Plugs are a narrow, deep container format common in the reclamation industry. Most native perennials are provided in this format.
Some people say planting plugs in the spring is best because roots have the full season to establish.
Some people say planting plugs in the fall works well because soil temperatures are still warm and the air is cool, so plants are not stressed by heat but the roots can still grow.
Planting in the heat of summer is never ideal, but with adequate and careful watering it can be fairly easily achieved in our climate.
Deep mulch is beneficial for plants added late in the season, as it protects the plants from winter winds.
Our experience is that there is higher mortality in the fall, especially on exposed sites. However, in sheltered locations it can be done fairly successfully. Another consideration is that frost heave may push fall-planted plugs out of the grown as they have not developed roots to counteract uplift. Gently pushing the plant back into the ground in early spring will remedy this.