Jamin, yes you can grow trees in berms. 94% of all plant roots get their chemistry and water from the top 6". Deeper roots are for support, not for sustenance.
Your berm needs to be compacted get the large pockets of air removed before planting your trees. Depending on your zone, you should also insert 3 OR 4" PVC pipe drilled with holes near the root ball for best watering. Fill the pipes with water during the first year of The berm itself will provide plenty of drainage to not allow roots to rot.
Remove the burlap off your balled and burlapped root balls of trees. Is your tree from the nursery in a pot or balled and burlapped...or heavens, bare root? Keep the line between trunk and roots above the surface of soil! Do not amend the soil. Do not dig any deeper than the depth of the rootball. Keep the subsoil below the root ball undisturbed. Chose a spot with plenty of room for this tree...walnut. Big deal about the difference between English and Black walnut?!
Do not plan on planting anything near this walnut. Trust me. A walnut tree is to die for! Those that know walnut, Juglans, are very careful just using walnut wood in a shop. A raised plant bed is perfect. That means double digging and raising the plant bed using just the soil you are double digging.
Please do not do hugelkulture. Ugh. Where do these fads come from? Not at all scientific. Raising the soil, just by double digging is all you should do if you are worried about drainage. NO amending the soil. NO foo foo beneath the soil. Hugelkulture is a perfect example of making a perched water table.
If you've got major drainage problems, it would be nice to see the site where you want to plant. The best things for trees is to plant them right into the spot with no amendments whatsoever. Most trees are grown in clay in the nurseries. They B & B them with a clay root ball.
Clay is wonderful soil!! Just need to know management practices! Trees grown in clay should be planted in clay! If you plant a clay root ball in a sandy or easily draining soil, your tree might die from lack of water. Water will run around the clay and down into the sandy soil. Very bad.
Do not amend your soil. Do not dig deeper than the depth of the root ball. Water regularly until established. Add a balanced fertilizer. There is no other alternative for ensuring proper chemistry for a plant. Decomposed compost is not fertilizer, is not to be used for soil. Should not be used in potted plants. Should never be used to 'amend' soil.
Compost installed on top of the garden soil is necessary to feed the soil. It has to be decomposed, dump on top of soil, never allowing the trunk's bark to be covered with soil, or mulch, or rocks...and that compost is taken into the soil profile by a healthy soil macro and micro life. Decomposed organic matter feeds the soil organisms. So very important. You have to know what your compost is made from, how far it is in the decomposition process to be able to discern the proper balanced fertilizer you need to add.
Not at the very beginning, later, in the new growth season following the planting of your plants.
Edit: 95% of all roots...are within 4 to 6 inches from the surface of the soil. The reason is water and chemistry necessary to absorb via roots for photosynthesis. Organic matter after it is decomposed can offer some minuscule chemistry (I will not say 'nutrients', replace chemistry with 'nutrients' might help make more sense...not nutrients, not food but chemistry).
A soil within a stable ecosystem has just enough life that is fed by just enough decomposed organic matter falling from this established ecosystem to be recycled to allow one new plant into the ecosystem or allow another plant citizen in the system to fight off an insect problem. Very finely tuned. We humans will never be able to mimic an ecosystem. We do have to understand our artificial gardens, especially the edible gardens.
We need to understand plant's needs; draining soil (all soil is wonderful just each type needs its own management techniques), proper watering, an alive soil which is where decomposed organic matter is critical, light, the correct amount not just any light will work, and dog gone it, soil doesn't come with fertilizer.
Walnuts, have a poison that poisonous to walnut trees... is primarily to stop their own progeny from competing with mommy/daddy. There are other plants are sensitive to it other than Juglans the genus. Usually those same plants are also needing lots of light and shouldn't have been planted beneath a Walnut tree. It truly is no big deal. Walnuts shade their root systems, keep other baby walnuts from competing for chemistry (nutrients) and keep other plants from competing because of their deep shade.
I had a Juglans full grown, once. What a wonderful wonderful tree to raise and know. Worth planting at least 3 of them, far apart from one another so that you will get at least one mature walnut tree. Better harvest in terms of calories and 'nutrients' than an apple.
I am assuming you will be purchasing nursery stock? Potted trees to 5' or Balled and Burlapped up 15'? A berm would be your best bet. When you make a raised by by 'fluffing' up the soil (it becomes huge when you start double digging), provide trenches to gather excess water and tell the water where to go, even if standing water were all around the base and the young tree had its roots in that berm out of the standing water, that tree, that plant will do just fine.
Tree roots, all plant roots, all of the 'feeder' roots are found within 4 to 6 inches of the surface.
Most trees from nurseries are grown in clay soil. That keeps their roots more confined so when they get dug up they keep most of the root system allowing for better survivability. Do not 'amend' the soil when planting trees. That berm will keep their roots from drowning. Hopefully we can help you improve your drainage. Make sure to not plant your baby trees too deeply. Only the root ball gets buried. No part of the trunk should have any soil, mulch, paper, nada touching the bark.
Send pictures, please. Tap roots are for support, not water or chemistry. There are a few trees that have tap roots that take up water and chemistry and that grow a mile beyond the canopy or down into the earth. That is a specialty. Not happening with walnuts.
Another misconception is root depth. Roots will grow wherever the
environment is favorable. They require water, oxygen, minerals,
support, and warmth. These requirements are usually found in the upper
few feet of soil. Roots rarely grow below four feet although there are
numerous cases stating the opposite. The major portion of a tree's
root system is in the top few inches of soil. This makes it easier to
understand why trees can be easily uplifted during wind storms or
other soil disturbances.
differences between types of roots