Light

Karen Delahaut, Outreach Specialist, UW-Madison

Proper intensity, duration, and quality of light are important to maintaining healthy plants. Too much or too little light can have adverse effects on the plants you grow. When a plant receives as much light energy as it can use, it has reached the saturation point. This amount will vary with the species and any additional light over and above the saturation point will not be used and may actually be injurious. Become familiar with the necessary light levels for the plants commonly grown and know that these light requirements will vary based on species and stage of development.

Plants grown under low light conditions typically have larger, thinner leaves. Sun-loving plants grown under high light have much smaller and thicker leaves. Insufficient light levels can retard plant growth and flowering. Etiolated plants have spindly stems with long internodes. They may also be pale. Be sure to provide adequate spacing between the pots to assure each plant receives the necessary light for optimal growth.

Excess light can cause plants to become chlorotic or necrotic if intense enough. Sunburn is the result of too much light and heat and is worsened when plants are moisture stressed. Grow plants in their recommended temperature range and provide shade when sunlight is intense. There are several ways to shade a greenhouse during the brighter months of the year. The least expensive and simplest method is to whitewash the greenhouse. However the downside is that the shading provided by whitewash can’t be adjusted if there is a stretch of cloudy days. Shade cloth is effective and adjustable but can be difficult to apply and will require periodic cleaning. On the high end, a mechanical curtain system that is triggered by the amount of light absorbed by a sensor in a computerized monitoring system. It is the most flexible but is expensive and difficult to install.

The quality, or wavelength, of light is also important. Light high in far-red end of the spectrum will produce longer internodes and larger leaves while reducing leaf and flower color.

Some of the characteristics of excessive of insufficient levels of light are listed in the table that follows. It’s important for you to understand that adverse environmental conditions such as light levels can cause symptoms that mimic pest damage and you must properly identify the cause before you can consider your treatment options.