Hi, 

Typically Green Giant Arborvitae are rather tough plants but we think they are not the right plant for this location and that is why they are not thriving. It is unlikely that a chemical used in home building was dumped in this exact location that is killing the trees. 

Our first thought is that it is too shady in this location. Green Giant Arb. is from the west coast and typically prefers full sun and fertile but well draining soils. They are adaptable trees but probably aren't very happy in the amount of shade they are getting from the spruce trees immediately behind them and then any shade from the house. Do you know if they get at least 6 hours or more of sun during the day? 

We can also notice that they seem like they are planted way too closely to the spruce trunks behind them. We are not even sure how the landscapers got them in the ground that close since there must have been numerous large roots given the size of the trees. If they cut them, it is likely that the spruce trees will die in a matter of time. Are they are your property or your neighbors? If they are on your property, you may want to have an arborist assess the health of the spruce trees so they aren't a safety risk of falling. 

We see english ivy on the ground and possibly climbing up the trucks as well. This is an invasive plant that at least should be prevented from coming onto your property if that is the property line, or you will be battling it for ever. You could ask your landscaping company about a rhizome barrier to contain it but you would still have to monitor the vines from jumping over the barrier and clip back periodically. The roots of ivy don't go do very deeply but the vine can easily reach under or over a fence.

What were your watering practices after the trees were installed? With the mature trees so close having larger root systems, they would have sucked up any rain water leaving your newly planted trees thirsty. You can refer to our watering trees and shrubs page for proper watering practices. 

Do you know if they landscapers removed the burlap before planting the root ball of the tree or if they were container grown, did they loosen the root system? When we zoom in on the photos it looks like the trunks are planted too deeply and we cannot see a root flare at the base where the trunk connects to the soil. The link explains the importance of this. 

Is there staking with a rope or wire wrapping around the trunks? This can girdle the bark and choke the tree and you would start to see die back from the top down. 

We would suggest some type of hollies for evergreen trees as they can handle more shade like conditions. We would also caution you from planting a monoculture of the same trees in a hedgerow. When you can diversify your plants, it will protect them from all failing due to a pest, poor site conditions, disease, or a weather event that could effect the same species all at once leaving a gap in your screen. You can explore our plant lists on our plants for mixed privacy screening page. 

Let us know if we can help further. 

Emily

Yes, feel free to reach back out with any questions.