Newly planted Thuja green giant soaker hose
Hello- We recently planted (7/27/19) a row of 20 (6 ft) Thuja Green Giants. We amended the existing soil with a topsoil/leaf gro mix. We have set up a soaker hose along the trunks/base above their root zone. I have been trying to find a definitive answer on how long/how often to run the soaker hose on these newly planted trees. Any advice?
There is no formula for watering with soaker hoses. There is too much variation among hoses and water pressure and soils. You may find that the hose delivers more water at the near end than the far end.
Once established, plants like about 1" of rainfall a week. Get a rain gauge (or a tin can) to measure you landscape's rainfall. Thunderstorms are notorious for spoty rainfall--one side of the yard can be different than the other!
While your trees are getting established (the next 2 years at least), you want to be sure that the soil stays about as moist as a wrung-out sponge. This includes the entire root ball of the tree. Water deeply when you water. Follow the directions on this page, especially about checking moisture down into the soil: https://extension.umd.edu/hgic/topics/watering-guidelines