Plants need to get cold-hardened

Paul McIntosh.
Paul McIntosh. Contributed

THE very recent turn around from our balmy autumn temperatures has struck again in the mid-May timing.

We went from above average temperatures to many areas receiving frosts and very cold conditions.

This abrupt change of course is what affects us most, changing from a warm to a cold air mass in the space of a couple of days.

Fast forward to mid-July and these cold conditions are the norm and we are well adjusted by then to those winter temps, however right now in the middle of May and we all seem to feel it a lot more.

Our plants in the paddock feel the same as we do.

What does cold hardened mean and is it really an agronomic term?

My explanation of cold-hardened is that plants have adapted or acclimatised to the winter growing conditions that they are in.

They need to change and develop internally of being able to handle these expected zero temperatures.

Before this point of time in many areas of regional Queensland, winter cereal crops with moisture were very happy and quite lush in their canopy.

Along comes a snapping frost or cold event and many physiological changes occur in the plant and from the outside we may well observe plants prostrated or collapsed on the ground and even colour changes to a lighter shade of green or even just a bleached white on leaf tips.

I have heard and used this term cold hardened for more than 35 years and each year entering a winter period can be very different in the transition stage received.

Also, these warm to snap cold transitions do play havoc with our maturing summer crops like cotton, sorghum, mungs and Lab Lab crops.

The obvious risk is that if these summer crops are not close to physiological maturity then the frozen leaf structures in the top storey of the plant are not going to be able to finish off the seed or lint production.

What defence do we have against this phenomenon?

Usually the planting date is our best defence against this getting more regular mid-May cold snap.

Farmers in northern parts of central Queensland are probably the luckier ones at this stage however you still need to calculate your own last planting date to reduce the risk of early frosts and cold conditions impacting on your finishing crop.

Even just the colder air mass has a slow down effect on maturing crops.

For example I have observed the black dot or physiological maturity point just takes longer to reach in sorghum crops with the lack of strength in the sun and lower temperatures occurring.

I have unfortunately inspected cotton crops following an Anzac Day frost on the inner Darling Downs some years ago which was not pretty and much unexpected.

Boll opening and quality were certainly adversely affected by this event and basically there is not much you can do about any of these situations except wait.

On the other hand, what about our newly emerged oats, wheat and barley crops?

Well the first thing is to hold off on post-emergent herbicides that you may want to use for weed control.

Topping the list of what not to use right now when our winter cereal crops are under this cold stress, are the Sulfonylurea groups with products like Metsulfuron, Chlorsufuron or any similar type chemistry.

I am even very wary about applying our phenoxy hormones like 24-D, MCPA and Picloram products at this active growth reduction time. These products are growth stimulation products and would not help in plant recovery, even though my description suggests otherwise.

All these products need this active growth from both the weeds and the crop to achieve a good result and I don't call flattening the cereal crop and doing a poor job on the turnip, mustard, milk thistle or parthenium as a good job.

Wait until they display some signs of new growth and upright structure before any spray events.

As for Lab Lab and other leguminous summer crops, I would suggest feeding/chopping before you lose the lot as frost does not leave you with any usable green crop for your production.

So whilst we shiver at our places of work and home spare a thought for outside plants and use some commonsense when making any spray decisions both in fallow and more particularly in-crop until we are all cold-hardened.

Topics:  plants rural toowoomba weather

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