Thuja ×soeegaardii (Cupressaceae) – a new name for an old hybrid

Thuja ×soeegaardii, a hybrid of Th. plicata Donn ex D.Don and Th. standishii (Gordon) Carrière, is described as a new nothospecies from the Kórnik Arboretum, Poland, based on plants obtained in 1988 from the Botanical Garden in Gothenburg, Sweden. The hybrid was found for the first time at the Hørsholm Arboretum, Denmark, in 1938, among seedlings grown from seeds collected from Thuja standishii. Plants grown from rooted scions of this hybrid were planted in several places within the Hørsholm Arboretum. Soon, the hybrid plants were gifted to the Botanical Gardens in Copenhagen and Gothenburg as well to the Poulsen’s Plant Nursery in Kvistgård. In 1967 seedlings were sent from Kvistgård to the National Arboretum in Washington, where one of them developed into a particularly showy, lush and vigorous plant characterized by a dense, narrow, conical crown. The offspring obtained from rooted scions turned out to be very drought and wind-resistant, especially suitable for the formation of tall hedges and windbreak screens. This plant, now very widespread in cultivation, especially in the United States, goes by the name of Thuja ‘Green Giant’. In terms of morphology, many features of Th. ×soeegaardii are intermediate as compared to the parent species. From the Th. plicata, it differs by the following characteristics: leaves not or only weakly shining above, lateral leaves with inwardly curved (not straight) apices, the apical part of lower central leaves thickened and slightly bent from the stem (not appressed), glands on central leaves only weakly visible, central leaves of primary shoots acuminate (not long acuminate) and seed wings sparsely papillose in the upper part (only rarely smooth). From Th. standishii, it can be distinguished by the less glaucous upper leaf surface, the less protruding apical part of lower central leaves, at least faintly visible glands on central leaves, acuminate (not acute) central leaves of primary shoots and less distinctly papillose upper part of seed wings.