***This is the second installment in a four part series discussing ideal tea growing conditions. For Part 1, please click here.
Marginal conditions occur when one or more of these ideal growing conditions is not met and where major corrective action needs to be taken to obtain a commercial yield. The symptoms of marginal conditions, however, vary from the obvious to the hidden, and the effects from debilitating to death. In the last 100 years of science based tea growing we have learned many techniques for minimizing the effects of less than ideal conditions – irrigation, shading, fertilizer, land drainage, soil acidification, all spring to mind – but be aware that though effective such techniques increase the cost of production, so have to be factored into your business plan.
Some examples and typical remedies for common marginal conditions are illustrated below under the heading Climatic. Soil Conditions and Genetic options will be covered in a future post.
a) Altitude/Latitude. The ideal conditions for Camellia sinensis, and particularly var. assamica, are found at high elevation (6000 ft) in equatorial latitudes. High altitude is always assumed to be best – “good teas come from high elevations” – but while this is true for quality it is not necessarily so for profit as yield often drops as quality increases. Even in Kenya yield is very altitude dependent (Stephens showed a 50% higher yield at 5,500 ft compared with 7,000 ft in the Kericho district of Kenya) and, increasingly, climatic conditions become marginal for growth as the latitude extends north and southwards. Thus, the most suitable height for growth declines away from the equator – 5,500 ft at 0 deg, 3,800 ft at 7 deg N, 2,500 ft at 15 deg N and around sea level above 30 deg. Latitude 40 deg N and S is about the limit for commercial tea, with marginal growing areas descending to sea level elevation – Turkey, Georgia, Tasmania – and then only using the cold hardy China jats with a six month season. Cold tolerance (this keeps your bushes alive) and season extension (this improves commercial yield) must rely on future cold tolerance breeding to overcome extreme temperature marginality.
Latitude for a given altitude affects daylength, light intensity and temperature, with local modifications depending on sea proximity. For example the embryo tea industry in North Pakistan at 36 deg N contends with frost and snow in winter, with cold dormancy between October and April but with drought and 100°F plus day temperatures before the July monsoon. The growing season is 180 days at 3,000 ft, but shortens to 165 days at 4,000 ft with a yield reduction of 10%. In this area the smallholder farmers can only grow frost hardy China types (raised as VP clones) as the Assam types die in the nursery despite using double layers of polythene as winter insulation.
b) Drought. Many tea growing areas suffer from a shortage of rain, poor rain distribution, and drought. In Pakistan for example the annual rainfall is marginal at 49” (last 5 year average) and not equally spread (the monsoon contributes 40% of this amount during six weeks). There is a hot drought in June, and cold drought in November. Also, there are extreme cyclic climatic variations to contend with. When originally tested for tea growing the 10 year average was suitable for tea at 65”.
Lack of water also reduces air humidity to the extent that bud growth is limited – no buds means no leaf which means no profit. The saturation deficit can rise to 7 kPA in the hot dry unproductive period in Malawi. Irrigation may be given in these rainfall marginal conditions (assuming water is available, suitable in quantity and in pH) but irrigation will not entirely replace natural rainfall neither can it adequately replace humidity. The type of irrigation employed (static or moving sprinkler, furrow, flood, drip) will influence its efficacy and cost.
b) Frost and Hail. These are adverse but intermittent climatic effects. Frost often affects areas edging into climatic marginality. The higher elevations in Kenya get frosted in spring as does the early season in Japan. Alleviation varies from thatching bushes with grass – used for example on young tea in the hills of Nepal and Darjeeling, to electric frost fans, used in Japan to circulate and mix the air over tea bushes at night. The use of suitable genetic material – China types rather than the broad leaved assamica – can also reduce frost damage.
Hail causes heavy losses of young flush (estimated to an average 10% of crop loss in Kenya). Very large hail cannon have been used for protection in South Africa – to drop hail away from the tea estate – but not particularly successfully.