Oranges and Lemons say the Bells of St Clement’s

CITRUS covers a wide range of wild, natural species, such as mandarin (Citrus reticulata) and pomelo (Citrus maxima), and ancient hybrids like sweet orange (Citrus sinesis) and lemon (Citrus limon) around for so long (thousands of years) that no one actually knows how and when they arose.

More recent natural hybrids like grapefruit (Citrus paradisi), thought to be an accidental cross between pomelo and sweet orange on the Caribbean island of Barbados, are cultivated alongside a myriad of man-made hybrids like Jamaican tangelo, also called ugli fruit and generated by crossing grapefruit and mandarin. All are paid-up members of the plant family Rutaceae. All very interesting you may say, but what is the relevance to arboriculture in the United Kingdom? Quite a lot actually; both historically and for the future, provided climate warming follows current predictions.

Lemon (Citrus limon) is a high-acid citrus grown in classic Mediterranean-type climates, but not in the Equatorial tropics where lime (Citrus aurantifolia) is grown instead.

Citrus was first grown in England inside an orangery (a large conservatory where trees were protected during winter). Peak period was 17th to 19th century, with first reference by John Parkinson who introduced the orangery to readers of his Paradisi in Sole in 1628. The oldest surviving example is the orangery at Kensington Palace, designed and built by Sir William Chambers in 1761. At 28 m (92 feet) long, it was the largest glasshouse in England at the time.

The orangery concept evolved into contemporary conservatories filled with a variety of warmer-climate plants now sold in containers at garden centres and superstores. These include custom-bred dwarf citrus varieties like the calamondins, laden with fruit despite the tiny tree size. Calamondins are called acid oranges, golden limes and also Philippines limes because they are widely grown in this Southeast Asian country in standard tree form and size. The fruit is used to season soups, sauces and seafood dishes. These container-grown trees can be placed on the UK patio in summer, although householders are strongly advised to bring them indoors during winter.

But are we, especially in southern England, too precious about citrus in general and its apparent inability to survive the UK winter? Indeed there are a number of well-established citrus trees in London growing outdoors and fruiting, including the famous Queenie grapefruit tree in the Chelsea Physic Garden in South London. A healthy citrus seedling named ‘Aunt Queenie’ was planted in 1948 and kept in a pot until 1990, when the original owner died. It is now growing in the Physic Garden, where it has fruited regularly since 1998. It is in the corner of a sheltered walled garden but is nevertheless 600 miles north of the nearest commercial citrus-growing area in the European sector of the Northern Hemisphere.

Kumquat (Citrus japonica) is ideal as container-grown plants due to the intrinsically small-sized fruit.

UK gardeners have already taken to olive, another warm temperate/subtropical fruit tree, with trees now adorning patios across England and surviving 365 days, year in, year out, so why not citrus? Olive is grown in English gardens with no real hope or expectation of harvesting the edible fruit, but for the novelty and beauty of the tree and its foliage. In the same way mainstream citrus plants like oranges and lemons will not, in most situations, produce edible fruit, the UK growing season is too short and not sufficiently warm – but there are other reasons for trying to grow citrus trees.

The evergreen leaves are absolutely superb, rich green in colour and covered in a thick layer of wax, making them bloom in the sunlight and hold onto beads of water after rainfall because the water is repelled by the hydrophobic wax bloom; and white citrus blossom offers some of the sweetest smelling flowers on planet earth.

At the end of the day, commercial citrus grown in Florida and Brazil will periodically experience severe frost. The current year’s fruit crop may well be lost but the trees will recover. No one is saying that sustaining citrus trees in the English climate will be easy, but is entirely possible nevertheless. I suspect that citrus as a candidate tree for UK arboriculture has never really been tried. And this is almost certainly because we have grown up being told these trees cannot survive an English winter, whereas Queenie Grapefruit and others in London clearly show the opposite, even if some careful and clever plant siting and protection have to be used.

Orange blossom – one of the sweetest and most fragrant flowers on planet earth.

And don’t forget there is wide variation in cold tolerance exhibited by different citrus species. For instance, lemons (Citrus limon), which are very popular in the suburban gardens of Adelaide, struggling in the Adelaide Hills only 20 km south-east of the city but rising to over 700 m above sea level at Mount Lofty and correspondingly much cooler. My daughter lived in the Adelaide Hills village of Bridgwater with a winter climate not much different to south-west England. She successfully grew cold-tolerant types of orange that produced barrels of fruit and made fine marmalade.

