In the spring of 2007, I planted fourteen arborvitae trees around an air-conditioning tower as a screen. The soil is mostly clay. I dug planting holes twice as deep as the root ball, and added quite a bit of compost both under, around, and on top. That first summer I watered them religiously and they did fine. The next summer, 2008 was fairly wet and so I was not regular in watering them. One after another they began dying. I was surprised when I pulled up one of those that had died to find that the roots had not grown at all, out from the original root ball. Three have survived until now. Is there some secret to growing these trees? Is our area not suited to them? Does one have to pamper them for several years? Why didn't the roots show any growth? I do not want to try replanting around the AC unit unless I could hope for better success.
I think there may be several factors at work here. First of all arborvitae plants are fairly drought tolerant once they are established--too much water would be worse than dry conditions. While they like water, the soil needs to drain well. Clay soil is not known for its friability. It holds water far better than most soils, so this could be a factor. Was the soil gray in color or have a sour smell when you dug them up? The fact that your roots didn't spread at all tells me a few things. Root rot could be a factor from too much water, but if the root ball was not disturbed at planting and the plants were root-bound, then they stayed that way in the ground--and were not able to take up nutrients and water any further away than there were roots. Planting too deep can also hurt them--we normally want the crown at the soil level, or the plants can smother. Amending the soil is fine, as long as the compost or amendments are mixed in with existing soil and the amending is two to three times as WIDE not Deep as the planting hole. Roots don't typically grow down very far, but they should spread far and wide, that is why we want to amend in a distance away from the plants. If you can't amend a wide area, then don't amend at all. Otherwise, you are basically containerizing the plants in the ground.
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