It will depend on the make up of the rootball and your soil. I’d start with a slow trickle next to the trunk for an hour and then check about an hour later by digging a 4-6 inch hole into the rootball and feel for moisture. How big is the tree?

tj

5 feet

Then do as I suggested.

tj

How often should I deep water? Every three days? What is a good frequency?

"What is a good frequency?"

No one can tell you that. It depends on your soil and your climate and weather conditions. You will have to physically test the soil moisture at root depth.

When the soil in the rootball feels dry. There is no schedule due to the variables I mentioned above.

tj

I have what may be a naive question: How do I check the soil in the root ball? Do I use a trowel and actually dig into the root ball from the side? How and what do I use to check the soil?

A narrow trowel, big spoon, whatever. Dig into the rootball and feel with your finger.

tj

Thanks everyone for the information. One more thing. I read that you are supposed to give a newly planted emerald green arborvitae 10 gallons of water every 3 to 5 days. Do you deep water the 10 gallons of water Or do you use a watering can to water at the trunk of the tree. If I deep water it, how long does it take to deep water 10 gallons? If I use a watering can,  I have a gallon watering can. Do I water with the watering can 10 times if I have a 1 gallon can?

if the water doesnt soak in.. but only runs off.. then you arent really watering the plant are you .. your watering wherever the water runs to ... ergo.. you trickle the water so it soaks in immediately ... barely puddling before soaking into the ground ...

this it basic garden science.. not limited to these conifers.. which are trees.. all plants need water where the roots are ... period

ken

Whenever I plant something like that, I put the hose right next to the trunk and let the water run for maybe 5 minutes. Not only does it soak the root ball. it also help settle the soil and get rid of air pockets. After that, I rarely ever need to water again. I usually plant evergreens in the fall. I have maybe 25 arbs right now and it worked for all of them. Arborvitaes don't seem to need a lot of water. They're pretty drought tolerant. My soil holds water really well and I get consistent precipitation year round in zone 7a. If you live in a hot arid location, what I do probably wouldn't work for you.