Box hedges….i want to grow a dense hedge around my garden, they are coming bare rooted and about 12" tall. Thanks

boxwoods come in two varieties (horizontal and vertical). Generally the variety you purchased would be horizontal in nature.
This plant will grow three to one. Three inches wide while growin one in in height. Whereas you have twelve inch plants they are approximately fifteen years in age. They will be sherred for shipping thus you’ll have a 12′ h by 8′ w plant on your hands.
Regardless allow the plant to assume it’s natural growth patterns. Prepare your bed to allow for both internal and external growth. Picture five years from now. Place the plants back away from the drive or pavement you want by at least four feet. Space them at least 30 to 36" center apart.
I recognize this differs greatly from previous advice, however, the key to this is what happens in the future versus immediate result. At the previously applied thoughts you will have plants that either smother each other or die to lack of water and fertilization. Boxwoods are great for training in growth. You can prune them to go taller (prune the sides) or horizontally (prune the top). Always prune with a hand pruner vs. the electric/powered pruner.
Prepare your bed by hand or roto-till. Again make it at least three feet off any pavement. Although evergreen they can tolerate neutral soil as well as acidic. Prior to planting add both a 10-10-10 with mineral granular (really cheap) and some bone meal to the soil. Do this on the last pass of roto-tilling or raking in the bed. Buy a double or triple shredded hardwood mulch for the plants.
Establish the original planting level by the difference in the root material. The level of planting should meet with where the bark is darker (this is above ground). Spread the roots, as best you can and dig a hole to support the spread roots.
Once in the ground tamp the soil to set it. Boxwoods are hearty and you will not hurt it making sure it is in place. Mulch around the exterior root structure, however, do no allow any mulch to touch the uprights. In general make a dam or well around the plant about six inches from the mainstem and do not mulch near that.
Water the dam/well you have made for a few minutes or until the perimeter is full of water. Let it be for a week or more depending on your rainfall. If there is none do it again but let the dam/well fill half way. If there is some leave it alone.
Sorry to be so detailed and longwinded, however, when properly grown the boxwood can become not only a hedge but a work or art. It easily adapts to pruning and is made into all kinds of shapes (letters, animals, waves). Hope this helps. I’m at gjgjobs@yahoo.com for other advise. Good Luck.