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Ground Temperature vs. Air Temperature


Arecaceus

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Since we are having cold spells all around the world lately (Australia, West + East Coast of Northern America, Werstern Europe etc...), I'm thinking a lot about a subject that from my perception is not so often discussed around palmnuts and exotic growers and I catch myself every now an then not taking it into consideration: The difference between the measured temperature at 2m above the ground and the temperature next to the soil. Especially in the winter this can mean dead or alive for some smaller plants like overhead protection or exposure can. I think it is an interesting topic because like all of the other climate parameters it's not universal and it can differ a lot depending on the weather conditions. But a measured temperature of just above freezing can already mean a few degrees below freezing next to the ground.

I'm pretty sure many here have their opinion on this and probably think about this, but how many of you actually do? Is anyone monitoring ground temperatures? Are you thinking about this whenever you plant something small? Do you think it makes a huge difference?

I hope somebody is as interested as I am in this.

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17 minutes ago, Hortulanus said:

I'm pretty sure many here have their opinion on this and probably think about this, but how many of you actually do? Is anyone monitoring ground temperatures? Are you thinking about this whenever you plant something small? Do you think it makes a huge difference?

My measurements are taken between 4 feet and 6 feet since that is the standard here.  I did set a sensor near the ground and can tell you it is definitely colder during radiational events.  Here in Florida the soil temperature usually stays relatively high during short duration events, and this tends to mostly nullify the lower air temperature.  As a case study, I vault planted hundreds of Ptychosperma elegans seeds and let the sprouts exposed to last January's cold.  None of them took damage or died.

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Lakeland, FL

USDA Zone 1990: 9a  2012: 9b  2023: 10a | Sunset Zone: 26 | Record Low: 20F/-6.67C (Jan. 1985, Dec.1962) | Record Low USDA Zone: 9a

30-Year Avg. Low: 30F | 30-year Min: 24F

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The ground here never ever freezes, even in 2021 when it hit 12F and stayed below freezing for an obscene length of time. Tropical things like jackfruit, Syzigium, Ficus, mango, starfruit, etc still came back from ground level. 

There was a radiational freeze last year where it hit 26F but the ground was still very warm from days in the 70s and 80s the week prior. No damage to tender stuff like hibiscus and jackfruit under canopy. 

From my observations, closer to the ground is better when it comes to zone pushing in a dense planting scheme with overhead canopy. Ground level is probably a lot colder on a bare field. 

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Jonathan

Katy, TX (Zone 9a)

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19 minutes ago, kinzyjr said:

My measurements are taken between 4 feet and 6 feet since that is the standard here.  I did set a sensor near the ground and can tell you it is definitely colder during radiational events.  Here in Florida the soil temperature usually stays relatively high during short duration events, and this tends to mostly nullify the lower air temperature.  As a case study, I vault planted hundreds of Ptychosperma elegans seeds and let the sprouts exposed to last January's cold.  None of them took damage or died.

This is very interesting! So the ground temperature in Florida is actually helping during cold spells? So the US has different standard for measuring temperatures? As far as my knowledge goes 2 meters (6.56 feet) is the international meterological standard. Or do you mean standard for normal people? Because I don't think that here in Europe people are measuring at exactly 2m either. Just somewhere around it. Many measure directly at the wall of the house which is also misleading.

From my experience radiation is also having the biggest impact on the temperature difference. If frost comes with an overcast sky the ground temperature seems to be close to the one measured at 2m. I also think there is a difference between seasons. Late frosts at the end of winter or in early spring are colder on the ground than in late autmn or early winter.

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3 minutes ago, Xenon said:

The ground here never ever freezes, even in 2021 when it hit 12F and stayed below freezing for an obscene length of time. Tropical things like jackfruit, Syzigium, Ficus, mango, starfruit, etc still came back from ground level. 

There was a radiational freeze last year where it hit 26F but the ground was still very warm from days in the 70s and 80s the week prior. No damage to tender stuff like hibiscus and jackfruit under canopy. 

From my observations, closer to the ground is better when it comes to zone pushing in a dense planting scheme with overhead canopy. Ground level is probably a lot colder on a bare field. 

 

So what I already take from the first 2 posts is that in lower latitude regions the ground is actually warmer often because tempertures in a cold spell go down from higher tempertures. This allignes with my observations that early frost in the beginning of the winter is often not as cold on the ground as in late winter here in a more temperate climate. All my observations are made in an overall warmer area, because my garden is inside of a city with a lot of buildings, wind protection and closely planted plants.

