Many herbaceous perennial plants tolerate a wide range of temperatures and can be found in three or more USDA zones. But all of them have a breaking point below which tissues freeze and rupture and above which tissues fry. The vast majority of herbaceous perennials that are suited for a particular zone go dormant to survive freezing temperatures. Others, however, are more susceptible to damage from high temperatures. They may go dormant in summer, spending most of the growing season tucked away underground. A handful of herbaceous perennials go dormant in early summer, but re-emerge in fall and bravely tout the freezing cold with strong, leathery leaves, like Arum italicum `Pictum,’ Japanese painted arum. Effective temperature and actual temperature are different. Effective temperature takes into account humidity, wind, cloud cover, and air temperature to produce the way a person (or plant) actually feels. The cumulative effect, for example, of extreme heat, full sun, excessive wind, and low humidity can result in major damage to perennials that are tolerant of the mid-range of all of these conditions, or the high range of one or two of them.
What can we control? Temperature can be modified by introducing shade or air movement. But too much shade will negatively affect herbaceous perennials that thrive in full sun. Plants susceptible to temperature extremes, particularly during the winter months, can be insulated after the first hard freeze. And plants that may be damaged by late frosts can be left covered or surrounded by neighboring plants to help protect them. But we can't turn a zone 4 plant into a zone 6 plant. No matter how much we might covet the real pampas grass, we'll have to settle for the substitute.