Visit Cambridge Botanic Garden and you will find a fine specimen of Citrus (Poncirus) trifoliata and called Japanese bitter orange though native to China and Korea. Citrus trifoliata is a deciduous shrub growing to 3 m by 3 m at a slow rate and bearing viscous spines. It is not frost tender and is known to survive temperatures as low as -20°C. Trees produce fragrant flowers pollinated by insects from April to May, autumn foliage is an attractive yellow and the fruit and seed ripen from September to November. But you don’t want to eat the small, yellow and slightly furry orange fruits, which are unpalatable due to the presence of bitter oil called ponciridin. However, there are clearly other reasons for growing this tree, which performs entirely satisfactorily in southern England at least.

Commercial citrus is routinely sprayed with copper fungicides such as cuprous oxide, the rusty-red coloured deposit of which is visible on these recently sprayed oranges.

Cambridge Botanic Garden where this tree is growing recently boasted the highest temperature ever recorded in England; 38.5°C (101.3°F) on 25 July 2019, beating the previous record set in Kent in 2003. By the same token, a temperature of -13.6°C was recorded in February 2012.

Citrus as fruit trees have been carried throughout the world over a colossal period of time, calculated at around 4,000 years. Mainstream citrus fruits like oranges and grapefruit are now grown in a wide range of climates, from the classic Mediterranean-type climate of Spain and Cape Province, South Africa through the subtropics of Florida and into full-blown Equatorial tropical environments.

Indeed, citrus is the most widely travelled and dispersed fruit-tree crop in the world, but this has consequences. Taken from their East Asian home, citrus trees and their leaves and fruits came up against insect pests and pathogens with which they had not co-evolved. As such they lacked natural resistance and, outside of their climate comfort zones, were even less able to cope with these new pests and diseases.

For instance, sweet orange grown in the wet and humid Equatorial tropics is prone to a range of very nasty-sounding diseases such as scab, melanose and greasy spot, caused by fungal pathogens which are Elsinoe fawcettii, Diaporthe citri and Mycosphaerella citri, respectively. Consequently, citrus growers are forced to spray their trees with fungicide, the most appropriate being copper-containing fungicides. Indeed, the main reason why UK consumers can buy blemish-free citrus fruit is because trees have been sprayed with copper-based fungicides which have been used perfectly safely in this way for at least 100 years.

Citrus suffers from a wide range of nasty diseases – oranges with scab disease on the fruit and greasy spot disease on the leaves.

Writing about fungicide spraying of citrus makes me wonder why FC Scotland have fiddled while Dothistroma needle blight burns our plantation pines – Corsican, lodgepole and Scots – and now the irreplaceable native, wild Scots pine in the Caledonian Forest. Aerial spraying trials using copper fungicide were initiated in 2013 and restarted in 2015, but apparently with no extension or mention since then.

Hesitancy is due to concerns over safety, but this is perverse and peculiar given that the British public eats citrus fruit from all over the world sprayed with the same copper fungicide as that trialled on pine trees in Scotland, or others with very similar specifications

But perhaps concern is for the integrity of tree epiphytes including lichen, algae, mosses, liverworts and even ferns. Tropical epiphytes grow rapidly and take over citrus trees, adding extra weight and causing physical and physiological damage. They are controlled incidentally by the copper fungicide sprays used to control true fungal pathogens and the dreaded fungus-like pathogen called Phytophthora. Not Phytophthora ramorum – that likes larch – but Phytophthora citrophthora and Phytophthora parasitica, causing a lethal bark necrosis called gummosis because of the resin-like ooze that issues from the bark. 

It would be perverse if UK plant health authorities fail to press ahead to save wild and plantation Scots pine for fear of upsetting the lichen lovers. If they do have a change of heart and resume the work then the most efficacious copper fungicide should be used. FC Scotland has used copper oxychloride containing 55 per cent weight/weight of fungicidally active copper. Cuprous oxide contains over 80 per cent weight/weight of fungicidally active copper as the divalent copper ion (Cu2+) and thus requiring less fungicide product to achieve an equivalent level of control.

Note: ‘Oranges and Lemons Say the Bells of St Clements’ is an old children’s nursery rhyme with roots in 18th-century London. It is thought to refer to a London church called ‘St Clements’ that was close to the docks where citrus fruit were offloaded.