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First of all, it is often a mistake to place topsoil over the existing soil, unless both soils are exactly the same in structure and texture. Placing a soil with different texture or structure over another type of soil reduces the downward movement of water, in most cases. The result is a high water table which can seriously harm a plant. For example, if a sandy loam is put over a clay soil, the water will move down quickly through the sandy soil, but once it reaches the more impermeable clay soil, the gravitational movement of water is restricted. Because the downward movement of water is so much slower through the clay, a soggy layer or high water table can build up above the clay layer. Any roots in this area can die from a build up of toxic gases which are released from those roots as they respire and use up the available oxygen in the soil. Fresh oxygen has no way of entering a saturated soil because all the pore spaces in that soil are filled with water. When topsoil or organic matter are added to a soil, they should be thoroughly mixed into the existing soil and the sub-layer should be loosened or broken up with a spade before planting any type of plant.
Arborvitae will often fail after they are planted because roots fail to grow from the rootball. This can be a result of desiccation (drying out) of the rootball in hot weather while the plants are being moved to the site or while they are waiting to be transplanted at the site. The tiny feeder roots can perish very quickly in hot weather if the rootball is not kept moist and cool. This is more common with balled and burlapped trees, although plants in black plastic containers can also dry out quickly near the edges of the pots if not cared for properly while at the site. If the rootball has been allowed to dry out, the trees will often decline slowly and then die.
If the rootballs have been plunged directly into the planting holes without the roots first being spread apart properly, growth and development of new roots is restricted. The tree is then unable to absorb sufficient water and desiccation and death of leaf scales is imminent. Girdling of the roots can also result from improper planting. If the roots were growing in circles at the bottom of the rootball and were not spread out properly when they were planted, they will eventually strangle each other as they grow. Dead roots result in dead plants. Death from girdling roots usually happens over a period of a few years.
Severe browing of leaf scales can result from over fertilizing or planting the trees to deep. A high concentration of salts in the soil from chemical fertilizer will kill young feeder roots by drawing water away from the roots. A tree that is planted too deeply will suffer from a shortage of oxygen.
Because arborvitae are shallow rooted plants, it is wise to mulch the soils surface during hot, dry weather to conserve moisture and maintain an even soil temperature. It is however, unwise to use green or fresh materials for mulching shallow rooted plants. Micro-organisms in the soil will use up the available nitrogen in the soil during the process of decomposing the green matter. Once the material is completely decomposed, the nitrogen is then released and readily available to the plant. Only well-decomposed materials should be used for mulching purposes. Futhermore, wood chips can settle and form an air-tight layer which prevents oxygen from entering the soil. As previously mentioned, a lack of oxygen in the soil can cause arborvitae leaves to brown very quickly. Watering practices have to be adjusted accordingly when plants are heavily mulched. It is very easy to over-water a plant when it is mulched, especially if the soil has poor drainage. Arborvitaes do not tolerate wet feet and root death can result from a lack of oxygen. On the other hand, a shortage of water during hot, dry, windy periods will result in desiccation of the foliage. During hot, dry spells the arborvitae is highly susceptible to heavy mite infestations as well. The mites can cause severe yellowing and browning of the leaf scales as well as death to the trees if their numbers are high. These insects are very tiny and may go unoticed without careful observation. A hand- held magnifying glass is useful in detecting their presence.
Overall, it is not uncommon for trees of this size to lose some leaves during the establishment period. Their new location may not be sheltered as well from the drying winds as they were at the nursery, for example. Until the tree develops a good, healthy root system, it will undergo a period of stress. Lower branches and inside branches located near the trunk will often shed leaves due to lack of light. This is normal, especially with older specimens.
Winter months and early spring are usually very hard on arborvitae because the leaves continue to lose moisture. Warm spells are especially hard on the trees because the rate of evaporation increases, but the roots are unable to replace that moisture because of the still frozen ground. To protect young or newly planted trees from desiccating winds in the winter and early spring season, place burlap around them in October (but not touching them), especially on the southwest side where the sun can thaw the leaves and stems. During warm spells in early spring, the trees should be sprayed with water from a garden hose to counteract the high evaporation rate.
