not a problem. among other things the product is listed for container production of nursery stock (trees and shrubs).

"We had planned to till in compost to amend the soil..."

Now the question is, why are you going to amend your soil? Are you amending the entire expected mature root zone? Do you have good drainage (amending soil often negatively impacts drainage)? Are you aware that you should amend very sparingly, if at all?

Here is a link that might be useful: Planting a Tree or Shrub

I am not so sure about planting into a pre-emergent barrior. It IS used for containerized plants but as an over-the-top application.

What does the label say?

This entire thread is confusing :-) Are you tilling up the lawn you just treated with fertilizer and herbicide? Or are you planting in an adjacent area to be tilled? If you intend to plant in an area that is now lawn, I'd caution you that tilling will not be sufficient to control the regrowth of grasses into the planting area and newly planted shrubs and you will be dealing with weedy grasses popping up forever after! You really need to eradicate the grass first, either by removing, smothering or otherwise killing with a herbicide.

If you are planting in another, non-lawn grass area altogether, then there is minimal worry about the pre-emergent migrating and contaminating new plantings. If the shrubs are of reasonable size - not tiny little starter things - pre-emergents are really of no impact anyway.

The plan was to use one or more applications of round-up as necessary on two portions of existing lawn then add shrubs.

I meant not to apply any fertilizer or herbicide to these portions of the lawn but out of habit forgot.

The link below is to the exact herbicide I applied. If the instructions where clear to me I wouldn't have to ask here :) After applying I know I can't overseed, and I know it is safe on established plants, but it is unclear if it is safe with transplanted bushes. As mentioned, the new bushes would be 24"+ for lilacs and 48"+ for arborvitae.

After the grass died off, I was going to amend the soil with 4 inches of matured compost to a depth of 12" and width of 60", tilling (or hand turning) in the dead grass with it. My county provides free compost from the leaf and yard waste clippings they collect. I planned to do this per research from several academic sites recommending amending, e.g. http://www.gardening.cornell.edu/factsheets/orgmatter/index.html#compost . Seems I'll have to look into this part more.

Here is a link that might be useful: Fertilizer with 0.013% Dimension

I found this on a Dimension techsheet: "For ornamentals...apply only after transplanting when soil around roots has been thoroughly settled by rainfall or irrigation or injury may result." Some of the information out there does seem a little confusing about this point, and I bet the results could vary quite a bit by type of plant and a bunch of other factors.

Here is a link that might be useful: Planting a Tree or Shrub (see section 7)