Ask Mr. Smarty Plants

It sounds like a lovely landscaping plan, and a good use of native rock. Even the rings of rocks around the trees is a nice touch. However, we do not recommend that you add soil and mulch within those rings around the trees because you will basically be doing damage to the base of the tree. It can rot out, attract disease and just generally have results you are not going to like. It is one place raised beds are not advisable. We recommend that you just restrict those beds to a light mulching, which will protect the tree roots from heat and cold, hold moisture in for the tree, and still permit the roots to "breathe," as well as the trunk of the tree. Don't pile it too high against the trunk, you will be asking for fungus, which can then make a access to the tree easier for other diseases.

While we're thinking about it, you can go to our list of Deer-Resistant Native Plants, Narrow Your Search on Texas, the General Appearance (herb or herbaceous blooming plant, shrub, tree, etc.) and the amount of sun or shade you have, and you will get a list of potential plants for that area. Remember, there is no such thing as deer-proof, they will eat your shoes if you don't keep moving, and in serious droughts or cases of over-population of deer, they will even chow down on the items on this list.

Most oaks are known to exude allelopathic toxins to eliminate competitive plants from growing beneath them. This usually is not manifested until the oak is fairly mature, so you might get away with it under a young tree for a year or so,  but eventually, the landlord will kick out the intruders. In addition, most flowering plants need more sun than they are going to get under those trees. The deer-resistant plants would do no better, and if they would grow under the trees, we can assure you the deer would find them there, too.

As for the Juniperus ashei (Ashe's juniper) on your property, there is some disagreement whether anything will grow under them. Again, you have the heavy shade, and the litter of needles, berries and twigs always on the ground, which will discourage many plants and inhibit seedlings. 

From this article  Biology and ecology of Ashe Juniper by F. E. Smeins and S.D. Fuhlendorf, we extracted this paragraph on the allelopathy of the Ashe juniper:

"There is little evidence that the accumulated litter of Ashe juniper in anyway alters the chemical nature of the soil as it relates to growth and development of other plants (Yager 1993). Soil chemical and physical properties are in fact "improved" by the presence of the juniper litter (Marshall 1995). No allelopathic effects have been shown to be produced by the litter. The major impact of the litter seems to be its physical presence and its alteration of hydrologic properties of the area under the canopy. The thick litter layer is a difficult physical medium for seeds of other species to germinate and grow in mainly because moisture either runs off (due to the hydrophobic nature of the litter) or it dries out very quickly after a precipitation event, which prevents seedling growth from reaching the mineral soil (Yager 1993)."

In fewer words, this is saying that while plants are not being poisoned by allelopathy, it is very difficult for them to grow under all that shade and litter. 

We don't know if this leaves any spaces in your proposed garden for plants not directly under the trees, so we are going to search the deer-resistant list we referenced above for herbaceous blooming plants  for "part shade" (2 to 6 hours of sun a day) and "shade" (less than 2 hours of sun a day).

Deer-resistant shade tolerant herbaceous blooming plants for Central Texas: 

Aquilegia chrysantha var. hinckleyana (Hinckley's golden columbine)

Asclepias tuberosa (butterfly milkweed)

Berlandiera lyrata (lyreleaf greeneyes)

Capsicum annuum (cayenne pepper)

Callirhoe involucrata (purple poppymallow)

Coreopsis tinctoria (golden tickseed)

Dalea greggii (Gregg's prairie clover)

Lobelia cardinalis (cardinalflower)

Melampodium leucanthum (plains blackfoot)

Salvia roemeriana (cedar sage)

Monarda fistulosa (wild bergamot)

Tetraneuris scaposa (stemmy four-nerve daisy)

From our Native Plant Image Gallery:

Aquilegia chrysantha var. hinckleyana

Asclepias tuberosa

Berlandiera lyrata

Capsicum annuum

Callirhoe involucrata

Coreopsis tinctoria

Dalea greggii

Lobelia cardinalis

Melampodium leucanthum

Salvia roemeriana

Monarda fistulosa

Tetraneuris scaposa