It's two different things. In the winter, they turn bronze, which is a physiological response to cold. If yours are turning brown it means they didn't get enough water. Depending upon how severe it is, they may or may not survive IF you can deeply water them right away. Usually once they get to that stage, the damage is already done. The brown part is dead and won't regrow. If the entire plant is brown, it's basically a goner, if its only part brown and part green you might be able to save them by watering deeply and they could grow out of it in a few years.

lesson for today ...

some will argue about this ... but VERY superficially ....

i think of shrubs.. as flowering shrubs.. and the main reason they are grouped together.. is how they are rejuvenated thru pruning ... among other things of course ...

arbs.. are conifers.. which are also trees ...

there is a conifer forum.. where you will be freely enabled ... if you wish..

but denn gave you the answer you need .. but i will rephrase somewhat ..

interior yellowing/browning is normal.. no conifer holds its needles forever .. and the oldest.. inside fall off .. it is increased by transplant and drought ... life is all in the growth tips.. and if they brown completely.. it is basically dead

a lot of this stock.. from bigboxstore are horribly rootbound in the pot ... and that can lead to watering issues.. until fully established ... if they make it that far ...

ken

ps: when i grew up.. dad called them shrubs and evergreens.. who knew words meant things.. you know the ones.. the ones planted in the late 50's and 60's one foot from the foundation of the house ... that i had to shear twice a year thru the 80's .. God i hate shearing ... but i digress

IME, after several decades of horticultural consulting, the primary cause of failure for an arborvitae is improper watering. These also have the common name of "swamp cedar", which should be some indication of how tolerant - even demanding - they are of moist conditions. Rapid browning of exterior foliage is generally a pretty good indication that the plant has received insufficient water. Unfortunately, by the time one notices it, the damage is already done.

Ken's lessons are Ken's lessons.....and they may or may not conform to accepted horticultural or botanical conventions :-)

The accepted botanical definitions of 'shrubs' and 'trees' is based entirely on size and manner of growth, not on ability to flower (or not flower) or whether they are evergreen or not or needled or broadleaved. If you refer to the attached link of definitions, you'll note the lack of the term 'conifer' with respect to ether growth habit or definition. A conifer is just a plant that is cone bearing and usually (but not always) with needled foliage. It can be a groundcover (subshrub), a shrub or a tree, both single and multi-trunked. Arborvitae can be successfully argued to fit either the shrub OR the tree definition :-)

Here is a link that might be useful: USDA definitions

some will argue about this ... but VERY superficially ....

==>>> didnt take long did it.. lol ..

the key word was superficial

ken

LOL!! I dunno what's superficial about using proper terminology......it is what it is and not necessarily up for any argument at all.

And besides, you know I have to call on you on these statements, Ken :-)

It seems to have happened over the course of two days of heat with 10mph or so winds. As it happens, the wind comes from the direction of our neighbor's yard and the brown-ness seems most pronounced on that side. I don't usually go over there every day-I just happened to go there to trim and found them going brown. I think the heat+hot dry winds was a bad combo for them.

So. I have been deep watering them about every other day since I discovered it. They are still green on the inside (closest to the trunk) and more so on one side than the other. I noticed yesterday some branches on the more wind-hot side are now green...so either they were somehow pushed into the middle and are now flopping back out, or somehow they are coming back. I'll report back as to how they do over the coming weeks.

Of course, yesterday was back into the 90's with 10-15 mph winds again :P and STILL no rain in sight.