It would be good to know what part of the country you're in and what type of soil you have (pH and texture) to get really good suggestions. I live in the NE where I have loamy, acid soil, so my suggestions will be best if you have acid soil. Heaths and heathers are great 4 season plants - you can get different ones that flower here in NH from when there is still snow on the ground right through until until late fall in shades of white, pink, purple, and red. They have small foliage, somewhat like junipers, and they come in many colors from silvery to dark green to golds and red. Several of them change foliage color as cold weather arreves. However, they wouldn't be happy in many areas of the southeast where there is a long season of heat and humidity. Once established, they don't mind dryish conditions, though they need regular water the first year. Rockspray Nursery in MA is a good source. Another plant to look into if you like rhodies is PJM Checkmate. It has the same bright pinky purple flowers as the regular PJMs but is very short - at 2 years, mine is only about 1 1/2 feet tall and it's supposed to top out at about 3 feet. It has smallish, olive green leaves that turn at least partly darker reddish in the cold weather. There are some dwarf or low-growing spireas. Also, bearberry (Arctostaphylos uva-ursi) grows about a foot high, Deutzia gracilis 'Nikko' tops off at about 2 1/2 feet and has white spring flowers, and there are some 1-3' Forsythisas that might be hardy enough to bloom in your area. Dwarf Leucothoe (L. fontanesiana 'Nana') might be small enough for your purposes and has evergreen leaves and white flowers. Vaccinium angustifolium (low bush blueberry) has reddish new growth, white spring flowers, scarlet fall color, and of course blueberries. It's about 2' tall. One of the maroon foliaged Weigelas, either midnight wine or wine & roses, is short and has pink flowers. There are also miniature and groundcover roses, and you could ask for info on the rose forum. If you want acid loving plants and are in an alkaline soil area, you can dig out your soil and replace it a couple feet deep with an acid soil mix - look on the azalea and rhody forum for suggestions. You'd need to water with rain water, though since your goundwater would be basic along with the soil. Also, you may want to look into perennials and ornamental grasses.

The answer to your question is really yes and no. Yes, I grow prairie plants known to be deeply rooted and no, they are not capable of breaking up compacted soil. For one thing, roots require oxygen to develope and grow; something compacted soils just don't have. Once again, we have to be careful that we don't take a fragment of information and apply it too broadly. It is well documented that many prairie plants can (the word is "can", not "will") develope deep root systems. Other plants do this as well; alfalfa being one receiving much study documenting root systems 15 to 30 feet deep! However, these same plants when grown in compacted soils just don't ever develope roots anywhere near those depths. In fact, the statisical evidence proves that root penetration stops at the layer of compaction -- whatever that level is. I have included a link to some documented studies of alfalfa root developement which is really quite representative of some prairie plants - they just haven't been documented as much. Alfalfa is well known for its deep roots -- provided the soil will allow this to take place. IronBelly Here is a link that might be useful: Root development of alfalfa.

Do not add soil to the top of what you have. You'll bury the root flare. You will need to do a full repot but not right now in the heat.

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.