The biggest mistake in these zones is planting too early and too late. In spring, wait for consistently workable soil before getting perennials and shrubs in the ground — frozen or waterlogged soil does roots no favors. In fall, stop fertilizing shrubs and perennials by midsummer so plants can harden off properly before winter arrives.
Mulching is your friend from both ends of the season. A 2–3 inch layer of mulch keeps soil warmer in early spring to help roots establish, moderates soil temperature through summer, and protects roots through winter freeze-thaw cycles. Shredded leaves make excellent, free mulch if you have them.
When feeding your shrubs and perennials, a slow-release fertilizer like Espoma Organic Plant-Tone applied in spring gives them a nutritional boost right as new growth kicks in. For Heuchera and other shade perennials, a light application every spring is really all they need. Hydrangeas and butterfly bushes appreciate a bit more food, especially during their first season of establishment.
One more tip that pays big dividends: water deeply but infrequently once plants are established. This trains roots to grow deep into the soil, making plants far more drought-tolerant and cold-hardy than they'd be with frequent shallow watering.