dchall_san_antonio, Thanks very much for the advice. Yes irrigation is going to be a problem, especially since this lawn is not at my house and the person living there is elderly. I can make sure its watered well when I'm there but thats only a couple of times per week. Last time I tried to seed it I used the Scott;s "Sunny Blend" from Lowes but that might not be the best choice. Its also possible that it didnt get enough water to really become established, and last year was well above average here for heat with near-drought conditions. TYG

Magnolias & Dogwoods (Cornus florida) also have shallow feeder roots and the areas under the canopy are usually lacking in sufficient moisture to permit growing many types of perennial plants. I have four Southern Magnolias (M. grandiflora) that are underplanted with several types of shrubs and perennials that must have additional irrigation, even with normal rainfall. The evergreen foliage acts as an "umbrella" and the plants underneath receive little moisture, even after a heavy rainfall event. I use imbedded soaker hoses to provide supplemental moisture for the plants there. Growing shade tolerant plants underneath Maples is not difficult, if a few conditions are observed. Add 4-6" of compost, leafmould and pine bark fines to establish a shallow raised bed and to provide nutrients for the plants being installed. Leave at least a 6" air gap around the Maple's trunk. Don't add any other amendments to the planting area. When encountering small tree roots, cut and remove them. Above all, do not add fertilizer to the planting area. If you determine supplemental nutrients are necessary, they must be applied to the entire area, under and around the Maples, well beyond the drip line. Same rule applies to irrigation. Don't water individual plants. Provide an equal amount of water to all of the area under and around the Maples. Add an annual layer of compost to all of the area during late winter or early spring. I have planted dozens of different shade tolerant perennials, shrubs & small trees under all of my Maples for more than 40 years and some of my greatest plant successes have occurred there. One of the most challenging adventures was establishing Native Azaleas near the drip line of Flowering Dogwoods (C. florida). Water, water and more water was determined to be the solution! Even then, it required several years for the Azaleas to become well established and compete with the massive near-surface root system of the Dogwoods. SATWT Rb

Roots extend well beyond branch canopies. At their extreme ends tree roots are little delicate things, not as fragile as roots of some herbaceous plants but still tiny. These can be affected markedly by hostile conditions, same as with a small plant. For example, a Douglas fir fell through the roof of a house up the hill from me last winter. I could see it from the street. The lot had surely been developed decades ago (ours was the last lot to be cleared in our neighborhood, and that was in 1965). Presumably the now-failed tree pre-dated the development, although it is possible it grew up afterward. Anyway, after it failed the root flair could be seen to have ended abruptly on the same side and at the same point where a strip of lawn began - as though the tree's roots couldn't cope with the conditions under the turf at all and had either died or never grown into that part. It fell away from the rootless side onto the house. Root barriers are another way to mess with tree roots, I'd think walling off much of an individual tree's root growth might become quite a problem for it eventually. I wouldn't want to do anything that might cause a native conifer of sufficient height to come loose and topple on my house. As it is any tree has the potential to fall at any time (floors of some native stands are criss-crossed with fallen trees), I wouldn't want to increase the chances of one coming down on me (engage a certified arborist to inspect and evaluate any that appear to be having problems, if concerned).

Let's address #5 first. This is an incorrect assumption. In fact, it is an incorrect assumption to conclude this subsoil is devoid of nutrients and is - by your words -'uninhabitable'. The depth of the root systems of the native trees may just as easily be influenced by the amount of water that was available in the soil. The primary quest for tree roots is going to be a source of moisture. That's why for almost any kind of tree, the bulk of the root system, including all the fine feeder roots that take up that moisture, is located in the top 15-18" of the soil profile, regardless of how deep the enriched/amended soil may be. The only way to know if the soil is infertile or nutrient poor is to have it tested. Any amending will be influenced by drainage. Does the subsoil currently drain well? You can check online for various perk tests you can do yourself to determine drainage. To build on the previous comment, among other reasons it's not advised, amending individual planting holes in slow or poorly draining soil just creates pockets of overly wet soil around plant roots - the 'bucket effect'. These can lead to root rots and other disease issues. Increasing the easily plantable depth by bringing in a sufficient layer of soil can help but the 'topsoil' or a sandy loam garden mix is preferable to just compost because of more closely similar textural characteristics to the native soil and so avoid creating interface issues between the two different soils. It also never an problem to prepare wide but quite shallow planting holes and plant high, so the top of the rootball is above grade. You then bring in the better soil to mound up to the proper level If you feel you need to add organic matter, use some compost as a mulch. And this is something you will need repeat annually to replenish OM, which is always degrading and decomposing. #6 Don't assume the roots of the plants you are planning on adding will remain concentrated in the original planting hole (your 'quantity' factor in determining size of planting hole). Tree and shrub roots can be very pervasive and penetrate through some pretty nasty, compacted soils. They are actually more likely to stay concentrated in an enriched planting hole, as there is less incentive to seek out soil moisture and nutrients. There's a couple of other misconceptions in your plans as well but this should at least get you started in creating a desired soil planting situation for the immediate future. Probably the most important take-away from this - and the one most folks find to be counterintuitive - is that whenever possible, planting trees and shrubs in the unamended indigenous soil is the preferred approach. Any amendments should be applied as a mulch or top dressing.