Adequate lighting is essential for plant growth. The amount of light needed depends on the requirements of the plant and varies with different species. Many weed species, particularly floating and emergent forms, grow well under a range of natural light levels but usually grow best in full sunlight. The amount of light and the spectrum received by the plant will often dictate the growth form. Water hyacinth development is related to the light intensity and the red/far-red wavelength ratio (z)
(Méthy et al.
1990). Low light intensity and low z independently reduce growth rate and ramet production and increase petiole length and lamina area
(Méthy et al.
1990; Méthy and Roy
1993).
Ideally, natural light is the best and least expensive form of lighting, particularly in tropical and sub-tropical regions where day length is long all year round. In this case most plants can be simply grown outdoors in pools or dams in full sunlight, or covered with shade-cloth to regulate light levels. In temperate regions supplementary artificial lighting may be necessary in winter when day length is shorter. Forms of artificial lighting used vary with the amount of light required and setting up and running costs. Fluorescent tubes are the cheapest form with a wide variety of different tubes available depending on the wavelength required.
'Horticultural' or 'Growth' tubes are the most common type, designed specifically for growing plants. Other, more expensive lighting types, with greater light output per unit area,
include metal halide and high density sodium lamps.
