The following conifers are among the cold-hardiest on earth!
1)   Abies balsamea                (Balsam Fir)

USDA Hardiness Zone 3: -40° to -30°F.
North American fir with range distribution as far north as Labrador, Canada.
Balsam fir is the most cold-hardy and aromatic of all firs.

2)   Juniperus communis                (Common Juniper)

USDA Hardiness Zone 2: -50° to -40°F.
The most widespread tree or shrub in the world! 

Very fragrant, early flowering wintersweet will make you smile. Experience it for yourself in the Arboretum’s Witt Winter Garden.

1)   Calocedrus decurrens                Incense Cedar

This native of Oregon and south to Baja California was first described by Colonel John C. Fremont in 1846.
Incense cedar is often confused with Western Red Cedar (Thuja plicata), but is distinguished by its branchlets being held vertically, its narrow pyramidal habit, and by the lack of white stomata on the leaf undersides.
Located north of the Wilcox Bridge (marked by a sign) and east of the Pinetum Loop Trail. 

For 13 years, the Director’s Holiday Open House was a tradition for all University of Washington Botanic Gardens staff (from the Center for Urban Horticulture and Washington Park Arboretum), Arboretum staff from the City of Seattle, all volunteers, Arboretum Foundation board members, and other friends and dignitaries. It began when John A. Wott moved to the Washington Park Arboretum in 1993 to become the first on-site director in many years. 

Since the late 1930s, the Puget Sound region has been regarded by some as the best rhododendron growing region in the U.S.A., with documentation for over 2000 hybrid rhododendrons. Washington Park Arboretum has always been a leader in showcasing rhododendrons, including species and hybrids. The hybridization of rhododendrons was one of the legacies of both the former curator, Joe Witt, and the former director Brian O. 

Except for their bright red fruits and similar common names, the Strawberry Tree (Arbutus unedo) and the strawberry (Fragaria spp.) have nothing in common.  This tree is valued as an ornamental broadleaf evergreen for gardens and it has a long history of appreciation in Western cultures.
The species name ‘unedo’ is attributed to Pliny the Elder who said of the fruit “Unum Tantum Edo” (Latin) meaning “I eat only one”.   

Betula Nigra, known as the river birch, has beautiful shaggy bark and is resistant to the bronze birch borer

1)   Araucaria araucana                Monkey Puzzle

Native to Chile and Argentina in the south central Andes mountains.
This long-lived tree is frequently described as a living fossil.
Large cones yield many edible nuts, similar to a pine nut.

2)   Berberis gagnepainii                                 Gagnepain’s Barberry

This evergreen shrub is native to China in the Sichuan and Yunnan Provinces.
Shrub is protected by many slender three-spined thorns. 

1)  Sorbus alnifolia                                               (Korean Mountain Ash)

Native to central China, Korea and Japan this medium-sized tree boasts showy 2-3 inch umbrella-shaped clusters of 5-petal white flowers in late spring.
As summer yields to autumn, clusters of purple-red to orange-red ½ inch showy fruits appear and persist into winter.

2)  Gaultheria mucronata                                      (Prickly Heath)

Formerly known as Pernettya, this southern Chilean native spends the fall awash with showy globose berries in shades from deep plum to pink to white. 

Few small ornamental trees offer so many attractive qualities in the landscape as the paperbark maple (Acer griseum). With its bright green leaves, coppery peeling bark, and vibrant fall color, this tree is highlighted in gardens across the country, and is specifically recognized as a Great Plant Pick for our region. At the University of Washington Botanic Gardens, we have six individual trees in our collections – one at the Center for Urban Horticulture and five at the Washington Park Arboretum.