Hello All,
I,m new to the community and haven’t even received my Rachio controller yet. Anyway, looking at setting up my zones. I have one that is a drip line for a long line of arborvitaes. What would be a good crop coefficient to use for this type of evergreen? Is there a place to find the coefficients of different plantings?? Also, am I over thinking this??? 
Thank for your help!
That’s a difficult question to answer. We don’t know your geographic area or climate zone. Are these trees newly planted or well established? What variety? Generally speaking, many trees benefit from deep infrequent watering. Arborvitae generally do best if kept moist, but not soggy. Some varieties require more water than others. If you begin to notice yellowing, it could be from too much water as well as too little.
When one of our cherry trees began to exhibit signs of iron chlorosis, the first thing we did was change that schedule to a fixed one that waters every seven days for more or less an hour. This year the trees greened up nicely and the growth was considerable. We just had our first substantial cherry harvest. 
Your best bet would be to consult your neighborhood nursery, aborist or university extension office. They should be able to give you specific information for your climate zone and the variety you have.
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P.S. Crop coefficient values have been determined from field research and apply to various agricultural crops and turfgrasses, but formulas for trees are not as reliable because of the many variables in the landscape setting. Microclimates, full sun, shade, plant spacing, wind and variety of the arborvitae would all be factors.
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Arborvitae is a shrub that is very opportunistic when it comes to water. I would use the shrub coefficient and trim it back if you can. Spider mites, bagworms, oh I don’t like this shrub anymore. Leyland cypress is a good alternative
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