Hello,

Arborvitae (Thuja species) prefer fertile, moist, well-drained soils in full sun, although they can tolerate partial shade. Read more here: https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=254006&isprofile=0&

You appear to have two concerns about your trees.

1. Foliage of the plant is turning brown. This is probably due to seasonal needle drop which occurs in most evergreens. The needles of evergreens do not last forever but eventually age and drop from the inside of the tree out. See the attached article. https://hortnews.extension.iastate.edu/faq/inner-growth-my-arborvitae-turning-brown-problem

If it is not the seasonal falling of leaves, a plant failing so soon after planting is likely due to poorly draining soil. Arborvitae grown commercially in fields of well-drained soils do not develop the same types of roots as when grown in clay. Often plants that are not used to heavy wet soils are transplanted into clay that drains poorly and the trees die. Water drains slowly in clay soils. When they are not planted in ideal conditions, they are susceptible to disease and insects. Environmental conditions may also affect the trees which may cause many trees to suffer stress and susceptibility to disease and insects. As cool weather approaches adequate water is essential to help the trees store up energy for the winter. To discover what is causing the browning of the leaves, you can have a sample analyzed by MSU Extension’s Plant and Pest Diagnostic Lab at: https://www.canr.msu.edu/pestid/

2. Fertilizing. It is not recommended that newly planted trees be fertilized. When it is just planted, a tree spends all its energy on the establishment of its roots. Fertilizer has been found to inhibit the growth of new roots. Read more here: https://extension.umd.edu/resource/fertilizing-trees-and-shrubs

And here: https://extension.unh.edu/sites/default/files/migrated_unmanaged_files/Resource000590_Rep612.pdf

In addition, most established trees in the landscape do not require fertilizing unless it is obvious that something is wrong with it. A soil analysis will help establish whether any nutrients are missing. To learn how to obtain a testing kit, go to: https://homesoiltest.msu.edu/get-started

You can also contact a certified arborist to assess your trees. Go to: www.treesaregood.org , the website of the International Society of Arboriculture to find a certified arborist in your area.

Good luck with your trees.

Thank you for contacting Ask Extension.