When a a potted plant has died during this extended heat and I pull it out of the pot the dirt is moist snd unusually hot, even though it has been in the shade
Maybe the roots just sort of cook.

Dirt in the beds is the same. The top 4 to 5 inces are moist and hot.

So is it better to water or let the soil be dry?
Which gives the plant a better chance of surviving?

Phoenix can have lows in the upper 80's and 90's, I here. I don't know about their ground being cooler. There I am ignorant. I do know that my plants did not do well this year. I give up.

This is a very interesting thread. I have lost most of my geraniums (some I have had for years), and I thought it was because of the intense heat (they are not in the sun), but now that I have read this about the ground temps, am beginning to think that may be the culprit.

It has been hotter at night than in past hot summers. This was on the news here because families without air conditioning can't get the house cooled down at night which can cause health problems. The same might be true for plants.

I water my basil at least once and sometimes twice a day. Every other day would not be enough to keep it alive.

The desert doesn't retain heat, so the ground cools at night with or without moisture.

I think a lot of plants can take maybe a week or so of 100+ temps and make it, but not 3 months, not even the natives. The lack of rain makes the ground and the daytime temps hotter. We will all lose plants that we have had for a while. The question is what to do if we have a dry winter and another summer like this next year? What will be left standing and what is worth spending our water on? I heard the weather people say we have a 50/50 chance of this again next year!!!!!

Carla in Leander

Oh no. And what will the freezing temps in the winter do to the already stressed out plants and trees?

My lows were 76 last night. That is hot for us.
Lows in Phoenix lastnight ranged from 86-79, tonight 90 forecasted tonight. Maybe their ground is cooler but their air isn't at night. Now I could go into the mountains and it would be different. I imagine their DEEP ground temps have been heating up under the solar deluge and the top radiates off but underneath doesn't radiate off to quick but it doesn't gain as much because it is deep. Interesting. I am taking a devils advocate position, not that I believe what I am saying 'cause it is all speculation on trips to weatherunderground on my computer. I was surprised too when I was corrected by people in Phoenix about my idea of the cool desert nights and their plants getting nightime breaks. That is true in the upper desert but the low desert is an oven night and day. True their humidity is higher because people water their lawns non stop compared to the desert in the same way that the humidity drops when one drives out of Austin. I hadn't thought that the ground might act in a different way in regards to their nightime temperatures.

Moisture keeps things from heating up as quickly too, so the DEEP ground if kept wet would not heat up as quick as deep dry ground during the day but retain heat theoretically at night. The accumulative heat gain, would then heat up more or less having never totally cooled during the night? Would desert ground be hotter underneath because it was dry and the top radiated off heat but the lower layer stayed hot, slowly gaining heat night by night. Can't say. Just scratching my head here.
Now if you let your top dirt dry off, then it would heat up more and so would the moist dirt in the deep recesses (what wet dirt) and that would not cool off being wet. Now if the dirt was totally dry top and bottom...There are lots of variables...aren't there. Heat transfer through a dirt mass needs to be looked at. I am just a dilettante who has gotten caught in an eddy of hot swirling suppositions.

In rammed dirt house the concept is to keep the sun off the dirt so they don't get heat gain here in Austin. In the upper desert no need or so the builders tell me. All I know its hot out there and I would keep the geraniums out of the sun. My hats off to all of you with gardens still going. Hats off to the endless supply of water and may it not run out. As for me, my basil is dead and I am trucking water in to flush my toilet and buying my basil. It is hot! And I am often grouchier this summer too.

My geraniums were NOT in the sun as I stated earlier, but they are dying nontheless. They have never been without an ample supply of moisture, so this is why the hot soil killing their roots makes sense to me. Perhaps someone very knowledgeable will weigh in soon, and we will have our answer (if there is one).

Carrie, my geraniums (I think my start was from you indirectly through Kristi) are barely hanging on but new growth is white. I've had these for 3 years.

Good discussion.

My point was that when the sun shines on the ground, the soil surface will be hotter than the air. Especially if the ground is dark colored. That's why you "fry an egg on hot asphalt", when that same egg won't fry in a shaded frypan held up in the same hot air.

