Technically, the word perennial only refers to a plant that lives longer than two years. So a perennial in Zone 7 could be the same as a perennial in Zone 4; however most likely the assortment of viable perennials in each zone is vastly different.
Generally speaking, a plant is classified as a perennial if it can survive to Zone 7. Above that range, plants become tender perennials or annuals.
So for the purpose of this article, we will only consider the "hardiest" of hardy perennials, plants that are hardy to Zones 4 and lower (or higher in latitude). Even if you aren't one of the brave few who garden in Zone 4 or lower, the plants on this list are definitely tough enough to survive your area's worst winters.
Alpine Rock Cress (Arabis alpina)Alpine rock cress is a low growing (6 to 12 inches) perennial that blooms light pink to white blooms and grows best in full to partial sun. Hardy to Zone 3, it is one of the first plants to bloom in early Spring even in very cold climates. It is easily propagated by cutting and also by burying shoots in soil to create new roots. Alpine rock cress makes an excellent rock garden plant or a ground cover among spring bulbs.
TiarellaTiarella, a close relative of Heuchera, is a beautiful shade plant that is a perennial in Zone 3 and warmer. Also known as foam flower, tiarellas bloom airy pipe cleaner-like spires of light pink blooms in late spring. It will spread well in good soil and functions well as a groundcover by making multiple clumps in semi- to full shade. Tiarella is a native to North America woodlands.
Sea Holly (Eryngium planum)Although some sources rate sea holly as a Zone 5 perennial, many gardeners report it is reliably hardy in Zones 2 to 4 as well. Sea holly has very interesting looking blue and grey green blooms from mid to latesummer. It can grow up to 4 feet tall and almost as wide. Sea holly is also grown for its large, veined leaves which are interesting in their own right. It will do best in full sun and will survive in almost any soil type.
Blue Dogbane (Amsonia tabernaemontana)Blue dogbane, along with all of the amsonias, is an excellent choice for a shrubby, native perennial. It will thrive in full to partial sun, growing up to 3 feet tall. It blooms beautiful light blue flower heads in early summer. It is hardy to Zone 3 and can survive in many different soil types while providing a nice back drop to other perennials.
Liatris (Liatris spicata)Liatris, also known as gayfeather, is a Zone 3 hardy and is a bee and butterfly magnet. It makes an excellent border planting and will establish a fine clump within a couple years. Liatris' grassy foliage is welcomed in early spring followed by dense, feathery spikes of purple, white or pink flowers late in the summer. It likes full sun and will grow in almost any soil.
Dwarf Goldenrod (Solidago sphacelata)Dwarf Goldenrod makes an excellent drought tolerant plant and is hardy to Zone 3. This dwarf variety will grow only 2 feet tall and stays generally compact. The blooms look like frothy yellow fingers that reach out from the top of the plant. Dwarf goldenrod will grow best in full to partial sun and somewhat poor soil.
AstilbeAstilbe is a great plant for the shady perennial garden and is hardy to Zone 3. It has high moisture needs and is a perfect companion plant to hostas. Astilbe come in varying sizes from 12 inches to 4 feet tall and will provide airy, textural plumes of flowers in early to mid-summer.
PenstemonPenstemon is a northerly gardener's close friend. Hardy to Zone 2, there are over 100 choices of penstemons for the home garden. Native to North America, penstemons come in a variety of sizes, colors and preferred conditions. The particular one pictured at right is 'Husker's Red' and is readily available commercially throughout the US.
Yarrow (Achillea)Yarrow is a versatile and very tough plant. Some would even say it is very invasive, even at its cold hardiness limit in Zone 2. If you have a spot that nothing else will grow due to soil or drought conditions, yarrow is a good choice. It comes in all shapes, sizes and colors and can even bounce back after a deer or rabbit mows it down.
Creeping Phlox (Phlox subulata)Creeping phlox forms a dense mat of flowers early in the spring as most perennials are just beginning to push through. It is very low growing and provides a nice carpet of color for your other hardy perennials. It is hardy to Zone 3 and spreads rapidly every year.
Bleeding Heart (Dicentra spectabilis)Bleeding Heart, a standard in gardens in most cooler regions, is hardy to Zone 2. Its lobed leaves and arched stems of flowers make it a classic choice for shady spots in a perennial garden. Once established, bleeding hearts are long lived and make an excellent companion to other perennials such as hostas, astilbes, and hellebores.
