Greenhouse - Response to Light Levels

Light levels can have dramatic affects on certain bedding plants. The following is an article on the subject from the Michigan State Greenhouse Alert newsletter.

Flowering of bedding plants can be influenced by greenhouse light conditions, specifically irradiance. Irradiance is the amount of light reaching a plant at any given moment in time. It is an instantaneous value and is measured in units called micromoles over a square meter of greenhouse space (µmol·m-2·s-1). Growers can use a number of hand-held light meters to measure irradiance in the greenhouse. Depending on the meter used, measurements may be available in footcandles or µmol·m-2·s-1. One µmol·m-2·s-1 of sunlight approximately equals 5 footcandles of light.

Bedding plants may have a facultative irradiance response where high light levels promote faster flowering or an irradiance indifferent response where high light levels do not affect flowering. Species exhibiting a facultative irradiance response flower faster because the juvenile stage of development is shortened under high light levels. As a result, plants form fewer leaves before initiating flowers than those grown under lower light levels. For example, Salvia farinacea has a facultative irradiance response. Plants generally form 24 leaves before flowering under long days yet form only 18 leaves before flowering when grown with an additional 150 µmol·m-2·s-1 (approximately 750 footcandles) of light.

It is important to note that temperature influences the rate of development, and plant temperature of species in both response groups can increase under high light levels. Plants exposed to direct sunlight can be 5 to 7ºF warmer than the surrounding air temperature. Although elevated plant temperature due to high light conditions does contribute to faster flowering, it does not influence the number of leaves formed below the flower.

Facultative irradiance plants

Centranthus
Cleome
Cosmos
Flowering tobacco
Gazania
Lavatera
Poppy
Silene

Irradiance indifferent plants

Ageratum
Stock
Amaranthus
Thunbergia
Celosia
Verbascum
Dianthus
Zinnia
Gomphrena
Lobelia
Mimulus
Statice

Reprinted from "Light and flowering of bedding plants" by Beth Fausey, OSU Extension ABE Center in the February 17, 2006 edition of the Michigan State University Greenhouse Alert newsletter.