Hyacinthus orientalis L. (NE Medit.) – A rather rare escape from cultivation (most often a mere throw-out). Locally sometimes temporarily persisting but usually in small number and probably nowhere genuinely naturalized (in contrast with taxa from the genus Hyacinthoides that are often considered invasive). Probably first claimed from the Osbroekpark in Aalst in 1977 and since then rather regularly seen in widely scattered localities. An up-to-date overview of Belgian records is available here: http://waarnemingen.be/soort/view/6881.
Hyacinthus orientalis is easily distinguished from species of Hyacinthoides on the basis of its perianth that is connate for about half its length (vs. connate at base only). It is fairly variable itself, especially in cultivation: cultivars are often multi-flowered, with variously colored perianth (many shades of blue, pink or white) and with wider leaves than in wild plants.
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Selected literature:
Koyuncu M. (1995) Hyacinthus orientalis L., Liliaceae. Karaca Arbor. Mag. 3(1): 45-46.
Persson K. & Wendelbo P. (1979) The artificial hybrid Hyacinthus orientalis x transcaspicus (Liliaceae). Bot. Notiser 132(2): 207-209.
Stork A.L. (1979) Hyacinthus orientalis L.: espèce sauvage et horticole. Rev. Hort. Suisse 52: 277-280.