In climates with low humidity, and low dew point, surface soil that is wet will cool especially rapidly at night. The surface soil temperature, if the soil is wet, has to approach the dew point, which in a desert climate is a lot lower than the air temperature, especially if it is breezy!

That means, for the same daytime surface soil temperatures, moist soil will cool off more at night. So the average surface soil temperature will be lower in dry climates.

Bear with me here!

The deep soil temps (where the deep roots are), will be exactly the average surface soil temperature. So, that all means that in desert climates, the deep soil temperature may be lower than in more humid climates for the same air temperatures.

Makes me wonder if, when it's hot, and the dew point is high, if I'd be better off covering my (well mulched!) soil with something white! Might be a LOT more sensible than putting shadecloth over the leaves.

In any case, my garden soil must be quite hot this year, even down deep.

Pam, a large container of these that I think are dead are probably the ones you are referring to. I have a few hanging on, but barely, and doubt they will much longer.
I really thought it was just the intense heat they have to endure each day, but it could also be the soil as the containers could just simply not cool off enough at night.

Carla, I heard that on the news the other night too. They said we are probably in for a La Nina winter which will be followed by a dry summer.

BTW, I think I just heard thunder. I rushed outside and there are clouds out there. *fingers crossed*

Dan - Thanks for the explanation of what you meant. I've lived in the CA desert and remember the immediate difference as soon as the sun went down at any time of year. If the daytime high is 118o and it drops to 90o at night that's a similar difference but you can sit outside comfortably at night where you can't here in San Antonio. A few weeks ago here we were not getting much below 90o at night and that was hard on a lot of things.

In case anyone wonders, 118o is the hottest I experienced while living in the desert and it is hot even without humidity! Palm trees aren't very shady either.

I'm also from the desert of CA. I also read up on how high temps kill plants that are well watered and even in the shade.
The overheated plant cannot take up enough water,fast enough,into the leaves to keep from wilting.( The expiration exceeds the rate of the plant's ability to take in water). A plant that is wilted isn't able to produce the needed nutrients to keep the plant alive. If the high heat is continuous the plant will eventually starve to death if opportunisic disease,pests, or hungy animals don't get it first. Small, hairy leaves, waxy leaves, and the lack of leaves (cactus) are all examples of extreme drought resistance found in dry areas. Most plants found in the garden have showy foliage and/or flowers. Even native ones :). I'm sorry so many of you are losing plants.

Pjtexgirl, I think that must be true that an overheated plant can't take up water fast enough. But what perplexes me is that I routinely see 20-30 consecutive days of over-105F here in the summer, and well watered stuff doesn't die. This year, with more than twice as many such consecutive days, it does. It seems unlikely that an overheated plant that can't take up water fast enough needs 70 days of overheating to decide to die. Well, but maybe, as you said, it just gets so weak that other things finally kill it.

I've been wondering about what Greybird said. What will happen to those that are barely hanging on when the temps get to 15 degrees?

I also think that every plant has its limit of 100 degree temps no matter how much water. Sort of like having a low temperature limit that it can't survive below. I just had two ligustrums die on me the other day that had lived in the same spot of 5 years. I've lost several potted plants that just gave up even with daily watering. And the 10 day still shows no days below 100! Welcome to September.

I always heard that desert gets cooler at night because the sand doesn't hold daytime heat like soil does. Even with cooler nights, however, plants won't live with continuous 100+ days and little water (which is what we've got now). I guess we will all find out what will survive especially if we have a dry, cold winter and another summer like this. One of my neighbors is letting her young oak tree die (she's not watering it at all) which kind of bothers me. It's more than 1/2 brown leaves with a little green left.

Carla in Leander

The constant wilting is a huge problem here right now. Plants like Hamelia that should be drought and heat tolerant are wilting daily even in part shade. When I check them the soil is still damp yet if I water them they will perk up but they might be overwatered at some point. Where is the line?

I've lost a number of drought tolerant plants like Lantana and Copper Canyon Daisy despite regular watering. Even Salvia Greggii looks like it's ready to give up.

Well, it's like this.

