Everyone knows that plants need light to grow. Whether it is natural sunlight or growing lights, it is pretty well understood that light, and enough of it, is a “must” for plants. What everyone doesn’t know is that not all light is the same, and that different phases of plant growth require different types of lighting.
As plants grow, their need for light changes. Once plants transition from their growth phase to their flowering phase, they need a different duration and type of light. Outdoor growers don’t need to worry about this, since most plants long ago in their evolutionary journey, acclimated and adapted to the changing amounts of light over the course of the year and use it as their cue to flower (or start to grow, stop growing, etc.).
Let’s look at the “whys” and “hows” of grow lighting during the flowering phase. You will enjoy a higher yield on your high value crops if you change your lighting to be in “tune” with natural cycles and types of lighting.
Why Plants Need Lighting During Flowering
What is so “special” about getting flowering lighting right? Until plants get 12 hours of continuous darkness, they will remain in the vegetative stage. Growing indoor crops requires mimicking the natural light cycle. Without doing so, buds will not properly form and won’t produce as many trichomes as they otherwise should. Inadequate or the wrong setup of flowering lighting will either not allow your plants to flower or will result in lower yields, poorer quality buds, and weaker plants overall that are more susceptible to disease.
All plants use what’s called the “phytochrome” system to ascertain the light they receive, including its intensity and duration. Creating lighting through photoperiod manipulation, that mimics the natural cues given by sunlight, will help your plants enter their flowering phase. Sugar leaves and bracts act as photosynthetic leaves. Within the tissues of these plant parts, energy is produced that helps to develop dense buds, complete with cannabinoids and terpenes. Adequate lighting will help to promote these secondary metabolites.
The correct duration and type of light for the flowering stage offers additional benefits to your high value crops. Indoor growers can manipulate the lighting during this phase to influence the quality, density and yield. This in turn means that the buds produced will be thicker and have more resin than those that do not receive the correct lighting during the flowering phase.
The exception to all of this is with autoflowering species. Autoflowering species of cannabis evolved in northern parts of the globe where waiting for light cues would mean buds would be exposed to frost. Instead,these varieties of plants have an internal mechanism that triggers the formation of buds regardless of light levels. They will flower based on age and not external factors.
When to Transition to Flower Lighting
Knowing why you need to switch up lighting when your crop is flowering is important, but when do you do it? Unfortunately, there is no one, single answer. It can depend on what end result the grower has in mind, what strain of plant they are growing, and their particular growing setup. Ask any two growers when they switch over to flower lighting for their indoor grows and you are likely to get two different answers. There are some key indicators though that can tell you when you should make the transition.
The age and height of the plants are often considerations for when to switch over to flowering stage lighting. If a cannabis plant has been grown from seed, many growers like to wait for at least 60 days of the vegetative phase before transitioning to budding phase lighting. If the switch is made too soon, some growers think that their plants will not develop as many blooms as they potentially could otherwise. They also feel that the amount of resin their plants produce will be reduced if the flowering phase lighting and lighting schedule is adopted too soon. This is definitely not the belief of all growers.
Some growers switch over to flowering lighting as soon as their seeds have germinated. This practice is for growers that want to keep their plant shorter and on the smaller side. This is called “12-12 from seed”, referring to the amount of hours of lighting and darkness the plants are allotted. Even when flowering lighting is set up on plants once they have germinated, it will still be a few weeks before the plants start to develop buds.
For growers using clones, flowering stage lighting can be set up once the plants have established a strong root system. Since clones grow faster than plants started from seed, the switch can be made pretty much whenever the grower wants to make the switch. This is a benefit for small scale growers that may not have a different set of lighting for each phase of growth.
Flowering-specific Lighting to Consider
With so many lighting options on the market, it can be difficult to select the right one for the flowering stage of growth. The best flowering stage lighting is any lighting that emits light energy containing wavelengths from the red-light spectrum. This means that the blue spectrum lighting that was great for the vegetative stage will have to be switched out.
Growers have historically used HID (High Intensity Discharge) lighting during the flowering phase. The two main types of HID lighting are metal-halide (MH) lighting and high-pressure sodium (HPS) lighting. HPS fixtures emit redder and warmer light and are preferable to MH lighting for flowering stage light fixtures. Metal-Halide lighting is bluer, and is great for the growing phase, but should be swapped out during flowering. There are dual spectrum HPS lights that can be used for the whole grow as it emits both types of light for each phase of growth.
In recent years, many growers are switching to LED grow lights for their flowers. Older models of LED lighting could not compete with the quality of light given off by HPS lighting, but now most growers who use LEDs would agree that the LED lighting available on the market now is superior to most HPS lighting for use during the flowering stage. Modern LEDs are able to penetrate the canopy, providing an even light spread throughout the crop. Not all LEDs are created equal though. There are cheap ones on the market that are not as good as the older HPS types or even the ceramic metal halide (CMH) bulbs that some growers swear by.
