Plants need light to grow and thrive. While most plants benefit from ample natural sunlight, too much light or the wrong type of light can actually harm plants. When it comes to lighting for plants, it’s important to provide the right balance of intensity and color temperature to meet the needs of each specific plant variety.
How Plants Use Light
Plants use light for photosynthesis, the process by which they convert sunlight into energy and grow. During photosynthesis, plants absorb light in the blue and red wavelength ranges. Blue light helps regulate growth and development, while red light aids photosynthesis and flowering. Together, these two light colors allow plants to produce the sugars they need for healthy growth.
In addition to the specific light colors used for photosynthesis, plants also rely on light for signaling when to flower, when to go dormant, and even which direction to grow in. Light exposure triggers complex chemical reactions within plants that control these growth and developmental processes.
Too Much Light Intensity
While light is essential, too much light can actually stress plants or even damage them. If a plant receives more light than it is able to use through photosynthesis, the excess energy absorbed can cause overheating and oxidative damage at the cellular level. This photoinhibition can disrupt photosynthesis and stunt plant growth.
Too much sunlight can scorch or burn plant leaves, causing browning, yellowing, or tissue death. Entire leaves may shrivel up and fall off the plant. Flower buds may fail to open fully. Lean, leggy growth combined with browning leaves are telltale signs a plant is getting too much sunlight.
Outdoor plants prone to sunscald damage include tomatoes, peppers, basil, lettuce, and spinach. Indoor plants sensitive to intense light include African violets, orchids, and prayer plants.
Excess UV Light
Ultraviolet (UV) rays found in sunlight can also harm plants when exposure is excessive. UV light speeds up the natural process of breakdown of organic matter through radiation. While plants need some UV light, too much can degrade plant tissues and DNA, much like sunburn in people.
Thin-leafed plants and young seedlings are particularly susceptible to UV damage. Signs of UV stress include discolored or curling leaves, stunted growth, and sunscald burns. To limit damage, UV sensitive plants should be shielded from intense midday sun either through shading or physical barriers.
Too Much Artificial Light
Intense artificial light from grow lights or direct indoor sunlight can also overload plants. Typical household bulbs and fixtures often provide much more light than plants really need to thrive indoors. Light intensity drops off rapidly the further you move from the source. Place plants an appropriate distance from artificial light to prevent leaf burn.
Excess Blue Light
While blue light is beneficial for plants in moderation, too much blue light can be harmful. Excess blue wavelengths can overstimulate photoreceptors and inhibit opening of stomates, limiting gas exchange. This leads to excessive reactive oxygen species that damage cells.
Leaf injuries like bleaching and spot lesions combined with stunted growth may indicate too much blue light exposure. LED grow lights should contain some red light and limit short wavelength radiation.
Too Much Green Light
Green light falls in the middle of the visible color spectrum. Most plants reflect green light rather than absorbing it, giving them their characteristic green hue. While not inherently dangerous to plants, excess green light can suppress flowering by drowning out the red and blue light colors plants really utilize.
Too much green light skews the light spectrum away from the ideal ratio of blue and red. This can limit photosynthesis and photomorphogenesis. Ensuring adequate red and blue light prevents green light from becoming problematic.
Excess Infrared Light
Infrared light, though invisible to the human eye, makes up over 30% of natural sunlight. While a portion of this light is beneficial, too much infrared radiation can overheat plant leaves and damage tissues. Infrared wavelengths also drive higher rates of water loss through evapotranspiration.
IR light is minimized through greenhouse glazing materials. Indoor grow lights can use filters to control infrared output to optimal levels. Managing leaf temperature and humidity prevents damage from excess IR light.
Too Much Light at Night
Just as too much light during the day stresses plants, excessive light exposure at night can disrupt normal plant processes. Plants rely on the day-night cycle for photoperiodism, initiating key functions like flowering only when days reach a certain length.
Nighttime light pollution from indoor lights or outdoor security lighting can confuse plants. Too much light at night leads to flower disruption, inferior fruit production, and leggy growth in many species. Light at night should be minimized for healthy plant development.
Signs of Light Stress
Watch for these common symptoms of too much light:
- Wilting, curling, or wrinkled leaves
- Yellowing or bleached leaves
- Dry, scorched leaf edges and tips
- Spotted or necrotic lesions on leaves
- Foliage sunburn or bleaching
- Flower bud drop
- Lack of flowering
- Leggy, stretched growth
Providing the Right Amount of Light
The optimal amount of light varies considerably between different plant species. Some thrive in full sun, while others require shade. Careful attention to a plant’s light requirements will prevent damage from excess light.
Outdoors, strategic placement can limit light exposure as needed. Shade cloth hung above sun-sensitive plants blocks too intense sunlight. Sheltering plants beneath tree canopies or north facing sites reduces direct sunlight.
Indoors, move plants to an appropriately bright window for their needs. Sheer curtains diffuse harsh light. Distance from the light source and timing of artificial light both help control intensity.
Here are the general light needs of some common plant types:
| Plant Type | Light Requirement |
|---|---|
| Cacti & Succulents | Full sun |
| Vegetable transplants | Full sun |
| Annual flowers | Full sun |
| Herbs | Full sun to partial sun |
| Tropicals | Bright, indirect light |
| Orchids | Bright, indirect light |
| African violets | Bright, indirect light |
| Ferns | Low to moderate indirect light |
| Philodendrons | Low to moderate indirect light |
| Pothos | Low to moderate indirect light |
Using Screens and Filters
Where excessive light cannot be avoided, screens and filters can help protect plants. Greenhouse shade cloth blocks full sun, reducing light to suitable levels. Moveable shade screens allow adjustable control over sunlight.
Indoors, sheer curtains filter intense sunlight. Apply window film to glass to reflect UV rays. Grow light filters tailor the wavelength balance based on the needs of specific crops during each growth stage.
Providing Shade
Strategically placed shade modifies light quality and quantity. Use shade structures like awnings, pergolas, and trellises to shelter vulnerable plants during peak sun hours. Position taller plantings and structures to cast shade onto lower growing plants.
Shelve indoor plants on lower light tiers away from direct sun exposure coming through windows. Clustering plants together allows the foliage of one to shade those below.
Supplemental Lighting
Where natural light alone is insufficient, supplemental lighting supports plant growth. Timed to turn on when ambient light dips too low, artificial lighting maintains optimal brightness. Grow lights enhance specific wavelengths tailored to the crop.
Outdoors, low voltage landscape lighting extends hours of useful light. Place floodlights to amplify sunlight on shaded areas. Indoors, full spectrum LED bulbs provide balanced radiance for low light rooms.
Monitoring Light Levels
Light meters measure light intensity to help provide ideal illumination and prevent excess light. Handheld meters test both natural and artificial light sources. Solar powered data loggers track outdoor light over time.
Place sensors at the same level your plants grow to assess actual light reception. Take readings throughout the day and adjust light accordingly. Target the minimum intensity needed for healthy growth.
Conclusion
While supplemental lighting benefits most plants, too much of even good light can be problematic. Watch for signs of light stress and take steps to filter, diffuse, shade, or reposition plants as needed. A balanced spectrum at the right intensity encourages lush, healthy plant growth.
