You might have over watered. Instead of watering on a schedule, it's best to stick your finger in the soil and check the dampness. Also, hot is relative...just because it is hot, doesn't mean the soil is dry.
OR... and I am just throwing out ideas, I can't see the plant... was the pot too crowded? I don't know how big your plants are, but I'd just plant one rosemary to an 18" pot. They can get rather LARGE.... so with that goes maybe you were UNDERwatering the plant. My private opinion is that you over watered it. Is it humid there? I don't think that rosemary likes humidity all that much.

lptexas

The symptoms of both overwatering and underwatering are very similar. If your rosemarys are root bound and you continue to water, root rot can set in. It's been so hot and dry down here that I am having to water my potted plants twice a day, and I'm making sure that they aren't root bound. Pull your dead rosemary and inspect the roots to see if it is in fact root bound. Here is some information on drought effects on plants.

However, as you know, Texas is in the worst drought in decades. I take care of one of the Master Gardener demonstration garden here in Rockport. I had a large 4' upright rosemary that had been planted in a raised, well-drained bed for about 4 years. I didn't realize that the irrigation was broken, and I recently went down there, and the rosemary was totally dead. Dead, Dead, Dead! I don't know how long it took for it to die since I hadn't been there in about a month. The soil around it was so dry. So there is no doubt in my mine that my rosemary died due to drought.

And you might want to check with the Bexar County Master Gardeners to see what they think. Good luck!

heathen1

San Antonio is not humid. I live right on one of the Texas bays, and it is very humid here. But amazingly all of my rosemarys do great, but then I have them all in raised beds. It is native to the Mediterranean region, Portugal, and northeastern Spain so I think that it likes being on water.

Thank you for your responses. I kept researching online and kept running into the idea of "fungus" being the culprit so I panicked and pulled both of them out of the pot. The roots looked normal, I think, but they were mostly brown (not a lot of white) although they did not seem dry. I don't know what healthy roots look like.

Anyway there were more roots than soil it seemed so I did my best to separate the two plants without destroying too much of the roots of either. Did loose about half the roots of the dried "dead-looking" one and about a fourth of the other plant's roots. I repotted the healthy rosemary into the same pot with a lot more soil and out of sheer attachment kept the "dead" one. I planted it in an other pot with more soil just in case it still had some life left in it. The roots did not have any rot and the soil was moist but not soggy.

Maybe it was root bound? It is not really humid in San Antonio and I figure that if I had over or under watered BOTH plants would have died. Our temperatures for the last month or so have been 100 to 104 with a heat index of up to 10 degrees more and we have not had any significant rain in almost three months.

I'm crossing my fingers and hoping that the healthy plant will not hate me for all the movement. So far it looks ok... we'll see if it survives that change.

By the way... the one that died is not brown or black... it is a dried green pale sage color. It's leaves are all mostly on the branches although all the shaking to seperate the two plants made it loose a lot of needles.

It's been hot and dry here in Missouri too. I wondered if the roots of the upright plant might have been primarily on the side exposed to the heat of the sun every day. Some of my potted plants in the past have died before I realized just how hot the soil in the pot can get when exposed to the sun.

I'm thinking you overwatered. Two in one pot was a bad idea to beging with. Rosemary likes to put down more roots anytime a branch touches the soil. Give the prostrate rosemary its own pot and let it go nuts. You need to water a rosemary when the leaves start to contract, otherwise rainfall should be fine.

Rainfall in Texas??? What's that all about? HA!