Climate expectations

From citrus crops to soil erosion, climate change promises mixed benefits for UK horticulture.

Frost: deciduous and woody perennials require winter frost to provide deep dormancy, which is being lost as the UK's average temperatures rise. Image: Geoff Dixon
Frost: deciduous and woody perennials require winter frost to provide deep dormancy, which is being lost as the UK's average temperatures rise. Image: Geoff Dixon

In Britain, increased levels of carbon dioxide raising photosynthesis should favour growth and yield. Additionally, a moderate temperature rise will support that increased growth. Water shortages are most likely to be horticulture's limiting factor. Soil evaporation rises by two to three per cent for each 1 degsC temperature rise. As a result, we should get hotter, drier summers and warmer autumns.

Winters should be later and milder but with vicious cold spells that are unpredictable and sporadic, accompanied by sharp desiccating north-easterly winds especially in February and March. Rainfall will become much more variable and unreliable, frequently delivered in very heavy downpours.

The variation between the past three seasons typifies this increasing unreliability. These conditions blur the divisions between winter and spring. Northwards and westwards in Britain, conditions become milder and wetter compared with the present. The key problem for horticulture is increasingly variable weather. In such environments, plants and businesses fail.

Cold shortage

Britain's deciduous woody and herbaceous perennials need winter frost to provide deep dormancy, which is being lost.

Flowering times of bulbous and deciduous woody species have advanced by a month in the past 30 years. Sheets of early flowering snowdrops look wonderful but bring uncomfortable consequences. Soft, early bud growth that has broken dormancy too quickly is killed by even the shortest spells of freezing weather. This growth is not replaced for 12 months and the current season's fruit and seed production is lost.

It is likely that "forest giant" trees - oak, beech, elm - suffering repeated shoot loss will die out, damaging still further the structure of English landscapes. The earlier and weaker flower set of top fruit might be mitigated by using high-density planting under polythene or netting tunnels. Traditional unprotected apple and pear orchards would disappear but could be replaced by apricots, peaches and nectarines with their lower dormancy demands. The far South and West might even see citrus crops.

Wind

Late winter to early spring periods of intensely cold, north-easterly winds are disastrous for plants, particularly emerging seedlings. Even relatively low-speed winds carry soil particles that then abrade against leaves and stems. Such damage results in lower yields and poorer quality at harvest.

Winds twist seedlings, damaging stems at ground level. Wounded stems are wide open to collar and foot rot diseases. Odours from damaged plants attract pests. Powerful wind gusts collect and transport soil and seedlings hundreds of metres. Spring wind erosion is a major hazard for East Anglian and Lincolnshire vegetable growers. Wind adds costs for redrilling and loses profit through wrecked harvesting schedules.

Spring winds cause substantial damage in parks, gardens and macro-landscapes, which may lay hidden for several years. Root damage to large trees causes shoot dieback followed by pest and pathogen invasion. Trees and large shrubs in full summer foliage are very vulnerable to wind damage. Shallow-rooted mature trees such as beech are felled and killed. Fruit near to picking is downgraded by whiplash damage.

Drought

Water shortages will be one of the earliest problems stemming from climate change. Parks and gardens can adopt Mediterranean styles. This means using bulbous or tuberous types, trees and shrubs with thick cuticles, hairy leaves, deep-seated stoma, roots with extensive mycorrhizae and deeply penetrating habits. Lush, well-watered English 18th- and 19th-century parklands will suffer. Retaining their rural Arcadian charm requires reversion to wildflower meadows managed with traditional hay-cuts without mowing until early September. Clinging to the grassy elegance of English lawns will be signals of moral and social decadence.

Commercial growers will need to respond to water shortages and rationing. Irrigation of field vegetables and soft fruit crops becomes the key to business survival and success. The rapidly rising cost of water will force the use of trickle irrigation and fertigation. Capital outlays on underground feeder pipes and maintenance costs are high but outweighed by effective water use measured by yield and quality per litre applied.

The business aim is finding higher added-value crops offering good returns. This means more variations on the themes of salads, baby leaf and other edible foliage crops. By 2050, there could be political disincentives against exporting water into Britain from countries suffering severe drought, such as Africa. Crop management ensuring minimal losses and maximum resource use efficiency will be enforced.

Devastating soil erosion results from modest rain falling onto bare soil. Soil erosion in Britain is set to double by 2050. Limiting the use of erosion-prone crops is one strategy.

Pests and diseases

Climate change has already brought alien pests and diseases such as diamondback moth (Plutella xylostella), bacterial black rot (Xanthomonas campestris pv. campestris) and various Phytophthora spp. The combination of climate and husbandry change is exacerbating indigenous diseases such as clubroot of brassicas (Plasmodiophora brassicae).

Warm, moist winter soils allow continuous microbe activity, maintaining soil inoculum levels that would otherwise drop in freezing weather. Numerous foliar pests and pathogens increasingly attack autumn vegetables, encouraged by milder conditions.

Vermin populations are rising fast. Warmer conditions for extended periods enable continued breeding without any need for hibernation. Rats are causing major problems in field vegetables. Means of control such as poisoning and shooting are limited, especially close to crowded suburban areas. Woodpigeons are similarly increasing. These animals spoil produce at the point of harvest with urine and excrement frequently containing bacteria pathogenic to humans.

Warmer conditions will encourage insect pests, such as aphids and caterpillars, to grow and reproduce more quickly. Populations of the common house fly (Musca domestica) - while not directly a crop pest, it is a disease vector and nuisance - will rise by 250 per cent by 2080. More aggressive strains of pest and pathogen strains will develop. Unpredictable variations in rainfall reduce the weather-windows for spray application, making control more difficult.

Crops

Provided irrigation is available for horticultural crops and landscapes then, until about 2050, Britain could enjoy an increased range of home-grown crops and greater diversity of environmental plantings.

The need for food security based on home supplies could well result from export restrictions where countries are suffering severe losses in cropping. One likely benefit, certainly in southern Britain, could be more viticulture. Already there is a well-established industry of 1,000ha deliberately targeting the premium-quality market. English vineyards, such as Nyetimber and Riverview, now have excellent wines. As Kent's terroir closely resembles that of Rheims, "champagne" production is a possibility. Salisbury Plain could contain extensive vineyards.

Science-driven mitigation

Climate change offers both British production and environmental horticulture substantial opportunities. Britain can use plants previously viewed as too tender and add to public enjoyment of macro - and micro - landscapes, and provide better diets.

The industry will be more vulnerable financially because of reduced net margins, population pressures and the use of rural land for housing and associated infrastructure. Increased exploitation of crop breeding linked with improved environmental and husbandry management is seen now outside Europe as crucially important in mitigating climate change. Britain already has considerable knowledge of the molecular basis of inheritance only waiting to be used. Deploying this basic knowledge into applied science and technology demands political understanding and social willingness.

For the past generation, governments worldwide have expected that the forces of a free market alone will provide knowledge transfer in horticulture. This has not happened as the free market system is incapable of providing knowledge transfer into micro and small enterprises effectively. Providing adequate numbers of properly educated and experienced crop specialists capable of translating science and technology into horticultural practice is an essential part of coping with climate change and one that the Government must shoulder.


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