Catalpa is a genus of 9 species, native to East-Asia, the United-States and the West-Indies (see Kirkbride & Olsen 2011). Five species, a hybrid and numerous cultivars are cultivated as ornamental trees in Europe (De Koning & al. 2000, Knees 2000, Roloff & Bärtels 2006, etc.). Their identification is often critical and saplings are hard to identify with confidence in the absence of parental trees.
Three taxa have been reliably recorded so far (see below). Catalpa speciosa (Barney) Engelm. is also much planted and probably also escapes. It most closely resembles Catalpa bignonioides but has most leaves in opposite pairs (not in threes), larger corollas (55-65 mm across), a less strong odour and leaves that are more hairy beneath, at least when young (see also Lamant 2006). Two additional species (Catalpa bungei C.A. Mey. and C. fargesii Bureau) are much rarer in cultivation and their escape is less likely.
Saplings of Catalpa are frequently confused with Paulownia. Both are indeed rather similar. However, leaves are always opposite, much larger (often ca. 50 cm across in young trees) and more hairy in Paulownia, while mostly in whorls of three, subglabrous (but often glandular-sticky!) in Catalpa. Moreover, in Paulownia twigs are chambered (or even hollow) while Catalpa has twigs with continuous pith (see also Nowack 1987 for differences between both genera).
The genus Catalpa nowadays is very popular in cultivation in western Europe. It is one of the most frequently planted trees in public as well as private greenery in Belgium. Most taxa seem to produce viable seed and, under undisturbed circumstances, a future naturalisation of several taxa seems very likely. However, saplings in urban areas are seen as undesirable weeds and readily removed. Records of self-sown plants are steadily increasing, not only in Belgium but in most of the surrounding countries as well, see for instance: Schmid (2005), Brandes (2011), Smettan (2011), etc.
1. Young leaves brownish to dark purplish, often slightly lobed. Corolla ca. 15-25 mm across === Catalpa x erubescens
1. Young leaves green, entire or shallowly lobed. Corolla ca. 25-55 mm across === 2
2. Leaves usually 3 to 5-lobed or with obvious teeth at base. Corolla ca. 25 mm across, ground colour often pale yellowish or cream === C. ovata
2. Leaves entire or rarely very shallowly lobed. Corolla 40-55 mm across, ground colour white === C. bignonioides
References:
Brandes D. (2011) Neufunde von Neophyten im Stadtgebiet von Braunschweig. Braunschweiger Naturkundliche Schriften 10(1): 79-96. [available online at : http://rzbl04.biblio.etc.tu-bs.de:8080/docportal/servlets/MCRFileNodeServlet/DocPortal_derivate_00021233/Brandes-Neufunde_Neophyten_Stadtgebiet_Braunschweig.pdf]
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.
Kirkbride J.H. & Olsen R.T. (2011) Identity of Catalpa tibetica (Bignoniaceae). J. Bot. Res. Inst. Texas 5(2): 625-631. [available online at : http://www.brit.org/sites/alienplantsbelgium.bedefault/files/public/BRIT%20Press/JBRIT5%282%29/625-631_KirkbrideOlsen_Catalpa_JBRIT5%282%29625-631_20c.pdf]
Knees S.G. (2000) Catalpa. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 345-346.
Lamant T. (2006) Catalpa speciosa et C. bignonioides, deux espèces americaines difficile à distinguer. Bull. Ass. Parcs Bot. France 41: 36-37.
Nowack R. (1987). Verwilderungen des Blauglockenbaums (Paulownia tomentosa [Thunb.] Steud.) im Rhein-Neckar-Gebiet. Floristische Rundbriefe 21(1): 23-32.
Olsen R.T. & Kirkbride J.H. Jr. (2017) Taxonomic revision of the genus Catalpa (Bignoniaceae). Brittonia 69(3): 387-421. [available online at: https://link.springer.com/article/10.1007/s12228-017-9471-7]
Paclt J. (1952) Synopsis of the genus Catalpa (Bignoniaceae) III. Candollea 13: 241-285.
Roloff A. & Bärtels A. (2006) Flora der Gehölze (2e Auflage). Ulmer, Stuttgart: 844 p.
Schmid M. (2005) Untersuchung zur neophytischen Gehölzflora im Stuttgarter Stadtgebiet. Jh. Ges. Naturkde. Württemberg 161: 178-257.
Smettan H.W. (2011) Gebietsfremde Gehölze auf den Fildern. Jh. Ges. Naturkde. Württemberg 167: 409-447.
Wharton A.P. (1981) The foxglove trees. Davidsonia 12(3): 57-61.