I can keep my plants happy for 30 consecutive days over 100F, but this summer, with more than twice that number of 100+ days, they're dying. If each day is like the next, then what's the difference?

Yes, it could be just accumulated stress, and they might look happy, but they really aren't.

But one other thing that is different is the deep soil temperature. Soil is a pretty good insulator. The deep soil temperature will be hotter after 70 100+ days than after 30 100+ days. It'll just keep going up. I was just wondering if that's what is going on.

I did a google search for soil temperatures and one of the first things that came up was a soil temperature map that Marti found in 2008.

What they don't say is how deep are they going down to take the readings.

This is a chart showing some west Texas locaions with soil temps taken as deeply as 8 inches.

http://www.mesonet.ttu.edu/latestobs/soil.html

The interesting thing is it doesn't seem to make much difference if the soil is covered with grass or not. The soil is still hot. I'm assuming this is native grass because the soil is dry.

Another site said that at a 10 ft depth the soil remains a constant 75 degrees, give or take a few 10th of a degree.

Why haven't we been smart enough to have gone underground years ago where it's cool in summer and warm in winter as some of our ancestors did? I know there have been some experiemental houses built along these lines with skylights and maybe built on the side of a hill with one side of the house windowed, but they were more expensive than the average home owner could afford.

Here is a link that might be useful: Soil temperature map zoomed into Texas ....

Dry areas cool off at night because the lack of humidity allows the heat to escape. Humidity acts as a buffer. It keeps the heat lower in the day (despite feeling like it's hotter,go figure!) At night humidity holds the heat in too. I've noticed with our low humidity these days, it is cooler in the morning.

Humidity also acts as a buffer with plants. It helps plants with expiration during a hot spell. (Downside it also encourages molds,mildew a lot more pests and fungus. For example nobody grew even the most hard scrabble oaks in the desert. Even with cooler nights and mornings they couldn't stay hydrated :( . So while humidity makes it harder for people to cool down. It's good for plants.

The only thing I've heard about soil temp is burning feeder roots of trees. The soil gets so hot down 8-12" the surface feeder/gas roots are cooking! EEK! I mulched the heck outta all my trees. Mulch can stop or slow down this process.

Does any one notice that we've had a 100 degree temp differential this year? It was 10f here in Fort Worth over the winter. Now it's 110. How incredibly sucky.

It looks like I will lose 3-4 dwarf indian hawthorne shrubs this summer. 3 of them were planted last year and one is new this year. There are 3 other shrubs in the same area that seem to be OK so far. The leaves are turning brown at the center of the shrub while the leaves at the end of the branches are still green. The wood appears to still be green so maybe they will survive? Since they are new they don't have a well established root system and I wonder if they are unable to take up enough water to survive? The soil is moist but not wet and they get some afternoon shade. My older Indian Hawthorne shrubs seem to be doing OK even though they are in a less hospitable area (pool side in the hot sun vs. north facing bed with afternoon shade).

When you really want your soil to cool off at night, what you want is moist soil and dry air. I can provide the former, but not the latter. A breeze wouldn't hurt. That should knock the top layer soil temperature down to the dew point, which can be pretty frigid if the humidity is low.

Yes, I too have heard that 10-feet down is where the temperature doesn't fluctuate much seasonally. That's a lot farther down than most root systems, except maybe for the largest trees. At that depth, the temperature is exactly the seasonally averaged mean air temperature. Here in central Texas, that would be something like 67F.

Of course, that's how a "root cellar" works up north. It's a cellar that is deep enough that things stay nice and cool year round. Root cellars don't work that well here in Texas, though, because our seasonally averaged mean temperature isn't that low. Of course, you get too far north, and you're into permafrost, which isn't great for storing veggies.

Good links on soil temps. Thanks.

It is questionable as to whether my 12 year old pecan trees will make it. They look pretty bad with some naked branches here and there that look dead. I don't know that I can do anything for them since I'm already giving them some water. Most everything else is either wilted or burned and getting worse. Although I have been looking forward to September, the weatherman said to expect some 100 degree days in September, too. Great.

Carla in Leander

Did your weatherman say to expect 105 + days in September? Will this ever end?

interesting thread- I am not sure one way or another- but like the title says- Man its hot! It is horrible when you are praying for rain.

