[Stonehenge]

I'm working on a large scale design and am wondering about what depth of good soil is needed to trees (deciduous and evergreen) to live and thrive. The situation is this - we may be filling in a very large, old water feature that was constructed mainly with concrete. We'll break out the concrete, but there will be a lot of spoils for this design, and much of it will likely be soils contaminated with broken concrete, crushed limestone and other misc debris. The water feature in question is 4-5' deep.

Is 18" of soil enough, or do we need more?

[dan deutekom]

I think you will be fine with 18" of good soil. Naturally the top soil in most places is only 6" to 12" deep and then you are into the subsoil. Trees and shrubs seem to grow in the most unlikely places with rock and crappy soil. The smaller plants never end up using more than 12" of depth.

[agla]

Yes, most of a trees root system is in the top 18" of soil. That is what we were taught in school, anyway.

[Stonehenge]

Excellent. That makes my job much easier (and affordable). Thanks for the info.

[Prolawns]

Consider mixing some of the topsoil with whatever the subsoil will be, at least for a few inches of depth where they meet, this will help with water movement in the soil by not creating a barrier between the different layers of soil.

[hnbullock]

Though I am a relative newcomer to the field of landscaping and landscape design, I would strongly advise, based on an excellent class from Gary Johnson, an arborist and Professor at the U of MN in Forestry, that you test your soil (including percolation) after filling in the water feature you mentioned. Armed with that information, you can make a tree selection based on facts, which will save money in the long run.

The debris may impact soil pH, and some trees have a higher tolerance for variability in soil pH than others. If drainage is a factor (i.e. slow or quick to drain) then that too should be part of your selection criteria. I suspect you know this already, but thought I'd put my thoughts out there as I am working towards increased involvement in the forums.

[Stonehenge]

Those are excellent ideas. Unfortunately, we have to design the new landscape (and sell them on it) before we tear up the old, which is about 60-70% concrete - so there isn't a practical way to learn what's under there until we start tearing things up.