If you already have a grow light system that is optimized for the vegetation phase, but not so much the flowering one, fear not. You can easily supplement your grow lighting by adding red-spectrum bulbs to your array. If you are just setting up for the first time, high quality LEDs are probably your best option. Look for ones that are full-spectrum white with additional blue and red wavelengths so that you only need one type of light for the whole growing cycle, from seed to harvest.
It is worth noting that growers who have used HID and are switching to LEDs need to adjust their watering schedules. It is very easy to overwater plants that were previously in grow rooms with any type of high intensity lighting because of the heat put out by these lights. Under LEDs, less heat will be generated, meaning less excess moisture will be evaporated and will remain on the plants for longer periods of time. This can of course lead to the risk of developing fungal diseases. Make sure that moisture levels are carefully monitored in any crop that is getting LED lighting for the first time.
One of the most dreaded fungal diseases of cannabis crops is bud rot. It can develop quickly and spread quickly. It has the ability to destroy an entire crop that is in full bloom right before it is time to harvest. A few other tweaks to the growing environment in addition to proper lighting will prevent this scenario from playing out.
The right lighting for the flowering phase is not the whole story to creating the right climate to keep disease at bay. Bud rot can set in if your crop is too wet, the temperature is too low, and there is not enough moving air. Make sure that when switching over to LED lighting, the temperatures don’t drop too much. You should still aim to keep the air temperature over 68 F (20 C) at an absolute minimum. Temperatures above that, along with ample lighting, moving air, and an irrigation schedule that is not operating as if there are still HID lights, and you will be able to keep most fungal pathogens from being able to take hold and flourish.
How to Set up Flower Room Lighting
Once you know why specific lighting is important for flowering and what kinds of lights will provide what your crops need, it is important to know how to properly set up the lighting where your plants will be going through their flowering phase. To get optimal light penetration in the canopy, it is important to achieve the appropriate distance between the lights and the tops of the plants.
During this final stage, the flowering phase, lighting should be between 16 and 36 inches (40-90 cm) above the canopy assuming you are using LEDs. Other types of lights have to be much higher so that the heat that they emit will not negatively affect the crops. Placing the light closer to the crops may boost photosynthesis, but it can cause the plant growth to sprawl too far and affect the flower set. For HPS bulbs, they can actually burn the plants if they are too close.
When transitioning from the growing phase to the flowering phase, it is good to gradually adjust the lighting height as opposed to doing it all at once. Once your crops are ready to enter the flowering phase, raise the grow light above your canopy a couple of inches every few days until the optimal height has been achieved. This will help your plants ease into flowering and not cause them undue stress.
This transition can begin once your plants have reached about half of their desired, final height. If the colas get too tall during this transition phase or even once your plants have fully switched over to the flowering phase, it is ok to bend them down and away from the middle of the plant. This technique is known as “low-stress training”, or “LST”. This may also have to be done if growing in tight spaces where there is limited overhead room. In any case, do what must be done to keep the proper distance between your light source and the top of the flowers. Your plants will reward you with a great yield if you do.
Your plants will give you some cues if the distance between the source and the canopy is not ideal. Besides the buds getting too tall, the leaves can get too tall, in a condition known as “praying”. Additionally, the margins and tips of the leaves can turn yellow. If the distance is not corrected, the whole leaf will turn yellow or bleached. They may also dry and start curling at the edges. All of these conditions can be corrected by raising the light high enough during the flowering phase.
Other Considerations for Your Flowering Phase Setup
Since the flowering stage is an entirely different metabolic phase of life for the plant, there are different nutrient requirements. It can be very easy to accidentally overfeed your plants if you don’t change up your formulas from the vegetative stage. The need for nitrogen (N) is significantly decreased during the budding and flowering phase. The amount of nitrogen being supplied through your nutrient solution should be cut in half. The ratio of NPK fertilizer during the bud stage should be one part nitrogen to two parts each of phosphorus and potassium.
Monitoring pH is still important during the flowering stage. Depending on the growing medium, the pH during the flowering stage should be kept as stable as possible. For crops growing in soil the pH should be between 6.0 and 7.0, with the preference being somewhere in the middle between 6.4 and 6.6.
For hydro or coco coir grows, the pH should be lower, and kept down to about 5.8. Rising pH is mostly a problem for growers who have an alkaline water source, which can often be in the 7.5 range. Watering with water from a reverse-osmosis system or using distilled water will help to avoid some of these fluctuations in pH.
If the pH gets too high during the flowering phase, and it is caught early enough, it can be corrected by flushing the substrate with pure water, such as distilled or reverse-osmosis water. Additionally there are many pH up and pH down products on the market that make pH adjustment easy. Be careful not to overdo it though. It is best to apply a little and let it sit before testing, and then repeat as needed until the correct pH has been obtained. If these are not available, peat moss can be added to lower the pH in soil crops. Baking soda or wood ash can be used to raise the pH also.
Armed with all of these tips and the proper lighting set up for your flowering phase, you are ready to achieve some of the most delectable flower and most rewarding yields your high value crop has ever produced. Happy harvesting!
