Soil Temps and Stoppage

[PSU4ME]

I wanted to start a new thread to avoid hijacking another one. We started talking about soil temps and what they could tell us about the “winterizer window”. I’m obviously not the expert around here so I’m hoping for others to chime in. I’m most curious about how you can use soil temps to better understand what’s happening with grass. I know the eyeballs are best but I’m way to analytical to rely on that alone!

I’m planning to start taking temps and recording them and ryeguy posted a good link to an article that I’ll try to referrence.

This morning I used my thermapen (I’m big into smoking meats with my big green egg so I love me some thermapen) and my front yard tested at 44 degrees. Still seems that I’m not in stoppage range yet, going to give it a cut tomorrow to see what it does.

[andy10917]

I’m most curious about how you can use soil temps to better understand what’s happening with grass. I know the eyeballs are best but I’m way to analytical to rely on that alone!

If you're analytical, you'll know that it takes a good number of measurement samples before you can come to any conclusions that stand a chance of holding up, and that you have to account for (more measurements!) all factors that may have an influence upon the outcome/conclusions.

In my experience, there are the following influences/factors that you'd need to measure (there are probably more, but these appear have direct influence on growth stoppage):

1. Soil temperature: the problem with this is that it doesn't decline in a linear fashion - it declines haphazardly, but the linear regression is downward in Fall. And does the 2" depth matter more than the 4" depth temperature?
2. Sunlight: as the sun angle gets lower, there is less light for photosynthesis. And the length of day means that the lower light is available for fewer hours as Fall progresses. So, if there is a cold snap in October, is the effect of soil temperature greater or less than the same situation in November?
3. Moisture: many or most members have their irrigation systems off by the time that growth stoppage becomes a concern for us. It's clear that the amount and timing of the rains after the irrigation cutoff affect growth stoppage dates. The interaction with other factors is unknown.
4. Grass Types/Cultivars: Different grasses and even different cultivars reach stoppage at different times. My Blueberry KBG back yard stops later than my Emblem KBG front yard.
5. Fertilization: grasses that are fed up to well in early/middle Fall tend to remain growing longer. The withdrawal of fertilizer for "The Pause" is specifically to avoid the unnatural forcing of the growth beyond the point where natural slowing and stoppage would occur.
6. Shade/Cooler areas: shadier areas tend to stop first and protected areas may hang onto growth later - these little microclimates affect stoppage dates.

I'm sure there are more factors, and I have no idea how they interrelate. But, unless you're prepared to analyze a big pile of data (20-30 years?) assessing high/low/average temperature and how it interacts with all of the other factors, a simple conclusion becomes a simplistic conclusion (where a complex item is treated as if it were much simpler than it really is).

I find it far easier to mow 50 ft of grass in a few places, and see if I mowed anything. Just like in the Spring - it's far easier to look at the Forsythia flowers in the back yard than it is to gather growing degree days that are a week old from a site that is 500 miles away and knows nothing of local microclimates.

[PSU4ME]

Thanks Andy and sorry to make you write so much. When I meant “analytical” I was more looking at it from a logical standpoint where “ soil temps above 40 degrees and growth is still happening” would be a gauge. Def not looking to be scientific about it but I would have thought there to be a correlation between the soil temps and the winterization window.

I’ll stick to guessing with my eyes.

[Green]

Awesome post, Andy. I've been measuring soil temps every year starting at around this time, until growth stops for a couple of years. I still don't have enough data to draw real conclusions that are like rules. But, it's helping my intuition improve.

[andy10917]

Thanks Andy and sorry to make you write so much.

Never apologize for making me (or anyone) write a bunch of stuff (that's a technical term) when they're trying to tell them you think something is straightforward. If I think it's more complex, then I owe it to you to explain my position.

I too wish there were a measure that would allow me to know that at 4pm EST today, I hit the point where it was optimal to drop the Winterizer, or to apply the Pre-M in Spring. It doesn't exist, and what does exist is no better than the mower-of-clippings or the Forsythia shrub (or worse).

[Leakytires]

I personally have not tried the winterizer application yet, but I have been keeping an eye on the timing of it here and it seems like the application window has been very narrow over the last two years that I have been watching it. It seems to me that my chances of hitting it perfectly seem pretty remote.

I understand that we all want to get our fertilizer down at exactly the right time to get the maximum benefit from the application, but just what are the consequences of missing the “exact right time”? Will there be horrible repercussions or will the application just not provide as much benefit as it could have?

[andy10917]

If you apply the final Winterizing fertilizer before the top growth has ceased, you may force new tender growth that will be prone to winter damage or disease.

If you wait too long and root growth has ceased, none of the Nitrogen will be taken up and stored in the roots of the grass.

[Leakytires]

Thanks Andy.

So there is some risk for lawn damage on the early side. I suppose either could become a noticeable stressor on the health of the lawn over the next growing season. Has anybody missed the window by enough to have regrets?

[RichS]

I typically look for 5 days of soil temps under 50 at the depth of my thermometer, about 3.5 inches. There's an article out there that says shoot growth stops at soil temps of 40, but I've been watching/tracking for 15+ years and prolonged 50 degrees has consistently been accurate for me.

I was at that point last weekend, even though I picked up some clippings in my mower (early in the week presumably), but I went ahead and dropped the last 1lb N. Everything still looked freshly-cut as of last night - no growth over the week.