Cannabis growing is divided into five stages: seed, seedling, vegetative, flowering, harvesting. During the vegetative stage, you’ll have to set the light cycle to at least 18 hours of light a day. Some growers prefer to go as far as 24 hours a day.
Fluorescent (T5) lights are not the best option for growing weed. Usually, they merely don’t have enough capacity to provide cannabis with enough light in the flowering stage. Though, as lighting for cannabis seedlings they are just fine.
When Do Marijuana Plants flower under LED Lights?
Under natural conditions marijuana generally begins flowering with the onset of fall, when days become shorter. This is the idea behind all the techniques of growing weed under led lights — you basically emulate natural light cycles.
Marijuana leaves that start wilting and turning yellow can be either a sign of nitrogen deficiency or light burn. If the latter is the case, then widen the distance between your plants and the lights.
It is total darkness that stimulates the splitting of the seed coat and the appearance of the root. As soon as a small white root becomes visible, it is time to carefully move the seed into the substrate for growing and put into the light. In the wild, cannabis seeds drop from inflorescences in the fall and are stored in the soil. With the onset of the spring, only those seeds that were not buried in the ground too deep, and were not exposed to light, manage to break through the top layer of soil.
With the new high output lights such as the T5, fluorescent lights can be used until the plants are about 24 inches tall, which is just enough to grow a very short plant through to the flowering stage when you control your marijuana plant's growth through the vegetative stage.
In recent years, we've seen the creation of new high-light-output versions of fluorescent grow lights which can actually be used through the entire vegetative stage.
The biggest problem with fluorescent tubing is that the light that they give off is only useful for about a foot or so. That's why they're kept so close to your plants.
For optimal results, for vegetative growth choose fluorescent light bulbs that are labeled either as "Cool White" or "Cool" colored (also labeled as "6500k" colored).
Some growers even use fluroescent grow lights such as the T5 successfully to flower their plants (if the plants are kept short/small enough through growth control methods).
If the temperature is under control they can be kept as close as an inch or two away.
The other great thing about fluorescent lights is that they are usually relatively low powered and won't make a huge mark on your electricity bill.
Full-spectrum quantum-sensor light meters measure light differently than meters that measure lumens or footcandles. Light intensity measurements of lumens or footcandles will significantly understate PPFD intensity. This is because they measure the light intensity based on how humans perceive visible light.
If your LEDs are integrated with a lighting control software like smartPAR™, you can program your lights to increase intensity incrementally until you reach your maximum vegetative light intensity.
If you don’t know your propagation area’s light intensity, you’ll need to take light measurements for this step.
STEP 2: Begin to Increase Light Intensity
See the next article in this series, “How to Transition Cannabis from Veg to Flower”, to prepare for the transition from the vegetative to flowering stages.
If you’re switching from a LED propagation environment to a LED veg environment, you won’t need to worry as much about this step.
Continue to increase your light intensity until you’ve reached the vegetative intensity level. The ideal light intensity for vegetative growth is 400-600 μmol·m −2 ·s −1 . Once your light intensity has reached the level you want for veg and your plants are happy, let them veg away!
After your plants have acclimated to the LED spectrum, you can begin gradually increasing light intensity by 50% every 1-2 days. This allows for a gradual but timely transition. If you are growing more sensitive cultivars, you can increase the light intensity as slowly as every 7 days. If you are growing hardier cultivars, you can increase light intensity more quickly to complete the transition faster.