I have almost no experience with gardenias or gardening in Phoenix so for what it's worth here are things I'd check into:

Poor drainage. Sometimes the drain hole gets clogged. I'd water heavy and make sure water flows out the drain. The soil looks like it's been watered alot, but hard to tell. Is that mineral build up on top of the soil?

Salt build up. This seems extremely unlikely after only a month or two.

High pH. Gardenia like acid soil. Easy to test. Looks like you used soil with alot of organic so I'd guess pH is OK (acid).

Yellow leaves with green veins always makes me think of a lack of iron. You can have your soil tested for iron and pH.

It's interesting that the yellow leaves are on the bottom part of the plant. Seems like one part of the root is being effected. Hopefully when you planted this the top of root ball didn't get covered?

I took this picture after I watered it. I actually did water it heavy today to make sure the water was draining out. The yellowish stuff on top of the soil is cornmeal which I read is supposed to control fungal issues. I used brand new potting soil so I think the soil is okay.

It might be the iron, but I added some iron a couple days ago and it doesn't seem to be improving. Maybe I didn't add enough?

I'm hoping the root ball didn't get covered either and I might do some digging to find out. This is a newbie question but how does this affect the plant?

I moved the pot to an east facing wall today so its getting more light. Was that a good idea?

Waterbug is correct about iron defiency making leaves turn yellow, but with green veining. It usually occurs in the new leaves. Gardenias are difficult in our climate, and seem to do best on the north side of the house. I had problems with one, and Baker Nursery said to use a high iron fertilizer. Be careful not to burn the Gardenia. Chelated iron is readily available for the plant, other forms of iron are slower for the plant to absorb. I have attached a bulletin from the Extension Office about plant nutrients. You might find some help on it.

Here is a link that might be useful: Plant Nutrient Deficiencies

Thanks for the link! From what it says it looks like it could be nitrogen or molybdenum, since the yellowing is starting from the bottom of the plant and the new leaves seem to be fine.

:UPDATE:

I just wanted to update and give a warning to other future gardenia owners. I've figured out that my gardenia has root rot. It seems like it was only one plant that was affected. The yellow leaves turned brown then crispy and now the entire plant is wilting. Yeah, so when you see yellow leaves don't always assume its just mineral deficiency and not the roots inability to absorb minerals. :[

That makes sense given the soil was so new. Did you figure this out by pulling it out of the pot? Was the pot not draining? Just curious.

I've gardened for a long time other places but I feel like I'm having to learn all over again here in Phoenix. Even simple things like watering seem to be very different.

You asked about the affect on the plant when potted too deep. The roots near the surface exchange gases with the air. So adding soil over the top of the root ball can suffocate the plant. Also the stem may rot if too deep. Depends on plant though. Tomato plants can be buried very deep.

It is too heavy that I can't pull it out of the pot. Also, I don't want to take the chance and damage the roots of the good gardenia thats still alive. I checked the holes and one of them seemed to be plugged. I seem to have a reputation for overwatering anyway.

I think what makes it hard to grow things in Arizona is that when something is supposed to be in "full sun", for us its just really "filtered sun" and things like that.

And thanks for the info. It doesn't look like it was buried too deep. I was digging around it to remove the (nasty-looking) cornmeal and found worms in the soil. It was kind of bizarre, but a good thing I hope.

So I'm still curious...how did you figure out it was rot rot?

Waterbug, sorry I wasn't clear enough. I checked the two drainage holes at the bottom and one of them was plugged. The soil at the bottom was wet too. So the yellow leaves can be explained by overwatering, and the wilted leaves can be explained by root rot. When I took it out today a lot of the roots were brown mush.

I bought a smaller 10" clay pot and repotted my gardenia today. Thanks for the suggestion, Mike. I never thought of that. I did soak the new soil in root growth hormone and acidifier, hopefully that won't add to the problem. I also picked up a water/pH/light meter and it was $8 but I think it will be worth it.

Thanks for the info. Reminds me of a friend with a Lion's Tail he'd had for years that suddenly died. Turned out it had sent a tap root thru the drain, plugged it and the roots rotted.

I've read alot about how clay pots are porous so water and air can easily penetrate. Water I understand. Capillary action (wicking) pulls the water through and is a very strong force. But capillary action won't occur with air. In physics, the path of least resistance is always taken by objects moving through a system. Since soil is way more porous than clay the air will travel through the soil. I don't know of any physical law where air would flow through the clay.

I use clay, glazed, plastic and foam pots. I like the patina clay pots can get but now that I'm in Phoenix I'm moving away from them just to increase the time between watering.

Perhaps I'm wrong but,I thought the plant was bought in June.
There really is not time for a tap root to aim + plug 1 of 2 holes in a "sealed pot (glazed")' pot it was re-planted into.
As for physics: yes soil is more porous than a clay pot.
However water will penetrate into the clay pot and then evaporate OUT into the Arizona air which not only reduces the chances for root rot and overwatering, the process of evaporation will have a cooling effect on the root system of the plant.....like a mini-swamp-cooler. ....and roots like this!

This process is breathing! ....Not quite like osmosis,where as our blood does not come out of our lungs,hopefully.
...but we do use swamp coolers to breathe with our lungs and our skin pores unclog themselves better...... AIR IS TRANSFERING.

Glazed or plastic pots will not do any of this. And they will significly increase the temperature in side the pot.
They are great for astetics, lazy, or water-saving efforts.....but never for healthy plants or root systems.

mike

Tap root story was unrelated.