A taxonomist would be most interested in learning that there are just over 30 different species of spruce trees (Picea spp.) in the world, all native to forests in the northern hemisphere. These cone-bearing trees also gain the interest of horticulturists, since they have many ornamental features to use in gardens. Three different horticultural categories for spruces center around their mature plant form (size and shape).
Most people would immediately recognize the classic upright form of a spruce tree. It has a singular, straight trunk with whorling branches heavily covered in green, gray or bluish needles. The silhouette of the spruce tree is cone-like or narrow pyramid-like, perhaps even like a slender spire. Examples of such classic upright tree forms include the white spruce (Picea glauca), Engelman spruce (Picea engelmanii), Serbian spruce (Picea omorika), Colorado spruce (Picea pungens), red spruce (Picea rubens) and the Himalayan spruce (Picea smithiana). Other spruces may look more like plump or fat/wide pyramids, such as the Sitka spruce (Picea sitchensis) and the Norway spruce (Picea abies). Spruce trees are used as windbreaks, hedgerows or as large solitary specimen trees in spacious landscapes.
Genetic mutations or rare seedlings of the wild, tall, tree-like spruce species can yield plants that are considerably shorter, more or less resembling shrubs. Horticulturists tend to assign cultivar names to these atypical forms of the spruce tree and propagate them for sale and use in garden borders, rock gardens or building foundation plantings. These dwarf selections of spruces retain all of the characteristics of the wild tree but often grow more slowly and much smaller in size at maturity. Examples of dwarf Norway spruces are the varieties 'Clanbrassiliana' (the Tolleymore spruce), 'Pygmaea' and 'Nidiformis' (bird's nest spruce). Dwarf black spruce (Picea mariana 'Nana'), dwarf Alberta spruce (Picea glauca var. albertiana 'Conica'), dwarf Colorado blue spruce (Picea pungens 'Montgomery') and dwarf red spruce (Picea rubens 'Nana') also demonstrate selections with much smaller shrub-like proportions, often growing only 5 to 10 feet tall and 3 to 6 feet wide at most.
Just like some spruce trees yielded dwarf, shrub-like forms, others were selected by gardeners because of their flimsy, pendent branches. These trees (or large shrubs) have more architecturally interesting trunks and branch silhouettes and are used as a focal accent in gardens standing alone or trained on a trellis, frame or as an espalier. Weeping Serbian spruce (Picea omorika 'Pendula') grows only 5 feet tall and 3 feet wide with curtain-like branches. Some large wild spruce tree species bear rather pendulous branches and twigs, such as the Norway spruce (Picea abies) and the weeping spruce (Picea breweriana).