The purple nutsedge,
Cyperus rotundus
L., is a widely spread weed and of difficult control. The effect of light quality on its growth was determined to improve its control. Bulbs were germinated under darkness in the laboratory and light at 15 days, with three leaves and 1-7 g, they were carried to the greenhouse. A completely randomized design was used. In greenhouse with sandy clay soils, three replications and three treatments. All treatments had a layer of white greenhouse mesh, two layers of white cellophane (MB), one green (MV) and one red (MR). Average temperature was 28ºC (20-35ºC), average relative humidity 60% (20-100%) and solar radiation 250 to 300 g. cal. cm
-2 day
-1. Five samplings were made each seven days from the aerial and underground sections of the plants. After 28 days of growth, blooming took place in the treatment MB and a smaller number of leaves and dry weight of the aerial and underground parts were observed in the treatments MV and MR. After 73 days, the number of leaves and foliar area were smaller in treatment MB, while in the other treatments neither inflorescences nor fertile seeds were observed but formation of a larger number of corms and leaves were stimulated, MR being greater. It is concluded that light quality is important in the reproduction and propagations of the purple nutsedge, hence the application of green or red shade in combination with an herbicide or biological control becomes an alternative for control of this species.
