Ag Instructor Vic Martin: Surviving Winter

Great Bend Tribune
February 4, 2018

First, there is nothing to report on the drought front.  The area growing from abnormally dry to moderate drought, as is the severe condition, is expanding north and eastward but overall little has changed.  With all the concern over possible wheat lost from cold temperatures combined with the drought, it might be helpful to understand how cool season plants such as winter wheat and canola adapt to survive winter.  First let’s tackle monocots, grasses, like winter wheat and then dicots, broadleaves like winter canola.

The goal of a winter annual like winter wheat is to establish itself before winter, protect itself during winter, and then resume vegetative and reproductive growth after the danger of winter is past.  And to produce physiologically mature seed before the heat of summer. So how does it do this and how does it know winter is coming?

  • As a monocot, the growing point where new growth originates stays below the surface until mid to late March around here.  It is at the planting depth which is why a depth of one inch is recommended as a minimum depth.  This provides a less variable, warmer environment that the atmosphere.  Dry, bare soil with poor vegetative growth and persistent cold air temperatures can lower soil temperature at the growing point below freezing and potentially damage or kill the growing point but the plant has other adaptations.
  • As the cues for winter are noted by the plant, several physiological changes occur.  Plant growth is prostrate, not upright.  The chemistry of the plant cells changes and essentially produces antifreeze to lower the freezing point of the cell to prevent the formation of ice crystal and rupturing of cell membranes.    The plant is most winter hardy at the start of winter and the level of protection decreases as winter proceeds and spring approaches.  The plant can regain a level of winterhardiness if temperature decreases aren’t too rapid.  And once the plant has jointed and the growing point is above the soil surface there is essentially little protection except for a lush stand.  So what keys these changes?
  • Two things cause the changes: daylength and temperature with daylength being more important.  The plant has to prepare for winter before winter arrives so temperature isn’t as useful to the plant as decreasing daylength.  The decrease in sunlight keys the changes listed previously and this is reinforced by decreasing temperatures.  If the change in temperatures is too early or abrupt the plant is in trouble.  So how does the plant leave dormancy?
  • As daylength increases after late December as we are seeing now, that keys the plant to resume growth and it is upright not prostrate.  Combined with warmer air temperatures and hopefully moisture, the plant breaks dormancy and heads towards flowering.  So how does it know it is okay to flower?  Winter wheat has to accumulate so much cold before it will flower.  The amount varies by variety but without the requisite cold accumulation it won’t flower.  All of this combines to allow the plant the best chance of being able to escape winter and not reproduce until it is safe to do so.

Surviving Winter Conclusion
Published February 11, 2018

Nothing to report on the drought front except that some areas west of Barton County did receive a few inches of snow.  Unless conditions change markedly and the area receives precipitation well-above normal, conditions at best will remain the same or likely intensify.    Last week’s focus was on how cool season grasses like winter wheat work around winter to complete their lifecycle, i.e. produce viable seed.  Briefly, the growing point is below the ground until spring, it won’t flower until being exposed to a certain amount of cold (vernalization), it changes physically and physiologically for winter with decreasing daylength as the key, and won’t flower until daylight increases to a certain level.  This week’s discussion focuses on winter broadleaves like winter canola.

Unlike grasses, when dicots like winter canola germinate and emerge from the soil, the growing point emerges and is at the soil surface.  As compared to grasses where the growing point stays at planting depth until late winter/early spring, this leaves the growing point vulnerable.  At this stage and until after growth is well underway in the spring, if the growing point is lost the plant dies.  It is vulnerable to damage/destruction by weather factors, primarily cold, and also to physical damage, livestock feeding or trampling for example.  So how has the plant evolved to have the best chance of surviving winter and produce seed and how do producers manage a crop like winter canola?

  • With winter canola a producer is seeking the “Goldilocks spot” heading into winter.  The not too hot, not too cold, just right level of development giving the plant the best shot at surviving the winter.  The idea, based on average weather conditions, is to plant the canola in the fall and allow it enough time to develop around six leaves and a good strong taproot system.  Too little growth leaves the growing point more vulnerable to freezing and too much means the growing point may start to elevate above the soil surface into the air and again leave the growing point vulnerable to freeze damage.  And excessive fall growth can deplete so moisture.
  • Like wheat, winter canola “senses” the approach of winter through decreasing daylength and temperatures.  This results in prostrate growth and the plant forms a rosette (think of what a dandelion looks like) with the growing point in the center.  Cell contents also change and increases substances in cells to lower the freezing point of cell contents to protect the growing point.
  • Even though the growing point isn’t in the soil, it benefits from the heat reserve in the soil compared to the atmosphere to help moderate growing point temperature.  Interestingly, no-till can lead to stand loss during cold snaps as it appears the insulating effects of crop residue traps heat in the soil and that heat isn’t available to protect the growing point.  And as in winter wheat, snow cover helps.  If conditions result in growing point elevation above the soil surface, it is vulnerable to cold.  And dry soil stores less heat to help protect the plant.  One last item, just like winter wheat, the canola plant can lose all its fall above ground vegetative growth and be fine as long as the growing point is undamaged.
  • Referring to the previous point, one of the challenges in growing winter canola is the temperature fluctuations Kansas experiences most winters.  Oklahoma has less problem since it is warmer and places where winters are more consistently cold are also better off.  However, the up and down nature of our winters often leads to more stand loss, especially under dry conditions.
  • As days lengthen and temperatures moderate, the plant resumes growth and switches to upright growth with the growing point elevated on a central stalk.  Cold here can cause severe damage but if the plant is developed enough, secondary nodes can take over and the plant can produce a viable crop.
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