The Effect of Daily Light Integral on Bedding Plant Growth and Flowering

Abstract

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 × semperflorens-cultorum L.), impatiens ( Impatiens wallerana L.), marigold (Tagetes erecta L.), petunia ( Petunia × 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 –2 ·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 –2 ·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·m –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.