March 26, 2025
Written by: Peter Newman
Breaking grass dormancy – sandpaper in the cold and dark
In my earlier days I prided myself on my punctuality. Always on time, and many an argument with the family if someone was holding up proceedings.
As I’ve aged, I’ve slipped a little. Geraldton time is within half an hour. Not quite as loose as Broome time, but you get the drift.
Our grass weeds used to germinate on time. Opening rains would come in Autumn, and fsssst, up she comes!
Nowadays, our grass weeds are on Broome time, often coming up weeks after the break.
What is driving it, and how can we break this dormancy.
Recent AHRI research by an international team including Aniruddha Maity from Auburn University, USA, with help from Dr Mike Ashworth and Roberto Lujan Rocha from AHRI, is shedding some light on this seed dormancy.
Or should I say, shedding some darkness on dormancy! (Dad joke)
The team investigated Barley grass, Wild oats and Brome grass from cropping fields in Western Australia to find, as has been confirmed in the past, that the seeds of these weeds are more dormant than those from uncropped areas.
They then went on to work out what is causing this dormancy and how to break it, finding that the winning combination was sandpaper, darkness and cold.
Pretty much all winter weeds are dormant straight off the plant. The plants have adapted to have some built in dormancy to avoid germinating on a summer rain when conditions are not conducive to their survival. This dormancy generally breaks down over the summer months, leaving the seeds ‘soft’ and ready to germinate when autumn/winter rains come.
There are two main types of dormancy;
- Physical dormancy – e.g. where the seeds have a water impermeable seed coat, impeding imbibition of water.
- Physiological dormancy – caused by chemicals in the seed such as hormones and other inhibitory chemicals.
Both of these mechanisms break down with time, often linked to the thermal time the seed is exposed to.
Why has dormancy increased?
Put simply, farming is predictable. We generally spray and seed our crops at about the same time each year. The plants have adapted to germinate a little later to avoid the herbicides and tillage at seeding.
This is a bad thing, in that it makes it harder to kill weeds with knockdown and pre-emergent herbicides and is a good thing, in that the weeds now germinate often after the crop, giving the crop a competitive advantage.
This study, along with other previous studies, found that weed seeds collected from cropping fields have more dormancy than seeds collected from non-crop (ruderal) areas as can be seen by the picture below.
Photo caption: Seed germination of brome grass, barley grass and wild oat seeds after 5 weeks of cold stratification.
What is driving it?
The research team set about investigating many of the known dormancy breaking mechanisms including Sodium nitroprusside, Excised endosperm, ethanol, gibberellic acid, potassium nitrate, sandpaper scarification, acid scarification, cold stratification, and germinating in the dark or light.
A combination of all of these treatments was tested with the highest level of germination across the various species summarised in the table below.
Table 1: Combination of treatments with the highest level of seed germination for seeds from cropping paddocks.
Barley grass in this study, from the in-crop populations, did not like germinating in the light. The in-crop barley grass was more dormant than the ruderal (non-crop) barley grass and germinating the in-crop barley grass in the dark did help to break some of this dormancy.
Sandpaper scarification
The sandpaper scarification in this study consisted of seeds being rubbed between two sanding discs for 3 to 10 minutes until about 30 to 40% of seeds broke out of their seed coats.
All of the weed species responded to this treatment, suggesting that physical dormancy from a seedcoat that water cannot penetrate, is one of the key dormancy mechanisms.
This is clearly not something that we can achieve in the field. However, tillage at seeding and/or as an autumn tickle may contribute to the scarification of this seed coat. Also, seeds that travel through the combine harvester may also be subjected to some scarification.
Summary
There are some common themes that come out of this study.
- As has been found in the past, many of our grass weed species are now more dormant and germinating later in the season than they once did.
- The combination of sandpaper scarification, cold stratification (at 4-8deg C), and seeds being placed in the dark, was the most effective combination of conditions that reduced dormancy across all of the grass species in the study.
It’s no surprise that weed seeds like some tillage, with cold and dark conditions to germinate, but it’s important to realise that this is now more important than it once was as our weeds become more dormant.
Papers
Check out these papers here:
Posted in: AHRI Insight, Herbicide evolution and technology