A greenhouse offers many benefits to a gardener, from extending the growing season to protecting plants from pests and harsh weather. But no matter how nice your greenhouse is or how hard you work to control the temperature inside of it, plants will not grow well if they don’t have sufficient light.
Many beginning greenhouse gardeners ask whether they will need to install grow lights in their greenhouse. The answer depends on your unique situation and a handful of factors such as: your local climate, the type of plants you want to grow, your desired growing season, access to electricity, and budget.
Although greenhouses are designed specifically to let light through, there are times when even the best glazing isn’t enough; winter and rainy days are just two examples of when your plants may not get the amount of light that they need. Keep reading to learn all about when greenhouse lighting systems are needed, and when they are not.
Factors that influence whether you need grow lights
Although all greenhouse gardens could easily benefit from an artificial light source, it’s not a strict requirement. However, it’s important to consider your unique situation and desires, as you may find that you need grow lights to achieve a particular goal. Here are some of the most important factors to consider:
Local climate
As you might imagine, climate is the number one factor that influences whether you need grow lights in your greenhouse or not. In the northern hemisphere, for instance, the further north you live, the more the available light varies depending on the time of year. In the winter, for example, the days get shorter and the nights longer. By contrast, days and nights are always 12 hours long at the equator. Therefore, you may find that you have to supplement with grow lights when the days are too short to provide at least six hours of bright sunlight.
The types of plants you intend to grow
Even if your greenhouse is receiving at least six hours of sunlight, that may not be enough for some plant species. Different plants have different light requirements, as you likely know. Some plants may even need as much as 12-16 hours of bright light each day to flourish or flower! Make sure to check what your plants’ specific needs are, then consider whether your greenhouse gets enough light or whether grow lights would be necessary for healthy plant development.
Desired growing season
If you want to grow outside of your normal growing season (which is typically spring to early fall, depending on your growing zone), you’ll need to manipulate the growing conditions inside of your greenhouse accordingly. For example, if you wish to grow tomatoes in the dead of winter, you’ll need to make sure that the temperature and light conditions match those of summer. This not only means providing heat to your greenhouse, but also increasing the amount of light. The only way to do so is through grow lights.
Access to electricity
A grow light is a type of electric light, and as such, it needs electricity to function. If your greenhouse does not have access to an electrical outlet, then installing grow lights is a nonstarter. If that’s the case, you’ll want to consider other ways to make the most of your greenhouse, such as using it as a season extender or for overwintering mature plants such as root crops or leafy greens.
Budget
While there are some budget-friendly grow lights available on the market today, a greenhouse (even a small one), will need at least several lights to provide enough light to plants over a wide area. Plus, they consume energy, so you’ll want to consider energy costs, and they may need to be replaced after several years of use, especially in a high-humidity environment. So before you buy grow lights, make sure to think about whether you have room in your greenhouse budget for them. Remember, you can always add grow lights later in your greenhouse gardening journey!
When do you need grow lights in a greenhouse?
The short answer to this question is: whenever your greenhouse is not getting the amount of light that plants need to thrive. Most plants need about six hours of direct, full spectrum light per day. This is easy to achieve at the height of summer, when the days are longest and brightest. Not so in the winter, when the days could be too short to grow anything but very low light plants. As a rule of thumb, if your greenhouse plants are not getting much sunlight naturally, they would benefit from supplemental lighting, aka grow lights.
If you’re not sure how much light your plants are getting, you can measure the light levels at various times throughout the day using a light meter.
If you want to grow plants in the winter months
Unless you live closer to the equator, winter is not typically a season when most fruits or vegetables thrive, as the temperatures drop and the days get shorter. Therefore, if you want to grow vegetables year-round, you’ll need to ensure that they have adequate light in order to thrive. Moreover, the sun doesn’t rise as high in winter, so shadows are longer and can decrease the amount of light your plant gets, especially if there are nearby trees, shrubs, fences, or buildings.
Depending on where you live, you might get enough natural sunlight for low-light plants or to overwinter vegetables such as kale and carrots, but to grow anything else you’ll likely need grow lights and supplemental heating.
If it’s often cloudy or overcast where you live
Besides the length of day, local weather conditions can impact how much light your greenhouse garden receives. Although clouds still let UV rays through, it’s less than on a sunny day. If you live in a climate that commonly experiences fog, cloudy skies, or rain, you may need grow lights to keep your plants happy.
If your greenhouse is often shaded
If your greenhouse receives shade for any part of the day (whether from nearby trees or buildings), you might need supplemental greenhouse lighting to ensure your adequate plant growth. Keep in mind, plants also create shade! Therefore it’s important not to overcrowd plants in your greenhouse.
During the early spring or late fall
Many gardeners use a greenhouse as a season extender. In other words, even an unheated greenhouse provides just enough protection from the cold and elements that it enables the gardener to start the growing season earlier in the spring and keep it going later in the fall or even early winter. However, during these shoulder seasons, days are still shorter and plants may not receive enough full-spectrum light to grow efficiently. That’s where grow lights come in.
For seed starting
Although young plants don’t need as much light as mature ones, it’s important that they get enough, as this growth stage is the most crucial to a plant’s success. When seedlings don’t receive enough light, they tend to be leggy and weak; they may be more vulnerable to disease, and the stems may be too weak to support the weight of later growth. Use grow lights during this early stage of the plant’s grow cycle to ensure a healthy start!
For plants with special light requirements
Greenhouse gardeners are known for growing all kinds of exotic plants that may not be native to their local environment. But before you go experimenting, make sure you understand what your plants’ light needs are. For instance, some plants require extra light to thrive, while others have specific light spectrum needs.
Additionally, if you’re trying to control when a plant blooms, you can do so by controlling the photoperiod, or the daily light integral. Essentially, you can create longer or shorter days using grow lights or shade cloths in order to trigger early flowering or promote delayed flowering. This is a technique used by many commercial growers, and it’s particularly handy if you’re trying to time the blooming of particular flowers. For example, poinsettias only bloom when the photoperiod is short, while carnations require a long photoperiod to bloom. Similarly, if you’re trying to grow long photoperiod vegetables in winter (e.g. summer squash, sunflowers, or tomatoes) you’ll need to increase the photoperiod.
While you can shorten the photoperiod by adding shading in your greenhouse, the only way to increase the photoperiod is through the use of grow lights.
To increase plant yield
Even if your greenhouse naturally receives at least 6 hours of natural light each day, you may still consider installing grow lights to boost the amount of light made available to your plants. This is what commercial greenhouse growers do, and it helps them increase their yields and thereby their profits.
When do you not need grow lights?
If you’ve read all of the above information and still aren’t sure whether you need grow lights, it’s possible that you don’t! Some reasons you may not need grow lights are: if your greenhouse naturally receives ample light, if you only intend to use the greenhouse during the main growing season, or if you’re only planning on using your greenhouse for storage or recreation.
