Well, this is Ohio and we just don't have that kind of heat. For plants that want cool roots, like roses, I make sure the soil is covered and that lots of of foliage extends down over the edge of the pot. Siting the pots so they're not on steaming hot brick (or whatever) also helps.
Even in Ohio, soil temperatures in containers can easily exceed 100 degrees in the summer. Plant roots aren't going to necessarily be affected by the temperature, per se, it's the effect the soil temperature has on moisture and nutrient availability. Obviously, higher temps will increase the rate of moisture loss. Most controlled release or "slow release" fertilizers guarantee their rate of release (3, 6 or 12 months) at an average soil temperature of 70 degrees. If you're using a "controlled release" fertilizer, the rate of nutrient release can increase by over 40X when soil temperatures reach 100 degrees. This results in a "flush" of nutrient which soon runs out of the bottom of your container or results in a dangerous buildup of nutrient salts in the media which can also be harmful to your plants.
Not necessarily, but be sure to regularly water your containers to leach accumulated salts. Also, if you buy a growing media that says it contains slow release fertilizer, be aware that it's been sitting on a pallet at some depot for months wrapped in black plastic, and then at your garden center's parking lot. Any fertilizer in that bag (and it isn't much) has already released. At first watering, it'll leach out. I do have an alternative nutrient product which isn't heat sensitive and holds water, as well.
Most roots grow best at 65-75*. When root temps reach 80*, function and metabolism is affected, so is growth. By 90*, most root's function/metabolism begins to be seriously affected. At 95* temperatures root function in most plants can be critically affected with root/plant death occurring around 115-120*. Root temperatures at the perimeter of the sunward side in black or other dark containers can easily reach 140-150* from passive solar gain under some circumstances if the container is unshaded, or particularly if it blows over and is lying on its side. I've often had plants in nursery containers that when lifted from the container reveal dead roots in a high % of the soil concentrated on the side with sun exposure.
Painting pots white, wrapping with something white or shiny, shading pots, or using a pot-in-pot strategy are all helpful.
I lined the inside of a very large clay pot with sheets of styrofoam then added soil... my plumeria did fine until I put it into the ground two years later. AZ/Phoenix weather is just about as hot as it gets in the USA.
I understand the roots won't grow and thrive if they're meeting up against heat, heat, heat.
I live in the Phoenix area too and I double pot and put bubble wrap in between the pots. It helps, but the heat here is unrelenting in the summer for months day and night. I feel sorry for the plants, even potted cactus.
I'm a new gardener and last year was my first real try at growing stuff in pots. I read that I should cover my self watering pots with black trash bags to keep the moisture in. In this west central part of Texas, the black cover almost cooked the plants in a couple of days before it really got hot outside. I changed the plastic to white and watered from the top to cool it off and the plants pulled out of it.
Point being, I think anything that causes the pot to receive less light in the hot part of the year will help the plant.
I ran a shop lite in the winter for some of my babies, so it makes sense to me that you should be able to do the opposite in the hottest part of summer. As long as you don't shock them too much in the transition.
I'm not sure about other areas, but in my zone 6 garden the soil temperarure seldom if ever goes below 30F nor above 45F, if there is a 2" layer of pine bark mulch on top. What this means to me is that I can bury large potted plants, (10-18" in diameter, and filledwith a good soil mix), below the soil line and then the roots will have the advantage of lower Summer temperatures. If the same pot was left above the ground level in summer it might reach the hot air temperature eventually.
I am in the DFW area so we get dry, extremely hot days here. I have just stuck with growing things that can tolerate the heat more. I also move my pots around, with difficulty that is. If we have a month of 100 degree weather I try to drag them to the shade. I might try some wrapping this summer to see if that helps.
I have a huge pot collection because here in my part of Austin, I'm in pure caliche so I either palnt in pots or raised gardens. I'm moved away from clay pots except for my cactus and succulents as the get too hot and dry out too fast. In Spring and Fall when it's a bit cooler and they hold moisture I use them without saucers but in the Summer heat I have deeper thatn normal proportioned saucers under clay pots...some times clay colored plastic one. I have almost everything else in expanded polystyrene pots. They have a skin of color or are stone toned, etc. They keep plants considerably cooler, hold moisture and for us older gardeners are lighter to move. There's so much variaty out there.
Here's some others in the polystyrene from Home Depot. The color is "Limeade" and comes in many shapes, sizes and colors. These are 28". I must add that I have equal success with glazed pottery pots, I have Mexican Talavera and Vietnamese cobolt blues.
Sylguy, Do you put anything between the soil and the glazed pots to try and keep the soil cooler? I have some glazed pots on my patio that gets so hot during our summers. Still experimenting with plants to find what works well. wondering if there is something else I can do to try and keep the soil from getting so hot...
In Phoenix even the succulents and cacti roots boil in pots in sun.
teacup754, it helps some to double pot and put bubble wrap in between the pots or, if using just the single pot put bubble wrap around the inside then the soil. At the very least, it is definitely important to use light colored pots to help reflect sun and not dark colors.
Teacup: no I don't line the glazed pots, and I've noticed that the fuller the planting the cooler the soil.
Newtons3rd: I mentioned that I do use clay for cactus and succulents...I didn't lose any to last years' brutal summer, but lost a couple to last years' excessive freezes.
I can't remember, if I ever knew, what was Newton's 3rd?
Now that I've done all my potting...it's too late, but I'm going to try both double potting w bubble wrap, and lining pots with it...where were you last week? Only yesterday I put in an 8 hour day planting, potting, and re-potting...I did 27 plants up to 24" rs...was I pooped, it was a motrin evening!
All y'all, I'm still with the upscale polystyrenes. They are like a cooler for your plants, and they are more atrractive every year with new styles and textures for any decor.
I like the bubble wrap idea. I'm going to see how this summer goes and if the plants i have now (a plumbago and a duranta) don't make it I'll put the bubble wrap in. For sure I'll do it next time I repot.
you can also plant part of the actual pot in the ground...two reasons...if you live in high wind areas it helps hold the pots in place...secondly...if you plant deep enough the ground stays cooler and keeps the dirt in the pot cool...now..i'm not saying bury the entire pot...but a good part of it. it really will help!
tobee43, my mother in law, a great gardener, used to set empty 12" plastic pots all the way into the ground in between plantings in her garden and pop in 10" pots of seasonal color, annuals, tender plants that she took in, and things like coleus. If one died she replaced without digging, and come winter she brought the tenders into or by the house. Down in the ground kept them much better, she explained, than plants in that sized pots above ground.