They also produce panicles of flowers in mid-to-late summer. The flowers are orchid-like in appearance with crêpey petals, usually white, yellow or pink, with attractive markings and often fragrant. Slender bean pods are formed later in autumn and can persist into winter.
The most commonly grown species in the UK are the two North American types, C. bignonioides Award of Garden Merit (AGM), known as the southern catalpa; and C. speciosa AGM, known as the northern catalpa. They are similar in appearance but speciosa is a slightly larger tree with correspondingly larger leaves, flowers and bean pods, while bignonioides is more floriferous. C. ovata is another popular ornamental tree. It is a Chinese species with yellow-tinged flowers.
There are several highly sought-after varieties. Perhaps the favourite is C. bignonioides 'Aurea' AGM. It has large, ovate, golden-yellow leaves that become light yellow-green in summer. A nice tree for small gardens is C. fargesii f. duclouxii AGM, which produces clusters of lovely rose-pink flowers marked with orange and purplish brown. It has elegant leaves that are brownish-black when juvenile.
A little care should be taken with positioning in parks and gardens. Because of the weight of their leaves, especially bignonioides, their branches can be pulled down and even break off.
This should not prevent horticulturists from planting catalpa, but take into consideration that large trees in an urban setting may need bracing with cables or frequent cutting back.
They respond favourably to pruning and even pollarding, so like paulownia plants can be stooled to produce an attractive head of foliage. Several varieties look particularly effective treated this way — the dark purpley-brown leaved C. ≈ erubescens 'Purpurea' AGM, C. bignonioides 'Variegata', which has green and cream margined leaves, and C. speciosa 'Pulverulenta', which has pale-green leaves speckled with creamy white.
Catalpas are generally hardy down to about -15°C and like an open, sunny site. They can be planted in a variety of soils but do best on moist, fertile loams. They are ideally planted in a sheltered position because exposed, windy locations will lead to the leaves being easily torn.
Protect young plants from frost and be aware that it takes some years before they produce flowers.
They cope very well with air pollution so are good city trees. An example of this is a row of mature specimens that stand outside the Houses of Parliament in Westminster. They have fabulously contorted trunks and are believed to have been planted in 1857.
What the specialists say
Mike Glover, managing director, Barcham Trees, Ely
"Catalpa is a great choice for exotic flower and leaf but only in sheltered and south-facing gardens. Too exposed a location and the large, fleshy foliage can bruise and shred in the wind.
"C. bignonioides 'Aurea' is the stand-out clone. There are few yellow-leaved trees on the market so this one is an important inclusion for this group. After flowering, true to its name it can set bean-like seed holders. If planted in paved or patio areas, these can be quite troublesome in terms of constant litter to be swept up. A well-drained soil and south-facing sheltered aspect are key for best results."
Stephanie Dunn James, assistant managing director, Trees for Life/FP Matthews, Worcestershire
"Catalpa are deciduous trees and shrubs admired for their showy leaves. They are very tolerant of urban pollution and thrive in towns.
"C. bignonioides 'Aurea' has a low-spreading habit with large, heart-shaped, golden, velvety, silky leaves and white flowers in the summer. There is also a more dwarf form,
C. bignonioides 'Aurea Nana'. It contrasts well with C. ≈ erubescens 'Purpurea', whose young shoots are dark purple, almost black, gradually becoming green to dark green.
"Occasional pollarding and hard pruning can contain the size of the catalpa plant and enhance the leaf size. Unripened wood can be susceptible to frost damage, so protect young plants."
Robert Player, proprietor, Garden Associates, London/Hertfordshire
"Catalpas are one of our favourite trees and I have planted them in nearly all my squares. Last year, to celebrate the jubilee, the journalist Kate Aide planted a terrific specimen of C. bignonioides 'Aurea' in St James's Square, Piccadilly. London squares suit them very well.
