I was just thinking about this and someone feel free to set me straight, but I don't think a mulch will help cool the container. If anything I think it may trap heat. It should help with water conservation, but heat rises and a mulch on top seems like it would trap the heat the same as it does water vapor.

The main source of heat in the container aside from the air temperature is going to be sunlight on the container or sunlight heating the wood/cement whatever the container is set on. I would think an open top would actually result in less heat trapping.

While it's true that the sun will heat up the container, especially if it is made of metal or concrete, it is also true that mulch will help offset SOME of that heat. Mulch works on the soil in the same way that a shade tree works. It also helps keep down weed infestations and does indeed hold moisture in. Mulch is a very good thing!
There are lots of things you can use in your pots: newspaper covered with straw, chopped leaves, shredded bark, even gravel (for certain types of plants).

The last two years, I mulched my tomatoes in containers with plastic garbage bags. The dark color helped to warm the soil in the spring, the plastic holds in moisture in the summer, and the plant itself provides shade to cool the soil as it gets larger and the sun gets hotter. I just cover the soil with the bag, tuck it in around the edges, and then cut about a four inch hole, into which I plant the tomato. Works well. (These are in self-watering containers, so I don't water from the top.)

Thanks for the replies. My potting soil itself is fairly dark, and (today at least) the sun is hot. I was outside for about 15 minutes and I was wearing a black tee shirt and my back felt like it was on fire!

I don't think I'll put any mulch down yet, but when it gets hotter, I've got a bag of pine mulch in my shed I might use in the tops of the containers. As far as the containers, the plastic ones are white, but the big ones with the tomatoes are a reddish clay (and heavy!), I may use my earthbox trick with them and wrap the sides with aluminum foil to reflect the sunlight.

As I thought about this question, I realized how much science is involved in the answer and it kind of amazed me. It sounded simple enough . . .

The biggest factor affecting relative heat gain in containers is the color of the container. Dark containers absorb more light and turn it to heat, which raises soil temperatures. The difference in soil temperatures between a black container and white container, both in full sun, can EASILY be as much as 50*.

The shape of the container affects relative heat gain. Containers that are wider at the bottom will have more surface area closer to perpendicular to the path of light striking it (angle of incidence), so it will absorb more photons and turn them to heat. Light will strike containers that taper toward the bottom a glancing blow, so less will be turned to heat. Round containers also minimize heat gain.

What the container is made from has (virtually) no impact on how much light is absorbed and turned into heat, or the net amount of heat generated, but it DOES have an impact on how much heat is conducted through the container walls and into the soil. Steel and other metals are far superior to concrete in their ability to transfer heat, so a greater portion of the passive heat generated will pass through the walls of a metal container, compared to a concrete, wood, foam, etc. container. Generally, the greater the volume of air space in a material, the less it will transfer heat. Concrete, urethane foam, wood . . . all have air, while metal doesn't, so they are more effective at slowing heat transfer.

Since a dead air space is a superb insulator, the containers of materials that contain air will absorb heat at their surface and lose it to the air instead of conducting it through the walls. Here is where I must disagree with Donna's assertion that the black plastic she used as a mulch helped to warm the soil. It couldn't have. Even though the black plastic warms quickly in the sun, the dead air space under the plastic (this applies even if the soil is in partial contact with the soil - it cannot be in full contact because of the soil's irregular surface) insulates the soil from conduction, so much more heat is lost to the air than makes it to the soil. In effect, the black plastic shades the soil and SLOWS the warming compared to either bare soil or clear plastic.

If we wanted to utilize a plastic sheet over the soil to maximize heat gain, it should be clear. Light would then pass through the plastic and be absorbed at the surface of the soil where it turns to heat and gets conducted through the soil. The dead airspace between the plastic and the soil insulates the soil and prevents the heat gained from escaping into the air.

When we solarize a garden patch, using clear plastic to raise soil temperatures is soo much more effective than using black plastic - and so it is in containers as well.

Mulch both shades the soil, helping to reduce relative solar gain, and traps heat after it makes it's way into the soil, so it serves as a moderating factor. It slows heat gain during the day, helping to keep temps lower than they would be w/o mulch, and it slows heat loss as temperatures cool to below whatever the soil temperature is. It's difficult to determine exactly how effective mulches are, because they also tend to negate a portion of the cooling effect of water loss from evaporation by decreasing air flow over the soil surface.

After considering all the above, I probably won't get much argument if I propose that I've supported the conclusion that mulches are marginally effective at moderating rising temperatures of container soils resultant of solar gain, but there is a final consideration to take into account before we decide to use them.

Their effect on the evaporative loss from soils can be considerable. We know that soils remaining saturated for extended periods are inhibiting root function/metabolism via anaerobic soil conditions. If you are using a soil that allows you to go more than 2 days between waterings, it's likely that the cooling effect of mulching will be offset by the deleterious effect of extended periods of soil saturation and the accompanying lethal effect on roots.

Reason tells me that it would probably be somewhat productive to mulch the faster soils, and counterproductive to mulch a slow soil.

Condensed version: Grow in light colored containers. Shade the containers whenever possible. Use a well-aerated, free draining soil so your plants grow up big and strong, shading the soil for you so you don't need mulch. ;o)

Al