Leaving grow lights on 24 hours a day is not recommended for most plants. While some plants can tolerate continuous light, most plants need a period of darkness each day. Here’s an overview of what happens when grow lights are left on all the time.
Photosynthesis Slows Down
One of the main issues with leaving grow lights on constantly is that it can slow down photosynthesis. Photosynthesis is the process plants use to convert light into energy and food. During the dark period, plants stop photosynthesizing and undergo other important processes like respiration and transpiration.
If lights are on 24/7, the constant light exposure can overload the plant’s photosynthetic processes. This leads to slower growth as the plant struggles to handle the excess light energy. The plant produces more sugars than it can use, which are then turned into starches. Over time, starch accumulation can hinder further photosynthesis and growth.
Flowering and Fruiting Problems
Another problem with continuous light is that it can disrupt flowering and fruit production in many plants. Most plants rely on a dark period each day to properly set flowers and fruits. For example, orchids need 10-12 hours of uninterrupted darkness to initiate flowering.
Tomatoes and peppers also require long dark periods between 8-10 hours to develop flowers and fruits. Keeping the lights on 24/7 confuses these plants, leading to delayed or abnormal flowering. For fruiting plants, this means greatly reduced or no fruit production.
Stressed Growth
Having grow lights on constantly puts significant stress on plants. Just like humans need time to rest and recover, plants need time without light to undergo important biological processes. Continuous light exposure keeps plants in a constant state of growth, unable to properly rest.
This leads to overall weaker, stunted growth as resources are diverted towards protective stress responses. Light-stressed plants often have thinner, elongated stems and foliage as they stretch upwards seeking relief. Such etiolated growth is more prone to disease and physical damage.
Increased Pest and Disease Pressure
The stressed growth caused by 24-hour light also makes plants more susceptible to pests and diseases. Weakened plants have fewer defenses against infestations. Continuous light favors certain pests that prefer bright conditions like spider mites.
Having no dark period also inhibits plants’ recovery from and defenses against diseases. Spores germinate more rapidly in constant light. Combined with slower, weaker growth, this means plants have a harder time fighting off infections.
Altered Plant Chemistry
Exposing plants to uninterrupted light alters their natural chemical processes and composition. Research shows plants grown under continuous light have lower nutritional value than those grown with regular light/dark periods. Key vitamins, antioxidants, and phytochemicals are reduced.
Medicinal plants and herbs may also lose potency as constant light changes their chemical profile. Essential oil production relies on day/night fluctuations. Without this, oil yields and qualities decline. Altered plant chemistry ultimately leads to less flavorful, nutritious, or effective plants.
Excess Energy Use
From an operational standpoint, running grow lights 24/7 also wastes significant electricity. Lighting accounts for a large share of energy costs in indoor plant growth. Doubling or tripling the daily light exposure without improving plant growth just drives up energy bills.
Using timers to control grow light photoperiods saves electricity and costs compared to continuous operation. Turning lights off when plants don’t need them reduces overall energy consumption and expense.
When Continuous Light is Useful
While problematic for many plants, there are some situations where having grow lights on 24 hours can be beneficial:
- During seed germination – Constant light helps speed germination.
- For mothers and clones – Helps maintain plant growth for cuttings.
- Leafy greens and microgreens – Promotes rapid leaf/shoot growth.
- Day-neutral plants – Some plants like tomatoes flower under any photoperiod.
Even here, the risks of light stress and reduced quality exist. But for fast plant propagation and leaf harvests, 24-hour light has its uses. Careful monitoring is required to avoid long-term harm.
Best Practice Lighting Schedules
For most plants, the best practice is matching light and dark periods to what the plant naturally experiences. Here are some general guidelines:
| Plant Type | Daily Photoperiod |
|---|---|
| Flowering plants | 12-14 hours on / 10-12 hours off |
| Fruiting plants | 10-12 hours on / 12-14 hours off |
| Vegetables | 14-16 hours on / 8-10 hours off |
| Succulents & cacti | 10-14 hours on / 10-14 hours off |
| Propagation & seedlings | 18-24 hours on / 0-6 hours off |
Timers and photoperiod controls allow customizing the light schedule based on each plant’s needs. Matching natural day lengths avoids the issues caused by lights being on 24/7.
The Bottom Line
Leaving grow lights on continuously often does more harm than good for plants. The excessive light stresses plants, slows photosynthesis, disrupts flowering/fruiting, and enables pest issues. For most plants, matching light to natural day/night cycles optimizes growth and quality.
The exceptions are young seedlings, propagation, and certain fast-growing leafy greens. Here, 24-hour light can speed up growth when desired. But these plants require close monitoring for signs of light stress over time.
Investing in timers and controls allows customizing photoperiods based on each plant’s needs. An efficient lighting schedule balances plant health with rapid, productive growth.