The following is an excerpt from The Year-Round Solar Greenhouse, with additional content provided by expert greenhouse grower Paul Alaback.
Year-round greenhouse growers, and all gardeners, often wonder whether plants are getting sufficient light. As we’ll explain further below, whether your garden gets enough light is somewhat subjective — it depends on your expectations. Fortunately, there are some general recommendations to give you a baseline.
Before you can use light metrics, it is essential to understand how light is measured. Light is like rainfall; it changes in both intensity and duration. The duration – or hours the plant is exposed to light — is very intuitive. Intensity is less considered, but extremely important, as it changes dramatically throughout the day and season. Even on a perfect sunny day, light intensity can vary one-hundred fold depending on the time of day. In other words, one hour of light late in the afternoon may equal just a few minutes of light in the middle of the day. Your climate (cloudy v. sunny) also greatly affects light intensity. For these reasons, simply knowing how many hours of light your garden or greenhouse receives is insufficient. Depending on your location and season, it can mean dramatically different light levels.
When it comes to measuring light, most metrics only refer to light intensity. Foot-candles, lumens, and lux all measure the intensity of the visible spectrum at a single point in time. (On a more technical note, plants use a slightly broader range of light, and slightly different colors, compared to what our eyes perceive. The spectrum and colors that plants use for growth are called Photosynthetically Active Radiation,’ or PAR light.
The huge drawback with foot-candles is because it only measures intensity, it does not give a full picture of light levels in your garden. Fortunately, gardeners and greenhouse growers have faced this problem for some time and created a metric that combines both duration and intensity. The Daily Light Integral, or DLI, describes the total amount of light that falls on a space over a 24 hour period. That makes it very useful for comparing light levels and describing how much light plants need, making it the most common metric in the horticultural industry. Moreover, the DLI measures the PAR spectrum (the light that plants actually use for growth), and not just the visible spectrum.
DLI levels vary from 0–60 depending on location and time of year, as you can see from the map below published by Purdue University. Note the huge seasonal fluctuations in light levels. In most US climates, the winter months have only 10%–20% of the light as the summer! This presents the basis for one of the major principles of solar greenhouse design: maximize light in the winter.
If a plant has evolved to grow in the long days, and now receives only 1/10th that amount of light, growth will obviously suffer. Transmitting enough light and heat in the winter is one of the main goals of a year round greenhouse, and one of the major topics of The Year-Round Solar Greenhouse.
How Much Light Do Plants Need?
Now that you are equipped to measure and discuss light with a sufficient metric, we can return to our question. Plants can grow over a wide range of light levels, but you will get the maximum growth and flower / fruit production when you meet its minimum requirements. (It is theoretically possible to get too much light as well, resulting in burning of leaves or decreased growth, but this is unusual in a greenhouse unless you are growing plants that are adapted to shade.)
The “how much” question also depends on your expectations. What you want to grow, and how robust you expect this growth to be, determines how much light you need. Though it’s a subjective question, there are a few studies that give some metrics for recommended minimum light levels. The drawback is that these were created for the commercial greenhouse industry which has very high expectations. Hobby growers and even many commercial greenhouse growers can choose to grow with much less light than the following recommendations.
Recommended Light Levels (in Daily Light Integral) for Commercial Greenhouse Growers
Seedlings, foliage* 5
Flowering seedlings 10
Ferns, house foliage plants: 6
Ivy geranium, schefflera, 10
Leaf lettuce 12
Geranium, hibiscus, snapdragons 14
Roses, gladiolus 22
Tomatoes, peppers, cucumbers 22
To summarize, if you have more than DLI of 10 you can grow most seedlings and starts. If you want to grow tomatoes, peppers and other sun-loving plants they will grow ok with DLI of 12-15, but will grow best with more than 20. Citrus seems to do ok with as little as a DLI of 10-12 in winter. Leaf lettuce can be grown with as little as 4-10 DLI but quality is usually low below 8.
Most commercial year-round greenhouse growers consider a DLI of 12 to be the minimum threshold for good growth of most crops. Again, some year-round greenhouse growers grow with DLI numbers in the low single digits. Eliot Coleman, growing in year-round greenhouses in Maine, is a great example of this, though he only produces cold hardy crops (some leafy greens). If growing in a year-round greenhouse, we recommend experimenting the first year by planting many different crops, then observing and recording what you find works best in your conditions.
Does my garden/greenhouse have enough light?
The answer to this question comes in two parts. First, you must understand the light levels in your climate and latitude, shown in the map above. (For more specific levels by area, you can also use one of the many online tools for estimating the production of solar panels. NREL has a good one called PV Watts.)
The second step is to get more site-specific, taking into account anything that could shade your garden. For outdoor gardens, this is usually trees and buildings. In a greenhouse, the framing and glazing material (i.e. glass or plastic) greatly impacts how much light is transmitted inside, reducing light by as much as 50% compared to outdoor levels. The specifications of your glazing material (e.g. whether it has a transmission rate of 50% or 90%) will give you some indication of light levels in your greenhouse compared to outside. (For more on comparing materials, see How to Choose the Best Glazing for a Year-Round Greenhouse.)
Because there are so many factors influencing light at your site, often the best method to estimate light availability is a light meter. Most meters are expensive, so a relatively low-cost option is offered from Spectrum Technologies. The DLI 100 scout meter (about $100) is a stick-in light meter that measures the DLI over a day. You simply turn it on in the morning, let it go for 24 hours, and read the DLI light range the next day. Then you can compare this with the minimum light levels
given above. Its good to do this on both a cloudy and sunny day to get an idea of the range of light your plants get at this place in your greenhouse. Ideally, you would also do it over different seasons, but that requires some considerable planning.
“From my experience, you do not get enough natural sunlight outside to grow the full range of fruiting plants in the winter unless you live in the southern US (southern California, Arizona, New Mexico, Florida). From February onward you can grow many plants in Colorado, Wyoming, Utah, across to the mid-Atlantic region. In northern areas, you should be able to grow greens and foliage plants in winter as long as you have warm enough temperatures.”
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