Veronica (Veronica spicata)Veronica, also known as speedwell, puts up bright spires of blooms in early summer. It comes in a variety of sizes from dwarf (6 inches) to shoulder high (4 feet) and is hardy to Zone 3. It makes an excellent companion plant to obedient plant and Campanula.
Obedient Plant (Physostegia virginiana)Obedient plant, while not well behaved as its name suggests, ia a very resilient perennial. Hardy to Zone 2, the obedient plant spreads rapidly underground and establishes itself from small plants quickly. If you have an area you want to fill in quickly and maintain color in late summer, this tough cookie is a sure bet. Even in colder zones, obedient plant can become invasive, so make sure you keep it in check by pulling unwanted offsets or by corralling it within a specific area. It likes full sun, moderate water, and will grow in almost any soil.
Ligularia (Ligularia dentata)Ligularia is a prized hardy perennial for its large leaves and daisy-like flowers. It prefers moist, well drained soil in partial to dappled shade. Ligularia, also known as golden groundsel, is hardy to Zone 3 and will establish a well-behaved clump within a couple years. It would make a great companion to a white blooming astilbe or tiarella.
Monkshood (Aconitum columbianum)Hardy to Zone 2, monkshood is a good alternative to hollyhocks or foxgloves in a less-forgiving climate. Its dark blue blooms, which appear on tall stalks from mid-summer to early fall, are attractive to bees and butterflies. Monkshood prefers moist, well drained soil in full sun and would make an excellent pairing with veronica or lady's mantle.
Lungwort (Pulmonaria)Lungwort comes in a variety of cultivars that each has its own interesting foliage pattern. They almost look like someone spilled or splattered bleach on their thick, hairy green leaves. They make an interesting statement in shade and present small blue, purple or pink blooms in mid-Spring. Lungworts are hardy to Zone 4 and require moist soil.
Basket of Gold (Alyssum saxatilis)Basket of gold is a shrubby perennial, hardy to Zone 3. There are many different cultivars which represent different shades of yellow blooms. Basket of gold, also known as gold dust, grows up to a foot high and will blooms continuously from mid spring through early summer. It is an excellent xeric planting and will work well in rock gardens. It needs full sun and dry, well draining soil.
'May Night' Salvia (Salvia x sylvestris)'May Night' salvia is a rewarding perennial, hardy to Zone 4. It blooms violet-blue spikes all summer long if deadheaded. It is a magnet for bees and butterflies throughout the season. It will increase in size every year and can reach up to 3 feet tall. It is one of the first plants up in the spring and will tolerate many different soil types and moisture levels.
Balloon Flower (Platycodon grandiflorus)Balloon flower or bellflower will reward you with lots of blue blooms even with very little care. Balloon flower is very late to emerge in spring but it will be quick to catch up with other perennials. It will thrive with full sun in poor soils and varying levels of moisture. It is hardy to Zone 3. Balloon flowers resent being moved so take care not to disrupt its rootball.
Grayleaf Geranium (Geranium cinereum)There are many beautiful cultivars of Geranium cinereum, such as 'Ballerina' and 'Purple Pillow' (pictured at right). All are hardy to Zone 4 and require full to partial sun with good drainage. Most varieties will stay compact, growing only a foot tall and a foot and a half wide. Another hardy geranium to try out would be Geranium sanguineum, bloody cranesbill, which is hardy to Zone 3.
You must be weary in higher zones (lower in latitude) though, because plants that are this hardy can do one of two things in your garden: they can flourish so well that they are invasive without having any winter cold to put them in check, or they can overheat in your warmer weather and hot sun so much that they will never survive. Generally speaking, if a plant is hardy to Zone 2, it won't be a good fit for a Zone 9 or above garden. Well, you can't have ALL the good plants down there!
Special thanks to dahlianut for her recommendations for this article!
Plant Files photo credits:bmuller - Veronica (thumbnail) Kennedyh- Monkshood Toxicodendron- Tiarella Tcfromky-Veronica Papijo- Lady's Mantle Wandasflowers- Penstemon "Husker's Red" Crimsontsavo- Yarrow Redsnowflake- Obedient Plant Art_n_garden- Sea Holly, Dwarf Goldenrod Dave- Bleeding Heart Sanannie- Ligularia Poppysue- Blue Dogbane Grampapa- Creeping Phlox Sherlock221- Astilbe Bootandall- Pulmonaria Todd_Boland-Geranium cinereum Fro_Bro26- Balloon Flower Goswimming-Lungwort hczone6-Basket of Gold Daryl -May Night Salvia, Rock Cress, Liatris