In most areas of the country, it's best to plant ornamental grasses in the spring, after the danger of frost has passed. This will allow your ornamental grasses a whole season to establish a strong root system before winter. In very warm regions, a fall planting is better to avoid intense heat and drought while the grasses are young and vulnerable.
Grasses can develop deep, complex root systems, so it helps to till the soil deeply before planting. If you're using grasses to fill a space in an existing garden, you can get similar results by digging a hole twice as wide and deep as needed and back-filling with loose dirt. Turn some compost into the soil as you prep to provide moisture retention and nutrients for the first growing season.
Dig a hole slightly bigger than the pot of your ornamental grass plant and retain the excess soil. Overturn your transplant pot while supporting the grasses with one hand. If the plant won't budge, you can use a sharp knife or box cutter to cut the thin pot off of the plant. Gently break apart any matted roots while keeping as much of the transplant's original soil around the root system as possible.
Next, lower the plant into the hole and check the level: You want to be sure that your ornamental grasses are planted at the same depth as they were in the pot, as deep planting can encourage rot. Pro tip: Use the handle of your spade to span the hole to help visualize where the soil line will be.
Once you're satisfied with the placement, back-fill your excess soil into the hole and press firmly, but without packing it. You will have leftover soil from the hole you dug that you can spread around elsewhere.
The first watering must be a deep one to make sure that there are no air pockets in your soil around the roots of your plant. Water your transplant to the point of creating a puddle around it — this is called "muddying in" your plant.
For the rest of this first growing season, your ornamental grasses should receive about an inch of rain per week. If you're not getting that much naturally, water your plant twice a week to make sure. Ornamental grasses are often resistant to drought once they are established, but will need some extra attention in their first year.