As for many other cultivated trees and shrubs regeneration of formerly planted conifers is poorly documented in Belgium. In recent years sexual reproduction has been detected in numerous species but some processes probably already started long ago (for instance Pseudotsuga menziesii). Several additional ornamental species doubtlessly self-sow in Belgium but additional research is required. Many conifers are reputed environmental weeds worldwide (see for instance Richardson & Rejmánek 2004).
The University of Hamburg provides a lot of useful additional information in their “Gymnosperm Database” (available online at: http://www.biologie.uni-hamburg.de/b-online/earle.
An extensive bibliography on conifers is provided by Farjon (2001). Farjon (2010) is a monographic work and essential for the identification of any coniferous plant species.
1. Leaves fan-shaped (up to ca. 10 cm wide), deciduous, dichotomously veined === Ginkgoaceae
1. Leaves needle- or scalelike, evergreen or deciduous, never dichotomously veined === 2
2. Small dioecious shrubs, usually ascending, rarely exceeding 50 cm, Equisetum-like with articulate twigs. Longest internodes up to 10 cm. Never resiniferous === Ephedraceae
2. Monoecious or dioecious shrubs or trees, erect and much taller than 50 cm at maturity, never Equisetum-like and with articulate twigs. Longest internodes up to 1 cm long. Resiniferous or not === 3
3. Largest leaves at least 10 mm wide at base, flat === Araucariaceae
3. Leaves needle- or scale-like, much less wide === 4
4. Leaves needle-like, spreading in one plane. Young shoots green. Ovules borne singly, mature cones berry-like, red. Not resiniferous (native) === Taxaceae
4. Leaves needle- or scale-like, in one plane or not. Young shoots brownish, yellowish or reddish. Ovules singly or numerous, at maturity usually in woody cones (if berry-like, then leaves never in one plane). Resiniferous === 5
5. Mature female cones berry-like, bluish-black (native) === Cupressaceae p.p. (Juniperus)
5. Mature female cones woody === 6
6. Leaves in opposite pairs or in threes at each node. Branchlets flattened === Cupressaceae p.p. (Chamaecyparis, Thuja, Xanthocyparis)
6. Leaves solitary or fascicled. Branchlets not flattened === 7
7. Female cones with bracts and cone-scales distinct at maturity. Leaves solitary or in clusters === Pinaceae
7. Female cones with bracts and cone-scales united at maturity. Leaves spirally arranged === Cupressaceae p.p. (Cryptomeria, Taxodium)
Auders A.G. & Spicer D.P. (2012) RHS - Encyclopedia of Conifers. A comprehensive guide to cultivars and species, 2 vols.: 1500 p.
Crook C.S. (1997) A (very) provisional checklist of conifers in the British Isles. BSBI News 75: 42-47.
De Koning J., Van den Broek J.W., Van de Laar H.J. & Fortgens G. (2000) Nederlandse dendrologie (13e druk). H. Veenman & zonen, Ede: 585 p.
Farjon A. (2001) World checklist and bibliography of conifers. Royal Botanic Gardens, Kew (2nd edition).
Farjon A. (2010) A handbook of the world's conifers. Brill, Leiden-Boston: 1111 p.
Howell C.J. (2019) Naturalised status of exotic conifers in New Zealand. New Zealand Journal of Botany 57(4): 227-237. [available online at: https://www.tandfonline.com/doi/full/10.1080/0028825X.2019.1626744]
Krussman G. (1985) Manual of cultivated conifers. Timber Press, Oregon: 521 p.
Richardson D.M. & Rejmánek M. (2004) Conifers as invasive aliens: a global survey and predictive framework. Diversity and Distributions 10(5-6): 321–331.
Vidaković M. (1991) Conifers, morphology and variation. Grafički Zavod Hrvatske, Zagreb: 754 p.
Walters S.M., Brady A., Brickell C.D., Cullen J., Green P.S., Lewis J., Matthews V.A., Webb D.A., Yeo P.F. & Alexander J.C.M. (eds.) (1986) The European Garden Flora, vol. 1. Cambridge University Press: XV + 430 p.