Most local nurseries in my area keep perennial plants outside during the summer months.
There is one nursery which grows and keeps everything inside a greenhouse all the time.
In zone 4 or 5, does this mean that those perennials should be acclimatized to the outside like I would normally do for seedlings started inside?
Is there a risk if those plants are planted outside directly?
If the plants apparently have never (or not for a while) been exposed to strong sun and wind, I would definitively recommend hardening them off or shielding them a bit:
If the plants look basically robust, e.g. if you suspect they have been bought by the nursery from another supplier that grew them outside, leaving them outside in dappled shade and with some wind protection might be already enough, no need to do the "full babying" routine we do for seedlings. If planted right away in full sun, consider shading them a bit for the first days.
If you buy and plant the plants during a spell of overcast weather and mild temperatures and they show no signs of etiolation or general "softness", you might be ok with planting them immediately, but consider transplant shock plus sudden exposure to full sun - that might be too much.
If the plant obviously is etiolated and weak, think twice before purchasing - it might also be more susceptible to pests and infections. If you have your mind set on such a candidate, yes, do the full hardening off routine. Come fall they should be established enough to deal with the upcoming winter.
