Perovskia has one of the longest bloom times of any blue-flowered plant. Semi-woody shrub emerges slowly in spring; cut back after frost.
Tolerates drought, loves heat and full sun, can’t bear standing water. Likes to spread out once it gets established, and it is so beautiful it deserves every foot of space it can get! Plant 24-36" apart. Grows 36-48" tall. I suspect that Zone 4 reports of winterkill are from poor drainage rather than cold, so a cautious Z4. (bare-root crowns)
When you receive your order, open the bags and check the stock. Roots and crowns should be firm and pliable. If they are slightly dry, add a little water or, if they are going to be potted up soon, wet the roots. Generally, a little surface mold is harmless and will not affect the plant’s future performance. If you cannot pot them up immediately, store them in a cool (35–40°) location for a short time.
Do not plant bare-root perennial plant crowns directly outdoors.
Pot up the rootstock using well-drained potting mix in a deep 6" pot or a 1-gallon container. Avoid coiling the roots in under-sized containers.Grow newly potted perennials for a few weeks in a protected location in indirect light at 50–60°. Wet and/or cold conditions for an extended period may cause rotting.Transplant outside once they show some top growth and the danger of frost has passed.
For more info:About planting bare-root perennials.