  

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16 minutes ago, Xenon said:

The ground here never ever freezes, even in 2021 when it hit 12F and stayed below freezing for an obscene length of time. Tropical things like jackfruit, Syzigium, Ficus, mango, starfruit, etc still came back from ground level. 

There was a radiational freeze last year where it hit 26F but the ground was still very warm from days in the 70s and 80s the week prior. No damage to tender stuff like hibiscus and jackfruit under canopy. 

From my observations, closer to the ground is better when it comes to zone pushing in a dense planting scheme with overhead canopy. Ground level is probably a lot colder on a bare field. 

 

 

That's the difference between Houston at 29N and my inland location at 51N. Usually my ground doesn't freeze either, but after the worst early winter freeze in 300+ years here, the ground has frozen solid for me. My lowest here in the rural countryside, 35 miles inland from the coast, was only -8C / 17F during this event, which was a fair bit milder than Houston in 2021.

However the prolonged nature of this freeze (7-10 days) with most days only reaching 1-2C / 34F, meant the ground was able to freeze pretty badly. So in some ways this freeze has been worse than what Houston dealt with, despite the fact that the ultimate low wasn't quite as cold. The duration has been the biggest issue for me here really.

I am dreading to see the effects on my Chamadorea, Dactylifera, CIDP, Theophrasti, Washingtonia etc. I doubt it has got cold enough to kill them dead, but the foliage is probably all smoked now. It is due to get WAAAAAAYYY milder now moving forward, but if we get another bad freeze this winter, it will probably kill a lot of my plantings here now.

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Dry-summer Oceanic climate (9a)

Average annual precipitation - 18.7 inches : Average annual sunshine hours - 1725

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20 minutes ago, UK_Palms said:

 

That's the difference between Houston at 29N and my inland location at 51N. Usually my ground doesn't freeze either, but after the worst early winter freeze in 300+ years here, the ground has frozen solid for me. My lowest here in the rural countryside, 35 miles inland from the coast, was only -8C / 17F during this event, which was a fair bit milder than Houston in 2021.

However the prolonged nature of this freeze (7-10 days) with most days only reaching 1-2C / 34F, meant the ground was able to freeze pretty badly. So in some ways this freeze has been worse than what Houston dealt with, despite the fact that the ultimate low wasn't quite as cold. The duration has been the biggest issue for me here really.

I am dreading to see the effects on my Chamadorea, Dactylifera, CIDP, Theophrasti, Washingtonia etc. I doubt it has got cold enough to kill them dead, but the foliage is probably all smoked now. It is due to get WAAAAAAYYY milder now moving forward, but if we get another bad freeze this winter, it will probably kill a lot of my plantings here now.

I could sign that. Same for me. Everything. Did your ground freez also underneath mulching? Because mine didn't just the mulch itself and parts that weren't mulched.

  

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36 minutes ago, Hortulanus said:

This is very interesting! So the ground temperature in Florida is actually helping during cold spells? So the US has different standard for measuring temperatures? As far as my knowledge goes 2 meters (6.56 feet) is the international meterological standard. Or do you mean standard for normal people? Because I don't think that here in Europe people are measuring at exactly 2m either. Just somewhere around it. Many measure directly at the wall of the house which is also misleading.

From my experience radiation is also having the biggest impact on the temperature difference. If frost comes with an overcast sky the ground temperature seems to be close to the one measured at 2m. I also think there is a difference between seasons. Late frosts at the end of winter or in early spring are colder on the ground than in late autmn or early winter.

So the US has different standard for measuring temperatures?

Yes, siting standards for temperature per our NWS is 5 feet +/- 1 foot - so between 4ft and 6ft.  There are other standards as far as open space and distance to a paved surface: https://www.weather.gov/coop/sitingpolicy2

To be clear, almost no personal weather stations meet all of these criteria unless you have a very large, open property with a straw hut for a house ;)

So the ground temperature in Florida is actually helping during cold spells?

Very much so, especially with recently planted small seedlings that hug the ground.  If you have canopy to hold the heat from radiating out into space  and stop frost from forming, you've effectively negated most of the damage from radiational cooling.  Additionally, if the canopy is high enough to let the sun in, you'll get a good enough temperature rebound in the morning to stop latent cold damage.  Leu Gardens in Orlando can grow a lot of palms more appropriate for USDA zones 10a and 10b partially due to their canopy.

There are also palms like Coconuts and Foxtails that start demonstrating nutrient deficiencies if the ground gets below 60F for too long.  If you see yellowing leaves in years without a freeze, it's a safe bet that the ground got too cold for them to take in nutrients.

Many measure directly at the wall of the house which is also misleading.

That happens here a lot as well.  One of the first things I do when I see a station on WUnderground or AmbientWeather posting inordinately high numbers is check its location.  Often times, you'll find it is near a solid black shingle roof or on a lanai.

