A shrub with procumbent or trailing main branches and erect, spreading or horizontal branchlets, often forming wide, low mats and rarely more than 2 ft high. Leaves green or glaucous, the adult ones scale-like, appressed, four-ranked, ovate or oblong, acute and usually sharply apiculate at the apex, with a conspicuous resin-gland on the back; juvenile leaves in opposite pairs, slightly spreading. Fruits blue, with a glaucous bloom, on recurved stalks, with two or three seeds.
A native of North America, with a wide range of habitats, from the coasts of New England to the dunes of the Great Lakes and dry slopes in the Rocky Mountains; introduced early in the 19th century. It is allied to J. sabina, J. virginiana, and J. scopulorum, and hybridises with the two latter species where it is in contact with them in the wild.
cv. ‘Alpina’. – Branches erect at first, becoming horizontal; branchlets erect, up to 2 ft high. Leaves awl-shaped, slightly glaucous. Rehder described this juniper from a plant received from the American nursery of Ellwanger and Barry. It may be the same as the J. sabina var. alpina of Loudon, described from a plant sold by Loddiges’ nurseries.
cv. ‘Bar Harbor’. – This name seems to have been used in a collective sense for plants found wild near Bar Harbor on the north-eastern side of Mt Desert Island, Maine. These grow ‘in crevices on the rocky coast and may frequently be found well within the reach of salt spray’; they are steel-blue in colour and of prostrate habit (H. Teuscher, in New Flora and Sylva, Vol. 8 (1936), p. 194).
cv. ‘Douglasii’. Waukegan Juniper. – Main branches prostrate, branchlets spreading. Foliage glaucous, scale-like or intermediate, becoming plum-purple in winter. Sprays densely set, pointing forward and upward. It makes a wide, dense mat and is a useful ground cover. The original plant grew near Douglas’s nursery at Waukegan, Illinois, on Lake Michigan. Similar plants are found wild in sand-dunes on the shores of that lake. ‘To see this Juniper as it grows naturally is a revelation. Its main branches are entirely buried in the sand, only the branchlets coming above the surface, where they form a dense, soft carpet which follows in every detail the outline of the dune.’ (H. Teuscher, loc. cit., p. 195).
cv. ‘Glauca’. – This variety was selected by Prof. Sargent of the Arnold Arboretum as the best-coloured form of J. horizontalis and sent by him to Murray Hornibrook, who named it, and described it as having ‘extremely blue – practically steel-blue – whipcord-like foliage, with a very flat habit’ (Dwarf and Slow-growing Conifers, 2nd ed., p. 120).
cv. ‘Plumosa’. – A wide-spreading, flat-topped shrub with prostrate main branches and spreading branchlets, ultimately 2 ft high. Leaves grey-green, awl-shaped, purplish in winter. It was found growing wild on the coast of Maine and distributed by the Andorra nursery, Philadelphia (Rehder, Journ. Arn. Arb., Vol. 6, p. 204). It is sometimes known misleadingly as the Andorra juniper.
cv. ‘Wiltonii’. (‘Blue Rug’). – Of completely prostrate habit. Ultimate branchlets slender, with small, closely appressed, bluish-grey leaves. The colouring is retained throughout the winter. The original plant was found in 1914 by J. C. van Heiningen of the South Wilton nurseries, Connecticut, growing on Vinalhaven Island, Maine (Den Ouden and Boom, Man. Cult. Conif. (1965), p. 179).