Answer: Junipers need full sun and a well drained soil, so they would be among my first choices for this difficult hot and dry bank planting. When you plant, be sure to loosen the soil over a wide area and as deep as the container to encourage easy rooting. Leave a rough edge on your planting hole, do not dig it polished and smooth as clay soil is prone to do.
After planting, water thoroughly to settle any air pockets and then mulch with an organic mulch; larger particled mulch such as bark nuggets will be less likely to wash down the slope. You may also want to create some horizontal contouring with boards or rocks to slow the runoff if it is very steep.
You will need to kill off the grass/weeds before you plant, then maintain a mulch layer over the entire area. YOu may be able to smother the existing growth with several layers of damp newspaper topped with a generous layer of mulch, and dig out the weeds at each planting hole. If you do that you could plant immediately. Or, you may want to use an herbicide containing glyphosate. Read and follow the label instructions carefully, including the recommended waiting period before removing the foliage.
Be sure to mulch the overall area as soon as you have removed the existing growth and maintain it in a layer several inches thick -- nature will fill in available open soil with weeds if you don't. The mulch will prevent the grass/weeds from growing back and also help protect the soil from erosion, over time it slowly feeds the soil as well when it rots down. Do not put plastic under it as this prevents air and rain from reaching the soil.
Also be prepared to water your new plants, as needed, to keep the soil moist like a wrung out sponge (not not sopping wet or saturated) while they become established. This means the first year (unless it is frozen), and in droughty spells the next year or two after that. Junipers are deep rooted so you need to water slowly and thoroughly so it soaks in deep. After watering, wait about eight hours or overnight, then dig down and see how far the water went -- it can be surprising. Clay soil is interesting in that it holds water for a long time, but once it dries out it takes a lot of water applied very slowly to re-wet it. To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet.
There are several low growing junipers you could use. J. procumbens nana is my personal favorite among the low ones, however it is very slow growing, just a few inches a year. At that speed it takes a long time to cover a bank. Juniperus sabina cultivars such as "Calgary Carpet" might grow a bit faster, there are also several cultivars of J. horizontalis that grow faster. Your local nurseries should be able to show you some of these to see if you like them.
Good luck with your project!
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