7) Characteristics and conditions of sweetpotato cultivation

(1) Land suitable for cultivation

[Temperature]

  • Sweetpotatoes receive chill-related injury at temperatures below 9°C, become dormant at 10-15°C, and are able to germinate and form roots at temperatures of 16°C and above. Growth improves as temperatures rise but declines at 35°C and above.
  • Conditions suitable for growing sweetpotato include having an average temperature in the hottest month of at least 22°C, annual average temperature of at least 10°C, and cumulative temperature over the growing season of 3,000°C. Regions suitable for commercial growing include Fukushima and Miyagi (in northeast Japan), Niigata (the Hokuriku area) and areas to the south of Niigata, Nagano and Gunma Prefectures located no higher than 700 m above sea level.
  • If planted early at temperatures of 15°C or below, sprouts will root poorly; subsequent growth will be even worse. At excessively high temperatures, root tissues lignify, storage root counts decline, and roots take on a pencil-like appearance.

[Sunshine]

  • Insufficient sunshine during planting stops root growth and delays storage root swelling.
  • Optimal storage root formation occurs with 11-12 hours of sunshine daily. With 7 or fewer hours of sunshine, storage root swelling is inhibited. Poor sunshine at the end of the growing season results in markedly lower dry matter production.
  • Reaction to sunshine hours varies by variety. Early growing varieties are less affected; late growing ones, more affected.

[Precipitation levels and soil moisture]

  • About 470 mm of rainfall over the growing season and soil moisture levels of 60-70% are regarded as optimal.
  • Sweetpotatoes have good resistance to dry conditions and can grow well in sandy land, volcanic ash soil or on hillsides, but dry weather after planting will cause poor rooting. Also, droughts in August (i.e., during the growing season) result in lower dry matter production.
  • Sweetpotatoes are extremely vulnerable to standing water and flooding. On excessively wet land, drainage must be improved by use of high ridging and abundant composting.

(2) Sweetpotato formation and environmental conditions

  • Roots generated from nodes swell and form tubers. However, not all roots form tubers; instead, they diversify depending on the development of the potato's cambium and storage tissues.
  • Roots consist of an epidermis, cortical layer and stele. Young roots, in which the ligneous parenchyma cells of the stele lignify less while the cambium develops well, differentiate into tubers and begin swelling.
  • If the cambium is active and lignification is abundant, pencil-like roots are formed. By contrast, if the cambium is inactive roots become thin - even at lower levels of lignification.
  • Insufficient sunshine, low temperatures, excessive humidity, excessive nitrogen and hard soil all prevent tubers from swelling. Adequate rainfall, high temperatures, abundant sunshine, potassium application and soft soil promote storage root formation and swelling.

(3) Soil conditions

  • For root growth and storage root development, it is necessary to maintain sufficient oxygen levels in the soil. Air permeability is important; if the soil is solid and hard, the oxygen supply becomes insufficient and excessive vine growth occurs.
  • In heavy, clay-like soil with poor air permeability, pay attention to ridge height and soil improvement, and apply compost.
  • When sweetpotatoes become dry, they tend to become round; when wet, they become long. In extreme dry or wet conditions, the skin color fades and the flesh deteriorates.
  • Sweetpotatoes have strong resistance to acidic soil. For pH levels from 4.2 - 7.0, growth levels and yields remain unchanged.
  • In thick, cultivated soil, the potato's above-ground and underground parts both grow well; storage root shapes are regular and quality improved. However, varieties less resistant to fertilizers may undergo excess vine growth due to surplus nitrogen levels.

(4) Progress of growth

  • In tropical regions, adventitious roots are generated merely by cutting the tip of vines and planting them in the soil; these extend to form tubers, because sweetpotatoes are perennial.
  • In temperate regions, by laying seed sweetpotatoes in a warm bed, maintaining fixed temperature levels and providing water, sprouting will occur in 5-7 days at 25-30°C; after 40-45 days, 20-30 sprouts can be obtained from a single seed sweetpotato.
  • If conditions are appropriate, planted sprouts will issue roots from the base of a petiole within 1 week and take root. After rooting, roots grow rapidly over the next 2 weeks.
  • There is a primordium located at the base of the petiole. While a large primordium grows quickly and thickens, a small one grows slowly. Over 3 weeks, the distinction between the two types becomes clear; at this point, thin roots become absorptive roots which absorb fertilizer.
  • One month after rooting, as the temperature rises and the potato's vines grow quickly, leaves become larger and more abundant to promote photosynthesis. Below-ground, tubers swell while the absorptive (thin, young) roots grow and absorb nutrients.
  • Vine growth peaks in August and September, actively sending photosynthesis products into the roots while tubers swell as they store starch.