If daninthedirt is correct about plants needing dry air at night, maybe I should buy them circulating electric fans to go with their shading umbrellas.

How interesting my yard would look.

LOL -- mine is already looking quite interesting with a combination of beach umbrellas, pasteboard boxes and sheets strung on poles. If a big gust of wind comes by it will all be in the neighbor's yards. Oh well .... :-)

I heard that the greater San Antonio area will see very extreme heat this weekend. Not as bad here in the hills, but it'll still be pretty bad. I've got practically all potted plants clustered together in the shade. I need to buy more shade cloth. I've got to water a lot before the heat gets that bad, too. I'd like to buy a really large umbrella thingy...but I don't know...we just spent quite a bit on our A/C.

Assuming we all lose quite a few plants before this all ends, are you planning on replanting or not? I think that may be the bigger question.

On another note, I have about 6 Indigo Spires salvias in two different beds. The ones watered with a soaker hose stayed hydrated longer compared to the ones that were watered by hand. Makes me want to think about a very large soaker hose project.

Carla in Leander

I have a 30-foot pecan down in the creek bed. Well, it's on a bluff about six feet above where the water table usually is. That water table is now down about three feet. That pecan has half of its leaves brown and crisp. The tree is dying. So I stretched my hose out and did it. First time I EVER watered anything down there. The roots on that tree would normally be bathed in water. Just insane.

That's really tough that a pecan tree down by the creek can't even make it. We have been watering our native oaks here. A 300 year old oak fell on a house in San Antonio yesterday, lost to the drought. The owner was scared but got out okay.

The soil is definitely steamy hot when I water the last few days.

Carla - I'm going to soaker hoses on timers. I went to a water wise gardening seminar and he recommended soaker hoses over drip systems for home gardeners because we move things around more than commercial landscapers.

Check out the new flat soaker hoses. I really like mine and they should last a lot longer than the old style. You can get them at Walmart, Harbor Freight, and other places.

When I replant there will a lot more Yucca in the hot zone and perennials will be treated as annuals placed in areas that are easy to rework. One Agave is going to be moved back so the late season hot sun doesn't hit it. I'm planting more Gomphrena next year, that stuff just laughs at the sun. I also picked up some Mexican Bird of Paradise plants last week to fill in the gaps.

My garden in general looks about the same. I lost a couple newly planted brugs to thrips and a couple other plants here and there but the "look" I want is still intact.(If wildflower madness is a "look"). I'm line feeding all of my trees (all are 3 yrs old or under), tented my veggies for a fall crop (assuming this lets up right?) and I water once a week for 40 ,ten minute, increments. A cheap humus and manure mix from lowe's has actually stopped the grass' yellowing in the front yard. I think the sod is taking up a lot more nutrients to compensate for the weather. If I hadn't organically fertilized back in spring the same way, I'm betting the sod would pretty much be all weeds, goat heads those horrific sand burrs. The back sod is 1/2 dormant but not a weed bed yet. I'm going to start the fertilizing regime back there too. Very few of my neighboors have inground irrigation or line hoses. They are using portable type sprinklers here and there but not the right way or long enough to really make a difference. Normally they have dormant, somewhat weedy lawns and some evergreen foundation plants accented by a few annuals. Nothing glamourous but looks fairly standard. Now it looks like thier landscaping was attacked by a nut with a blow torch. It's awful. I'm guessing Lowe's and home depot is going to be flooded with folks looking to replace at least some of what they lost over this summer just to make thier home look normal.

When out holding the hose the other evening, I saw a couple of small stipa plants that have sprouted from seeds. Amazing. I have some seeds I gathered last fall from the muhly grass so I think I'm going to make one of the beds that is the hottest this time of year only muhly grass--meaning I will start some new clumps and hopefully have about 5 big clumps filling that one area (with a soaker hose under them all). I believe I can run soaker hoses to everything but one bed by the driveway. I'm still thinking on that one, assuming the roses there actually survive. All but two of them are really burned. Except for this bed, I think there is a way to reach all the other ones with soaker hoses. Then I just have to worry about the trees.

Carla in Leander