When to Light or Shade, and When Not…


Plants use light to increase mass through photosynthesis. More mass usually means higher quality and longer postharvest life. However, in many cases, more light enters a greenhouse than a plant can use. In the spring, extra light is great because it helps heat the greenhouse. However, in the far south, or during the summer, we often shade because this extra light heats a greenhouse above what is desirable. We can see that our plants are not growing as well; in addition, it’s often uncomfortably hot for us.

In recent years we have measured the effect of light, temperature and carbon dioxide (CO2) on photosynthesis of many bedding and potted plants. Using that data, we can make informed decisions on when we should light, or conversely, when we should shade our crops. This research has received support from the Floriculture and Nursery Research Initiative and the American Floral Endowment.

In this article we will present some standards that can be used for lighting or shading in different parts of the country, as well as during different stages of production. Some recommendations are based on new data, some from old data, and some from just seeing what works in commercial greenhouses. Initially, we will discuss the importance of light quality in some plant developmental processes, then the importance of light quantity on plant growth.

Light and Seed Germination

Some species require light to germinate. A light requirement for seed germination ensures that a germinating seed is near the soil surface, maximizing its ability to survive. Alternatively, some species will only germinate when not shaded by other plants. Why? In both instances, the color of the light that a seed is exposed to is important. Red light (660 to 680 nm) promotes seed germination in species that require light for germination. Sunlight naturally contains red light in it and will, therefore, promote germination. Leaves, on the other hand, filter out red light, which is the reason that leaf shading inhibits germination of light-requiring species.

Examples of light requiring species include gloxinia, some nicotiana, verbascum, some primula, carrot and some lettuce. In contrast to light requiring species, some species require darkness for maximum germination. Dark-requiring species (or species in which germination is enhanced by dark) include cyclamen, salpiglossus, nigella, datura and a number of other species.

We provide light in most seed germination chambers in order to maximize germination of light-requiring species. In general, we add fluorescent lights. Why? Because fluorescent lamps emit a lot of red light. The commercial standard for lighting in germination chambers is 2 to 10 footcandles from fluorescent or red LED lights. 

First Three Days After Germination

We have found that high light intensity immediately after seed germination, either directly through too much light and/or indirectly by heating up the media surface, can kill germinating seed or create a lot of non-uniformity — especially in mixes. Specifically, when direct sunlight shines on a plug tray, it can heat to temperatures above 90° F. When this happens, small-seeded species can have difficulty completing germination and/or establishing in a cell. For this reason, we strongly suggest that light intensity not exceed 2,000 foot-candles (400 umol∙m-2∙s-1) on small-seeded species the first three days after germination.

Light and Cool Temperature Induction of Flowering

Many species, including many perennials, Martha Washington geraniums and Easter lilies, are induced to flower by cool temperatures (32 to 63° F). ‘Vernalization’ is the term used to refer to the cool temperature induction of flowering. The common commercial treatment used to induce flowering on plants that require vernalization for flowering is six to 12 weeks at 42 to 46° F.  

We have found that whether or not a plant receives light during vernalization can affect whether a cooling treatment induces flowering in that crop. Red light during cooling can reduce the required cooling time to as low as four weeks. In contrast, far red light (given off by incandescent lamps; 720 to 740 nm) can slow flower induction resulting from cooling and increase the time plants need to be cooled, from six up to 12 weeks for complete induction to occur. For this reason, we recommend that you light only with fluorescent lamps (up to whatever light intensity you can, but a minimum of 10 foot-candles).

Light Intensity During Rooting

Does an unrooted cutting photosynthesize? We found that yes, an unstressed cutting does photosynthesize! So, how much light can a cutting utilize? As long as a cutting is regularly misted to stop leaf drying, an unrooted cutting can use up to about 2,000 foot-candles (400 umol∙m-2∙s-1). We recommend that during early rooting — immediately after unpacking and sticking a cutting — set shade curtains to provide no more that 1,000 foot-candles of sunlight the first two to three days, then no more than 2,000 foot-candles from days three through 10 (until fully callused), and finally, up to 3,000 foot-candles once cuttings have rooted. 

Light When Finishing a Crop

Different species can utilize different amounts of light. Whether extra light is good or bad depends on the time of year and your location. In general, extra light is great in the north. However, you must remember that this extra light heats a plant, and this may be beneficial or detrimental depending upon the crop. For instance, photosynthesis decreases in pansies when plant temperatures exceed 68° F, but does not decrease in poinsettias until temperatures have exceeded 86° F.

We can divide species into two groups: those that are low light–saturating and high light–saturating. Low light–saturating species are those that can not use light above about 1,500 foot-candles (300 umol∙m-2∙s-1; Figure 1a). Examples of low light–saturating species include many tropical plants, as well as Reiger begonias and some geraniums. High light–saturating plants are those that can use light up to 3,000 foot-candles (600 umol∙m-2∙s-1; Figure 1b). Most of the potted and bedding plant crops we grow fall into this group. 

Any light a plant receives above where it photosynthetically saturates cannot be used! That’s why it is important to understand how much light your crop can use. Conversely, it is important to realize that we may be often over shading. Many shade curtains reduce light by about 55 percent, which is too much in many cases. Therefore, completely closing a shade curtain so that light levels drop to 1,000-2,000 foot-candles in a finishing environment can actually result in plants ‘starving’ at some level.

Plant Spacing

Leaves at the top and in the middle of the plant canopy are actively photosynthesizing. If you just provide the optimal light intensity to the top leaves, the middle leaves will receive less light due to shading from the higher leaves. The closer plants are spaced, the harder it is to get saturating intensities of light down to the middle leaves. Therefore, you will need higher intensities of light to achieve this. Ideally, light intensities provided to a crop will increase as the plants grow together to achieve the optimal light intensities throughout a plant canopy as a crop grows. We recommend that the light sensor that you use to control shade curtains or supplemental lighting is placed half way down in a crop at mid leaf level. 

Adding Carbon Dioxide

In all cases, we find that increasing CO2 in a greenhouse can allow plants to use additional light. We believe that we will once again be looking at adding extra CO2 in greenhouses, like we used to do 50 years ago. In general, most crops benefit from supplementing existing CO2 levels (which can range from 250 to 380 ppm in a greenhouse) up to 800 ppm total CO2. In addition, the extra CO2 allows plants to use higher light levels, thus allowing for even higher photosynthetic levels and increases in plant mass.

Optimizing Your Shade/Lighting System in a Greenhouse

A summary of temperature optima and light and CO2 saturation values on some of the crops we have looked at can be found in Table 1. In general, to optimize your light environment: 1) place a light sensor half way down the canopy in a crop, 2) set the shade curtain settings to start closing only when your crop reaches light saturation or only add supplemental lighting up to that level, and 3) stage lighting and/or shade curtain controls as much as possible to not over-light or over-shade your crops. Remember, optimal light intensities will change with development. Therefore, move plants among areas with optimal light levels at different stages in development, or change the lighting/shading set-points to match the optima during different stages in development. Lastly, reconsider the possibility that supplementing with CO2 may benefit your crops and allow them to use even more light!  

*To view detailed charts related to this layout, please select the PDF option to the right.
About The Author:

John Erwin is a professor of horticulture physiology, Jennifer Boldt is a doctoral candidate and Esther Gesick is a research fellow in the Department of Horticultural Science at the University of Minnesota. Erwin can be reached at [email protected]

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