If frost comes with an overcast sky the ground temperature seems to be close to the one measured at 2m.

If we luck out and have overcast skies, we typically do not get frost or a hard freeze here.  Often, you'll know if there are clouds overhead because the temperature will jump 3-5F for no plausible reason.  When I see this happen on my weather station, I immediately step outside and look for clouds.

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Lakeland, FL

USDA Zone 1990: 9a  2012: 9b  2023: 10a | Sunset Zone: 26 | Record Low: 20F/-6.67C (Jan. 1985, Dec.1962) | Record Low USDA Zone: 9a

30-Year Avg. Low: 30F | 30-year Min: 24F

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17 minutes ago, UK_Palms said:

 

That's the difference between Houston at 29N and my inland location at 51N. Usually my ground doesn't freeze either, but after the worst early winter freeze in 300+ years here, the ground has frozen solid for me. My lowest here in the rural countryside, 35 miles inland from the coast, was only -8C / 17F during this event, which was a fair bit milder than Houston in 2021.

However the prolonged nature of this freeze (7-10 days) with most days only reaching 1-2C / 34F, meant the ground was able to freeze pretty badly. So in some ways this freeze has been worse than what Houston dealt with, despite the fact that the ultimate low wasn't quite as cold. The duration has been the biggest issue for me here really.

I am dreading to see the effects on my Chamadorea, Dactylifera, CIDP, Theophrasti, Washingtonia etc. I doubt it has got cold enough to kill them dead, but the foliage is probably all smoked now. It is due to get WAAAAAAYYY milder now moving forward, but if we get another bad freeze this winter, it will probably kill a lot of my plantings here now.

Just a minor nitpick, but I'm in the western suburbs of Houston. Most of Houston "proper" recorded a low of 14-16F, 16-19F at the coast/bay. For further context: the week following the freeze had daily highs around 80F, it was also quite mild before the freeze. As far as survival, Houston proper has a lot more living pure W. robusta than where I am (which is next to none, just skinny wannabe mutts). 

Good luck with your plants! Cold damage can take a while to manifest. 

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Jonathan

Katy, TX (Zone 9a)

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21 minutes ago, kinzyjr said:

So the US has different standard for measuring temperatures?

Yes, siting standards for temperature per our NWS is 5 feet +/- 1 foot - so between 4ft and 6ft.  There are other standards as far as open space and distance to a paved surface: https://www.weather.gov/coop/sitingpolicy2

To be clear, almost no personal weather stations meet all of these criteria unless you have a very large, open property with a straw hut for a house ;)

So the ground temperature in Florida is actually helping during cold spells?

Very much so, especially with recently planted small seedlings that hug the ground.  If you have canopy to hold the heat from radiating out into space  and stop frost from forming, you've effectively negated most of the damage from radiational cooling.  Additionally, if the canopy is high enough to let the sun in, you'll get a good enough temperature rebound in the morning to stop latent cold damage.  Leu Gardens in Orlando can grow a lot of palms more appropriate for USDA zones 10a and 10b partially due to their canopy.

There are also palms like Coconuts and Foxtails that start demonstrating nutrient deficiencies if the ground gets below 60F for too long.  If you see yellowing leaves in years without a freeze, it's a safe bet that the ground got too cold for them to take in nutrients.

Many measure directly at the wall of the house which is also misleading.

That happens here a lot as well.  One of the first things I do when I see a station on WUnderground or AmbientWeather posting inordinately high numbers is check its location.  Often times, you'll find it is near a solid black shingle roof or on a lanai.

If frost comes with an overcast sky the ground temperature seems to be close to the one measured at 2m.

If we luck out and have overcast skies, we typically do not get frost or a hard freeze here.  Often, you'll know if there are clouds overhead because the temperature will jump 3-5F for no plausible reason.  When I see this happen on my weather station, I immediately step outside and look for clouds.

Yes same here. Especially in Europe very few people even have the space to measure this properly. Interesting I didn't know that the US has a different standard. But I don't think this makes a big difference because neither the US or Europe are measuring close to the ground or extremely high up. I don't know what the reason for the 2m standard is but I think it could actually be a bit lower and be measured at the height where the average human has their face. 😂

You have completely different growing conditions down there. The Northern American climate is very interesting and strange from a European perspective because of the huge land mass without enough mountain ranges to protect very low latitude regions from occasional cold spells. But in general that is something I'm trying to to right now in my garden. I plant some Eucaylputs and then cut it in a way that it acts like a roof for certain parts of my garden. So that radiation is reduced but sun can also come in.