"Coming from America's Deep South, they need a bit of warmth and shelter. They're not a good tree for the colder climes of up north and won't tolerate a windy position as the leaves shred, but take air pollution in their stride. However, they rarely make old bones, most only reaching between 60 and 70 years, although there are exceptions. Hyde Park Gardens recently lost one well over 100 years old and it only fell over due to regular fox diggings finally undermining it.
"In an ideal world I would plant a very young specimen of maybe one or two years old as they exhibit so much vigour and the needs of the canopy would be reflected by the size of the developing roots.
"Pruning of these trees is important. A young tree can develop extension growth of up to 2m in a season, but most of this is quite late so it doesn't have time for the final 30cm or so to fully harden off before the winter, so consequently it is often without a terminal bud resulting in unwanted lateral growth and an unwanted amount of dead wood — a magnet for fungal activity.
"Winter pruning of these trees has become popular but care is needed if coppicing. In my experience they are not as responsive as paulownias but they do favourably respond to hard pollarding, as can be seen in the south of France where they are a street tree, their huge leaves giving much needed shade to those foolish enough to try and cross the road."
Species and varieties
C. bignonioides AGM (H6), or the southern catalpa, is a medium-sized deciduous tree with a spreading habit. It has broad, pale-green, heart-shaped leaves that are usually entire and end in a tapered point as well as showy white flowers resembling crêpey orchids, marked with orange and purple. The fruit is a long, slender, bean-like pod. Height and spread: 15-20m.
C. bignonioides 'Aurea' AGM (H6) is a form with lovely, velvety, bright-yellow leaves. Young foliage is a bronze colour. It is a medium-to-large deciduous tree with a spreading habit. Tall panicles of white flowers appear in mid-to-late summer, usually followed by slender pods.
C. bignonioides 'Nana' is a compact rounded bush or small rounded tree if grafted as a standard. It has large, heart-shaped, mid-green leaves and in ideal conditions yellow- and purple-marked white flowers in summer followed by slender pods.
C. bignonioides 'Variegata' produces soft, cream-edged leaves that are slow to appear. Pale purple and yellow marked foxglove-like flowers appear in June and July. It becomes a small, attractive, asymmetrical tree if left to grow but can also be stooled for large foliage.
C. bungei or the dwarf southern catalpa is a small tree with a rounded habit that produces white flowers with purple spots between late spring and midsummer. Does not produce masses of flowers. Only really thrives during hot summers. Height: 9m.
C. ≈ erubescens 'Purpurea' AGM (H6) bears highly attractive, large, chocolatey-purple young leaves that turn dark green as they age. It flowers freely even as a young plant, with masses of creamy white flowers flecked with yellow and dark brown in June and July. This variety is resistant to honey fungus. Requires a sheltered position in the north of the UK. Height: 10m. Spread: 8-10m.
C. fargesii f. duclouxii AGM (H5) is a rarely seen but spectacular small tree. It produces rose-pink flowers in midsummer marked with orange and purplish brown. It has elegant leaves that are slightly lobed with a narrow, pointed tip. They are brownish-black when they first emerge. Height and spread: 8m.
C. ovata, or the yellow catalpa, is so named because its flowers are creamy white with a definite yellow tinge. Borne in July and August, they are perhaps not as showy as the flowers of bignonioides or speciosa. Smaller than speciosa at 10m.
C. ovata 'Slender Silhouette' is a rarely found, narrow, upright form with dense foliage.
C. speciosa AGM (H6), the northern catalpa, is native to the Midwest USA. It is medium sized with large, heart-shaped leaves and white flowers marked with yellow spots and purple stripes. Its flowers are larger than the southern catalpa's, though there are fewer of them, and it produces slightly broader seed pods. Height: 15-30m. Spread: 12m.
C. speciosa 'Frederik' produces masses of white flowers in May and June. It has glossy, mid-green foliage. Height: 12-15m. Spread: 15m.
C. speciosa 'Pulverulenta' produces large, pale-green, heart-shaped leaves speckled with creamy white. Mature trees produce masses of creamy white, foxglove-like flowers with a purple and yellow speckled throat in June and July. Height and spread: 3m.