The effect of daily light integral on bedding plant growth and flowering

Daily light integral (DLI) describes the rate at which photosynthetically active radiation is delivered over a 24-hour period and is a useful measurement for describing the greenhouse light environment. A study was conducted to quantify the growth and flowering responses of bedding plants to DLI. Eight bedding plant species [ageratum (Ageratum houstonianum L.), begonia (Begonia x semperflorens-cultorum L.), impatiens (Impatiens wallerana L.), marigold (Tagetes erecta L.), petunia (Petunia x hybrida Juss.), salvia (Salvia coccinea L.), vinca (Catharanthus roseus L.), and zinnia (Zinnia elegans L.)] were grown outdoors in direct solar radiation or under one of three shade cloths (50, 70 or 90% photosynthetic photon flux (PPF) reduction) that provided DLI treatments ranging from 5 to 43 mol(.)m(-1.)d(-1). The total plant dry mass increased for all species, except begonia and impatiens, as DLI increased from 5 to 43 mol(.)m(-2.)d(-1). Total plant dry mass of begonia and impatiens increased as DLI increased from 5 to 19 mol(.)m(-1.)d(-1). Impatiens, begonia, salvia, ageratum, petunia, vinca, zinnia, and marigold achieved 50% of their maximum flower dry mass at 7, 8, 12, 14, 19, 20, 22, and 23 mol(.)m(-2.)d(-1), respectively. The highest flower number for petunia, salvia, vinca, and zinnia occurred at 43 mol(.)m(-2.)d(-1). Time to flower decreased for all species, except begonia and impatiens, as DLI increased to 19 or 43 mol(.)ml(-2.)d(-1). There was no consistent plant height response to DLI across species, although the shoot and flower dry mass per unit height increased for all species as DLI increased from 5 to 43 mol(.)m(-2.)d(-1). Guidelines for managing DLI for bedding plant production in greenhouses are discussed.