I seal all my clay pots inside and out with Mod-Podge, but there are sprays now, too (I haven't tried them yet):

I actually started doing it years ago to protect decorative paint on my pots and found it helped with moisture and heat, too.

I don’t know the variety of Zinnias you‘re growing, but any I’ve ever had were very, very happy getting afternoon shade.

Another tip I’ve seen was putting smaller pots inside bigger pots for insulation, or grouping your pots together for a bit of protection.

For mulch, I’d use leaves, grass cuttings, straw, or other organic, breathable material.

Good luck!

It seals water but what about soil temperature? My pots aren't wet from watering the plants, it's the top of the soil on down that gets blistering hot, while damp.

What about bark for mulch? I have some of that. I think as the plants get bushier I won't have the problem. I have a big pot of Lavender and it covers the soil and it's not hot.

We still have patches of last Fall's oak leaves, would that be better than bark? After all these years I can't believe I'm having this problem. I usually put Impatiens in the clay pots but I didn't plant any this year.

Unsealed clay pots absorb moisture. Sealing them on the inside, as Jinx suggested, is a good starting idea. A shredded bark mulch on top, 2” is ideal, will help a lot as well. Of course, avoid any type of gravel mulch, as that would just add more heat. Plastic pots instead of clay would also keep the excessive heat down.

My preference for mulch in pots would be your leaves ... only because the wood mulch in my beds absolutely bake (and reflect heat) in the sun. And leaves have so many nutrients. But you could try the bark if you’d prefer it.

You’re reminded me I need to get some Profusions! :)

Oops, didn’t see your reply before posting, Lynn ... I was hoping you’d chime in! You’re another very experienced with extreme heat/sun and terra cotta.

Gardening in our hot states can be very challenging! :D

It sure is! That’s why I went back to “school” once I moved out here from Michigan and became a master gardener. I was killing everything, and watering weeds ( I found out later- LOL)!

I thought about plastic pots but I just love the clay pots. :( Howzabout if I put a potted clay pot inside of another clay pot? Would that help?

Jinx, I'll definitely get the old leaves. I do have one large plastic pot, it's sky blue. Would stick out like a sore thumb.

You should see the soil in the two large beds in front of the porch. Black as can be and full of nutrients. All because I never rake leaves out of the beds and over the years they made the soil great.

I also have rows of Zinnias coming up in front of rows of Cosmos that I sowed, they're growing like weeds. I'm too old to do that again. My back! lol

Wow, I just took a peek at the Zinnias in the clay pots and there are buds on two of them, both in the same pot! They were begun indoors in the same clay pot, 2nd week of April!

Awesome! I love zinnias. :)

Oakley, does the lavender in the pot overwinter or just an annual? I love lavender! Would love to see pictures when it's looking good.

We grow a long row of Lavender and I put some in a large pot in the flowerbed. When I went to move it the roots had gone through the pot hole into the ground. Which means it over winters in the pot. lol

Here's last year's Lavender.

Oakley, that's gorgeous! Does it turn brown in winter and then come back in spring?

Wow, Oakley! that is a beautiful bed of delights for the nose!! I love lavender, but haven't been able to grow it.