbicide resistance has been found in green foxtail.
2. Have the weeds been damaged by frost?

When targeting perennial or winter annual weeds, frost can reduce the efficacy of a fall-applied herbicide if it is hard enough to damage leaves. Weeds should be actively growing so that the systemic herbicide is translocated to the roots as the plant moves its energy reserves below ground in preparation for over-wintering. If the frost event has been a hard and/or sudden frost, then the plant may shut down. However, if there has been a slow decline in temperatures and/or a light frost, then the plant may remain active. All weeds are different in their tolerance to frost so the best way to verify damage is to check plant leaves approximately 3 – 5 days after a frost event (brittle or brown/black in appearance indicates damage). Application rates should be adjusted to the higher rates if 40% of the leaves are damaged.

3. Are the weeds in question escapes from earlier in the growing season?

If the weeds that are present are escapes or misses from earlier in the growing season, it may be a good idea to check for herbicide resistance if the weed species is known to have herbicide resistance issues (e.g. Glyphosate resistant kochia, Group 1 resistant wild oats, Group 2 resistant cleavers, etc.). This should be taken into consideration when planning rotations. Provincial diagnostic labs can perform this analysis.

4. For soil-applied herbicides, what is the temperature/ moisture/ organic matter content of the soil?

According to herbicide labels, fall application of soil residual herbicides should take place when the average soil temperature is 4°C or less, at 5 cm depth and within 3 weeks of soil freeze up. Caution should be used this fall as excess moisture may result in the movement of herbicides away from the upper soil layers and germination zone. Soil-applied herbicide rates vary based on crop type as well as organic matter levels in your soil, therefore a soil test may be required if you do not have this information. For other important application information, check the herbicide label or with your local agronomist.

5. Rotational considerations for fall-applied herbicides?

Flax can be seeded the following spring after applications of: Heat LQ, diquat, Lontrel, Kerb, Avadex, Eptam-8E, Fortress. For re-cropping restrictions, always check the herbicide label or in your local Guide to Crop Protection.

Fall-Applied Nitrogen Fertilizer

Generally, applying nitrogen fertilizer at seeding reduces risk of loss by keeping the product in the root zone and available for the crop. But with many on-farm activities in spring and with rising spring fertilizer prices, applications in the fall may be an attractive alternative.

Here are some key considerations when applying nitrogen fertilizer in the fall:

  • Banding fertilizer applications is recommended. It means less is exposed to microbial activity which converts the ammonium form (anhydrous ammonia or urea) to nitrates. The ammonium form is relatively stable, and at less risk of loss to the environment.
  • Broadcasting in fall can lead to the highest amount of loss compared to all methods of nitrogen application.
  • Late applications, or when soil has cooled to 5 – 7°C will result in reduced rates of nitrification from microbial activity. When ammonium is transformed into nitrates, it is more prone to leaching (movement from fall or spring rains) or denitrification (gaseous losses under anaerobic conditions).
  • Late fall-applied nitrogen is still at risk of nitrification/denitrification if the following spring is wet and warm. For this reason, fall-applied nitrogen tends to work best on drier or well drained soils.
  • Slow release forms like SuperU or ESN may provide some benefits by slowing down microbial activity, keeping nitrogen in a more stabilized form.

Please note that fall nitrogen application practices may differ slightly across the three Prairie Provinces. For more specific information on fall nitrogen applications in your area:

For more information on fall weed control:

For information on herbicide resistance testing:

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