Oh yeah I know that one! Also same here. I often use these private weather stations online just to check what's measured in my area. The offical weather station at the airport is not only very exposed but also seems to be at a coldspot. But then I have to check the private stations because often they show incredibly high temperatures sometimes or in summer extremely cool... Yet it's still sometimes a good indicator because one of the reasons the temperature is so different in my part of the city is the presence of walls and sealed grounds.

In my particular region hard frost also appears only with clear skies. But sometimes light frost can occur even with some clouds. In normal winters I think this happens because I'm influenced by the Western winds from the Atlantic which gives me a mild climate, but just a bit further east the climate starts to go more and more continental. When the air masses meet colder air gets sometimes sucked into the Western winds and then cold air can appear even with clouds. But those are the harmless frosts we have every winter. Not cold enough to freeze the ground or radiate all the heat. Most of the times you don't even see or feel anything you're only seeing the temperature measured that says -2°C ore something.

All in all I still think it's interesting that at your latitude the ground even acts like a heat vault.

Edited by Hortulanus
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7 minutes ago, Xenon said:

Just a minor nitpick, but I'm in the western suburbs of Houston. Most of Houston "proper" recorded a low of 14-16F, 16-19F at the coast/bay. For further context: the week following the freeze had daily highs around 80F, it was also quite mild before the freeze. As far as survival, Houston proper has a lot more living pure W. robusta than where I am (which is next to none, just skinny wannabe mutts). 

Good luck with your plants! Cold damage can take a while to manifest. 

I am going up to 15-16C / 60F on Monday, which may be enough to start showing damage. I'll get the covers off tomorrow morning. Next week looks mild enough to get the spears moving again on most things here, if they are still alive. There won't be any more frosts after tonight for the foreseeable future now. However nothing like the warm-up that you guys experience either side of your freezes in Texas however (80F+). I'm not sure how much of a good factor that is though, as obviously it allows good recovery rates for you, but it also means palms don't go dormant between freezes and may be susceptible to more damage when the freeze actually hits. Hard to say really.

I know London and many coastal areas have really dodged a bullet here due to UHI and coastal influence. Foxpalms has posted photos today of central London Bougainvillea with the leaves and flowers still, as well as undamaged Kentia's, Ensete, Canna's etc. So despite the severity of this freeze overall, it can't be dropping below freezing still in the mildest parts of central London. The far southwest and southeast coasts didn't go below about -2C / 28F either. The palms there, and in central London, won't even be showing damage after this event, unlike here in my rural inland location. I am actually the coldest location right now in the whole of southern England typically. A real frost pocket here. London and the south coast are all above freezing as I type this. Many places are already up to 7C / 43F at 1am now as the milder airmass begins moving in. The hills to the north of Guildford are at +2C / 36F. Yet I am reading -4.6C / 24F here. I am in a proper frost hollow. Although I will be above freezing come sunrise. Big change coming now.

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Dry-summer Oceanic climate (9a)

Average annual precipitation - 18.7 inches : Average annual sunshine hours - 1725

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13 minutes ago, UK_Palms said:

 know London and many coastal areas have really dodged a bullet here due to UHI and coastal influence. Foxpalms has posted photos today of central London Bougainvillea with the leaves and flowers still, as well as undamaged Kentia's, Ensete, Canna's etc. So despite the severity of this freeze overall, it can't be dropping below freezing still in the mildest parts of central London. The far southwest and southeast coasts didn't go below about -2C / 28F either. The palms there, and in central London, won't even be showing damage after this event, unlike here in my rural inland location. I am actually the coldest location right now in the whole of southern England typically. A real frost pocket here. London and the south coast are all above freezing as I type this. Many places are already up to 7C / 43F at 1am now as the milder airmass begins moving in. The hills to the north of Guildford are at +2C / 36F. Yet I am reading -4.6C / 24F here. I am in a proper frost hollow. Although I will be above freezing come sunrise. Big change coming now.

Careful with banking too much on UHI, central Houston at one point had zone 10 lows for 13 consecutive winters and the big mango trees, royal and foxtail palms, etc started popping up. Of course reality hits sooner or later 😅

We're due for a zone busting freeze next week (the third below average winter in a row....🤬). Hopefully the scales will balance and we'll get another warm stretch (same thing I hoped for last year LOL). We haven't even had a ground frost...okra still blooming, peppers blooming theirs heads off, etc and the forecast is calling for 19F and 22F next week hahahah. 

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Jonathan

Katy, TX (Zone 9a)

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I live in Atlanta. Soil temps bottom out late January around 29°F according to USDA maps.  The higher the bud, the higher the risk to freezing.

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