There are often plants in grand parks and gardens that puzzle and fascinate. Swamp cypresses, which are trees that can grow in waterlogged soils and produce knobbly growths around their base, certainly fit into that category. They are also conifers but do not have that traditional evergreen look. Rather, their leaves look fern-like, turn a lovely russet brown in the autumn and fall in the winter.
Taxodium is a genus consisting of just two species. Both are generally deciduous in Britain, but the trees can be evergreen in their native habitats of southern USA, Mexico and Guatemala. The main species grown here is T. distichum, commonly known as the swamp cypress. It becomes a conical tree, up to 40m tall, and is happiest grown by water. It has soft, pale-green needles that because of their placement resemble a pinnate leaf. The leaves of Taxodium are spirally arranged along the shoots.
People often confuse the swamp cypress with the dawn redwood (Metasequioa glyptostro - boides). But rather than spirally-arranged leaves, the redwood has opposite leaves and little pegs remain after they have fallen.
The most distinctive feature of Taxodium is the "knees", or pneumatophores, found on mature specimens. These are woody growths that arise from the roots and project above the ground and that for many years were believed to be an adaptation to swampy conditions, allowing the tree to access more oxygen. However, they do not develop structures like lenticels, which would help them do this. Recent research, published in the journal of the American Conifer Society, suggests they instead provide greater stability.
The other, very similar species is T. mucronatum, the Mexican or Montezuma cypress. It is not widely grown in Britain, as it is hardy only in the mildest areas. A third, T. ascendens, the pond cypress, has been reclassified as a subspecies - T. distichum var. inbricarium - and gives rise to the popular variety 'Nutans'.
Taxodium prefers moist, acid soil, but it can tolerate a range of soils, except one that is chalky. While T. distichum is hardy, the shoot tips are vulnerable to frost damage so frost pockets should be avoided, though the tree will recover if damaged.
Unlike most conifers, Taxodium responds well to coppicing, so it is used as a timber tree. But perhaps the most startling fact is that a specimen of T. mucronatum in the town of Tule, Mexico, has one of the widest trunks in the world - in 1982 its circumference was recorded as 36m.
WHAT THE SPECIALISTS SAY
JONATHAN TATE, owner, Lime Cross Nursery, East Sussex: "The larger Taxodium species and varieties are good trees for an arboretum and the dwarf types are good as collector's items. In the States, they are also often used as street trees.
"It's a small genus and there aren't that many cultivars worth getting excited about. The exceptions are 'Peve Minaret', a slow-growing, compact, narrowly pyramidal form. It's a pretty little deciduous tree with beautiful brown autumn colour. It's a good plant for the garden or landscape.
"'Cascade Falls' is certainly worth a mention. It would look best next to a pool, to show off the weeping habit. I also really like the pond cypress, T. ascendens 'Nutans'. It's a very pretty, slow-growing ornamental tree. T. mucronatum can probably only be grown in southern areas. It wants a sheltered spot and is semi-evergreen."
STEVE DANCE, nursery manager, Burncoose Nurseries, Cornwall: "Taxodium is a lovely, statuesque tree, which ultimately grows up to 40m. They prefer full sun or partial shade, don't suffer much in terms of diseases and are one of the rare conifers that enjoy damp conditions.
"I like the variety 'Nutans', which was awarded an Award of Garden Merit (AGM) and has more pendulous branches than straight T. distichum. I also recommend newish, pendulous variety T. distichum 'Falling Waters', which has nice autumn colour, and the variety 'Secrest', which is a slow-grower, suitable for heather or rock gardens. It's wider than it is tall."
ROBERT VERNON, co-owner, Bluebell Nursery & Arboretum, Derbyshire: "Taxodium is a lovely plant and grows to be a handsome tree, particularly in wet areas of the garden. Waterlogged conditions increase the likelihood they will form pneumatophores, large woody 'knees' that are eye-catching and ornamental in their own right.
"Once established, they are quite hardy, although young plants can suffer from a little dieback after cold winters. Frost pockets should be avoided to reduce the risk of this winter damage.
"We produce and sell a number of smaller cultivars that make great garden plants, especially for planting near ponds or streams, although they tend to grow too slowly and stay too small to be suitable for mass landscape planting."
SIMON TOOMER, arboretum director, Westonbirt, Gloucestershire: "Swamp cypresses are dramatic trees, especially by water. They are an excellent arboretum specimen tree and look good planted in clumps. They have beautiful fibrous bark and tawny autumn colour
"The most commonly grown is T. distichum. We grow it here and it does pretty well. We have more problems with T. ascendens. They are trying to grow it around the pond at Bedgebury. The effect of climate is quite subtle. T. distichum is hardy, but the trees like quite a bit of sunshine.
"T. mucronatum is not widely grown in this country because it's tender in many areas. It becomes a big tree in its native country, Mexico, but will not reach the same stature in Britain."
SPECIES AND VARIETIES
T. distichum AGM (H4), or the swamp cypress, is a tall, conical tree with fibrous, red-brown bark. It has soft, pale-green needles, arranged into pinnate leaflets spirally arranged on the shoots. They turn a rich reddish-brown in autumn before they fall. Height: 30-40m.
T. distichum 'Cascade Falls' (PBR) is a weeping form that originates from New Zealand. It has soft, green foliage in the spring that turns rich golden-brown in the autumn. This variety looks particularly lovely when allowed to weep gracefully over a pond or stream. Height: 3-4m. Spread: 2m.
T. distichum 'Falling Waters' is a new introduction with an attractive pendulous growth habit and excellent autumn colour. Height: 4-7m. Spread: 3-4m.
T. distichum 'Hursley Park' is a dwarf, bushy swamp cypress. The green summer foliage turns a deep bronze in autumn. Avoid dry soils. Height: 1.2m in 10 years.
T. distichum var. imbricarium (previously known as T. ascendens) is a small to medium-sized tree of narrowly conical or columnar habit, with spreading branches and erect branchlets. The leaves are bright green at first, then rich brown in autumn. Slower growing than T. distichum. Height: 10-20m.
T. distichum var. imbricarium 'Nutans' AGM (H4) is a columnar tree with spreading or ascending main branches. Its crowded branchlets are erect at first but later nodding, which is the meaning of 'Nutans'. The leaves are bright green and turn an attractive shade of bronze before falling in autumn. Height: 10-20m.
T. distichum 'Peve Minaret' is a beautiful, slow-growing tree with a columnar habit. It remains small, so can work in the smaller garden. Has bright-green foliage and rich-brown autumn tints. Height: 2m. Spread: 1m.
T. distichum 'Schloss Herten' is a compact, shrubby swamp cypress with bright-green foliage. It also has good bronze autumn colour. Height: 1m in 10 years.
T. distichum 'Secrest' is a flat-topped, dwarf variety with bright green foliage that ends up wider than it is tall. Suitable for heather or rock gardens or as a bonsai plant. Height: 90-120cms in 10 years.
T. distichum Shawnee Brave = 'Mickelson' is a perfectly pyramidal tree with delicate, feathery foliage and ascending branches. Great orangey-red autumn colour. Not widely available in the UK.
T. mucronatum, or the Mexican swamp cypress, has soft, green ferny leaves that are semi-persistent in warm areas. Not fully hardy in Britain. Height: 3-4m in 10 years, ultimately 20-30m.