Question
When I purchased my home, about seven years ago, I created a series of three circular raised beds of varying height. The soil of each garden rests against the blocks of the higher bed next to it. The exception is the topmost, highest and deepest of the gardens. The soil rests against the trunk of an arborvitae which I had cut down earlier in the spring. A stump remains.

When I built the gardens I used garden soil which I purchased from the nursery in pre-packaged bags manufactured by a reputable company.

I have never had success growing woodland plants (hostas, huchera, etc.) in the topmost garden after the first year. I attributed the failure to the trunk of the tree and the detritus which fell constantly from the tree.

Now, since I have removed the tree, I was going to replant the top garden with plants transplanted from the middle garden, and plant a lovely Japanese maple in the middle garden. This morning I was removing the arborvitae roots which had invaded the top garden from the trunk of the arborvitae when i noticed, under the top layer of soil, and going quite deep, a white powdery substance which has a chemical smell. Most of this surrounded the roots of the arborvitae, but some was free-standing.

Is this a mildew or a fungus? Do I need to replace all of the soil in the topmost garden? It is the largest and deepest. Is it caused by poor drainage? Should I add sand to the top garden? I water the gardens with embedded soaker hoses made from recycled tires.

I am considering digging down to the base of the stump and placing stout plastic between the stump and the soil to prevent further roots from growing into the bed. Will plastic affect the soil? The drainage? Will the stump help or harm the drainage?

Thank you in advance for your advice.

Answer
Hi Jennifer,  If the stump is still green, it will rob the surrounding soil of nitrogen as it decomposes.  You'll need to fertilize anything planted near the trunk with a water soluble fertilizer sprayed on the foliage several times a year for the first year.  That's the only concern I have about the stump.  The white stuff is probably fungus (some grows on decomposing organic matter and some are beneficial mycorrhiza), but probably nothing to be concerned about.  I'm surprised that the deep soil placed over the roots, and in contact with the trunk, didn't kill the arb fairly quickly.
Loose, bagged type, garden soil is not the best choice for elevated beds.  The higher the elevation, the quicker it drains, the harder it is to water evenly and the more likely you are to sustain winter kill.  If it were mine, I'd find a source for some good garden clay and blend it with your garden soil.  It holds water and nutrients better, your plants will do better and be less trouble to care for.  I normally use 2/3 clay to 1/3 fine bark, garden soil, cotton burr compost or any coarse organic soil amendment.  If it's only annuals and perennials, or shrubs that need excellent drainage, then about 1/2 and 1/2.
I'd remove as much of the trunk as you can, but there is no need for plastic.  Jim