Cannabis Plant Does Not Flower
4 min

What To Do If Your Cannabis Plant Does Not Flower

4 min

Having a problem getting your cannabis plants to flower and you don't know why? We'll go over the most common problems, provide you with some simple solutions and supply extra education along the way. Let's go!

You've nurtured your cannabis plants through weeks of vegetation, carefully tending to their every need. You've watered them, fed them, showered them in light and protected them from harm. Now, they won't even throw the first pistil. What's a grower to do?

This guide will walk you through some of the most common flowering issues, so you can help your plants pack on some big, frost-covered blooms.


Female Cannabis Plants

Dumb question, right? But, if you have a male and don't know it, it will never produce buds, no matter what you do.

You can easily identify a male cannabis plant once it starts to show its gender. Look at the nodes where the leaves and side branches emerge from the main stem. Females develop tiny pre-flowers and males develop pollen sacks. They're hard to tell apart when they first show, but within a day or two, the pre-flowers shoot out two tiny hair-like stigmas. The pollen sacks will continue to multiply until they look like miniature clusters of green grapes.

Wait a minute. I bought feminized seeds!?! Such seeds are bred using feminization techniques. This process is very reliable, and seeds turn into female plants 99.9% of the time. If you get a male plant from a pack of feminized seeds, you've either hit the bad luck lottery and fell into that 0,1% category, or there was an error at packaging stage, and regular seeds got mixed up with feminized beans. Mistakes happen, so do yourself a favour and watch for signs of sex no matter what you buy.


Autoflowering Cannabis Plants

Do your plants contain autoflowering genetics? Are you trying to get them to bloom under 24 hours of light? If so, try cutting the light back to an 18 hours on, 6 hours off schedule. Old-school autos, with a high percentage of ruderalis, bloom like crazy starting at the third week after germination, even when the lights are on around the clock. However, breeders have been answering demand for stronger, higher yielding automatics by decreasing the amount of ruderalis in newer automatics. It's simple math and science. Increase the ruderalis, and darkness requirements go down along with THC and yield. Decrease it, and the opposite happens.

If you're growing photoperiod plants in the same environment as your autos, don't worry about reducing light from 24 hours to 18. It won't hurt the plants one bit and it might do them some good. If you still don't see pistils starting to form after two weeks of 18/6 lighting, maybe your plants are actually photoperiod instead of autoflowering. Again, mistakes happen.


Photoperiod Cannabis Plants

Photoperiod cannabis plants will not flower, no matter how big or how old they get, unless they receive at least 12 consistent hours of uninterrupted darkness in each 24-hour period. Professional growers leverage those rules of nature to maintain their best mothers in a vegetative state for years so they can make clones. Most plants will start to form buds somewhere between 10 days and 3 weeks into a 12/12 lighting schedule, and here's why.

When the lights are on, photoperiod cannabis plants produce phytochrome red and phytochrome far-red in about equal amounts. When the lights are out, phytochrome far-red changes into plain old red. When red outnumbers far-red by the right amount, the plant will start blooming. To jumpstart this process, some growers turn the lights all the way off and drown their plants in complete darkness for 36 hours before they set their timers to 12/12.

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The amount of time it takes for blooms to start forming once switched to 12/12 is strain-dependent to a certain degree, but how long you allow for the vegetative stage also makes a difference. You can try to force plants to flower at any age. You can even put seedlings in a 12/12 cycle right after germination. But, in general, the younger the plant, the longer it will take for buds to show. If you grow your plants for about 6 weeks, long enough for them to naturally reveal their gender, they'll be raring to go when you cut back the lighting and can throw pistils as fast as 7 days.

If you're growing pure or near-pure sativas, and they still won't bloom after 3 weeks of 12/12 lighting, try 11/13, or even 10/14 for a week to see if you notice any change. Natural, landrace sativas evolved near the equator, where the longest day of the year isn't very different from the shortest, and they often have an extended flowering time that takes up to 14 weeks. Sativas can be stubborn, but they usually take the hint with a little more darkness than an indica or a mixed hybrid would require.


Cannabis Plants Dark Cycle

Another dumb question, right? Of course it's dark. The lights are off, duh! Some strains, though, are so sensitive that if you don't use a timer to make sure the lights turn on and off at exactly the same time each and every day, they won't be happy and won't start to bloom.

If the timer is set right and working properly, check to see if you have any leaks. Some strains can tolerate light leaks, but others go haywire with just a few stray rays. Check even if you're using a brand-new tent. Almost all, even the expensive ones, will usually have a light leak somewhere.

With your grow lights on, and the room lights off, closely inspect the tent and, with heavy tape, seal every spot where you see light shining out. Don't forget the top and the zippers. Keeping the lights off in the room that holds your tent can help, as will running lights on during the day, lights off at night.

Fortunate enough to have an entire grow room? Stand inside with the lights off during the day. Look for light creeping in from covered windows, around the door and anywhere else you can think of. Finally, put a lock on the door. Curious eyes could be taking a sneak peek during your plants' dark cycle.

If you want to be extra safe, cover any type of electronics in the grow space, so no LED indicator lights show. This could include digital timers, thermostats, thermometers, fans or anything else that runs on electricity.


Flowering Cannabis Plant

If you're growing your plants outdoors, you don't usually have to worry too much about available light. Mother Nature takes care of everything for you. Plant your seedlings in the spring and they'll start showing signs of flowering sometime in August, shortly after the summer solstice, and be ready to harvest during the fall.

However, you can get your plants to bud early outside too. The easiest way is to grow autoflowers. The not-so-easy way is to rig up a tarp or other covering that blocks out the light completely for 12 hours a day. Growers on a budget will have to pay close attention to the clock, so they can cover and uncover on a schedule. Those who can afford it, can buy automated systems that will do it for them.

If you're in an area where the weather is warm enough that you can plant cannabis outdoors in the late winter to early spring, you should know about re-vegging. Because the plants receive such little amount of light during the early season, they could start to flower, then stop and revert to their vegetative state as the hours of sunlight increase. You can achieve this artificially with your indoor plants too. Just leave a bit of foliage on your plants after harvest and turn the lights back up. Try it and see!


young cannabis buds

This is, without a doubt, the silliest question you could ask a seasoned grower, but newbies might not be able to distinguish the first sign of buds from the rest of the plant. In the beginning, it can be hard for an unpractised eye to see where the leaves end and the flowers start, and absolutely nothing looks remotely like all the sparkly pictures you see all over the internet.

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If you're patient, you have female plants, and you have the light/dark cycle dialled in, you'll soon see little white hairs that are clearly different from other foliage. Soon, calyxes will form and swell, crystals will appear and fill with THC and terpenes, and you too will have budporn-worthy blooms that you can be proud to say you grew.

Luke Sholl
Luke Sholl
Luke Sholl has been writing about cannabis, the wellness potential of cannabinoids, and the positive influence of nature for over a decade. Working with several cannabinoid-centric publications, he publishes a variety of digital content, supported by strong technical knowledge and thorough research.
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