If your weed plant only begins to flower at the end of August, there is a good chance that it will end up in bad autumn weather. To speed up the flowering period, you can put a weed in the dark. You force your weed plants to enter the flowering phase earlier by giving them less than 12 hours of light every day. You will not only be ready sooner. Putting outdoor weed in the dark can promote the flowering of the weed plant. Better weed and a quicker harvest, isn't that what you want?

In this article:

Beginning of flowering phase weed plant

Before we can tell you about putting your weed plants in the dark, you first need to know more about when a weed plant will start to flower. Most photoperiod weed plants (regular, feminized) enter the flowering phase when it is dark for more than 10 hours a day. The difference in the number of light hours gives the plant a signal that it can start spending its energy on making buds, instead of growing bigger all over the plant. 

Around the end of August it gets darker outside (in the northern hemisphere) for more than 10 hours. For most strains this means the beginning of the flowering of the weed plant. When growing indoors you decide when the plant will flower by adjusting the light schedule. Many growers set the lamp to 12 hours of light and 12 hours of darkness. Enough darkness for weed plants to proceed to the flowering phase.

There are a few exceptions to the rule. Autoflower strains have the characteristic that they automatically go into the flowering phase. They don't need a change in light hours per day to do so. Then there are 'special' photoperiod strains that are extra sensitive to changes in the number of hours of light. This has to do with the origin of the plant. Sativa strains that are extra sensitive have their flowering start earlier in the year (can be as early as mid-July). Indica strains can start flowering later in the year (mid-September, for example).

A weed plant starts to flower when it is in the dark for more than 10 hours a day.

What is putting in the dark?

Putting your plants in the dark is nothing more than forcing them to enter the flowering phase earlier than they naturally would. If you want to put a weed plant in the dark, you have to make sure that it doesn't get more than 12 hours of light a day. Normally this only happens at the end of August, but if you put your plants in the dark for 12 hours a day already in July, they will enter the flowering phase sooner. You imitate the sun, as it were. You are creating a difference in the number of hours of light, which means that your plant will start producing buds sooner. When you have finished putting your plants in the dark in August, they can bloom when the weather is nice and you will not end up in bad weather.

You can put your weed plants in the dark by putting them in total darkness for at least 12 hours every day. This is not difficult, but it does take a lot of time. You have to do it every day. You can do this by putting your plants in a grow tent, putting a big bag over them or dragging them to a completely dark grow room or greenhouse. More about this under the heading "how and when to put a weed plant in the dark”.

An example of a home-made blackout tent. Every day, the black tarpaulin is pulled over the tubes and fastened, leaving the plants in complete darkness for 12 hours. 

Putting outdoor weed in the dark = stimulating flowering

Now you know what putting in the dark is, but why should you do it? Well, to stimulate the flowering phase. When you're growing weed outdoors, your plants usually don't enter the flowering phase until the end of August. The average weed plant needs about 10 weeks to flower. If you take this into account then your plants will have to spend the last weeks of flowering in wet and dreary autumn weather. The lack of (sun) light will degrade the quality of your weed and the wet weather can cause you to suffer from bud rot, mildew or other problems. For this reason many growers want to speed up the flowering of the weed plant (starting earlier) in order to improve it. 

A good way of stimulating the flowering phase is to put your weed plant in the dark. When you have finished putting your plants in the dark in August, your plants will have a head start on the flowering phase and will therefore benefit more from the good weather. The sun is now at its most powerful and the quality of the weed is therefore better. By putting it out in the dark the right way you get better weed that is ready for harvest earlier in the year. This way you can enjoy your home-grown weed well into autumn, without the headache of moulds and storms.

Some growers try to grow two plants in one summer by putting them in the dark. By starting your first grow in May and starting to put it in the dark after a few weeks, you can be ready by the end of July. If you start pre growing your second plant during your first grow you will be ready well before autumn.

Disadvantages of putting weed in the dark

By putting your plants in the dark, you can stimulate the flowering phase. But there is also a downside to putting outdoor weed in the dark. Earlier flowering also means that your plant and its root system have less time to grow. As a result, you will see that the plant remains smaller than it has the potential to be. After all, the function of the growth phase is to ensure large and strong weed plants. Putting them in the dark too early can therefore result in a lower yield. So you only darken outdoor weed when the plant is big and strong enough. If you want to grow two plants in one summer (as described above) you will find that the plant will not grow as big as the characteristics of the strain would lead you to believe.

Another big disadvantage is the fact that putting the plant in the dark is very time-consuming. You just have to have the time for it: put your plants in the dark at the same time every day and then open up the proverbial curtain again 12 hours later. It is simply not possible for everyone. 

How and when to put your weed plants in the dark

Enough theory. On to practice. How should you put a weed plant in the dark and when is the best time to do so? What do you have to pay attention to?

How to put a weed plant in the dark?

You can put your weed plants in the dark in various ways. Choose from the following options:

  • Dark tent. You can make your own dark room tent by making a solid frame under which your plants will fit. Pull thick, sturdy black and white foil over the frame (black side inwards). Also fasten the bottom to make sure it is completely dark inside the tent.

  • Plastic bags. Another way is to put a large bag over your weed plant. The bag must be made fully lightproof. If you use bin bags, for example, you should pull several of these over each other.

  • Growing room. Do you have a growing room for indoor weed? For example, your shed or a large grow tent? Then put your plants in there every night.

  • Green house. Putting your weed in the dark goes well with growing weed in a green house. When it is time to put them in the dark, all you have to do is pull tight black-and-white foil over the greenhouse and make it lightproof.

Weed plants in a green house. Put a tarpaulin over them and leave them in the dark!

When to put my weed plant in the dark

If you want to grow one plant per season, we recommend that you start putting it in the dark around mid-July. After a few weeks your plants will enter the flowering phase. They can enjoy the powerful summer sun during the day. You can then also stop putting your plants in the dark at the end of August. Then in the Benelux countries, Germany and the United Kingdom for example, the nights are long enough and your weed plants can continue to flower on their own.

If you want to grow two plants in a year then it is best to start your first grow around mid-May. Let them grow for five weeks and put them outside at the end of June when the weather is nice. Start putting this plant in the dark right away and start germinating your second grow.

The time of day when you put your plants in the dark is pretty much up to you. We recommend putting the plants in the dark in the evening between 8 pm and 10 pm. It is important that you put them in the dark for 12 hours every day. If you put them in the dark at 8 pm, they have to go out again at 8 am. If you choose 10 pm, they go out again at 10 am. If you stick to these approximate times, they can enjoy the sun during the day.

Important! What do you need to look out for?

There are a number of things to watch out for when putting outdoor weed in the dark. Read these points carefully. If you don't, then you will have gone through a lot of trouble for nothing. In the worst case, you could ruin the weed plants.

Light leakage is fatal. Make sure there is complete darkness. Light leakage is not only bad for the rest a plant needs. When you disrupt the darkness the plants get stressed and this disrupts flowering considerably. It can cause leaf problems and in the worst case female weed plants can become hermaphrodites. A lot of stress and light leakage can disrupt the process of putting your plants in the dark and she will not flower. So make sure that your blackout tent, greenhouse, grow room or bags do not let in a single ray of light.

Make sure no light reaches your weed plants when putting them in the dark.

  • A fixed routine is important. Just like people, weed plants have a routine too. Try to get your plants in and out at the same time every day to maintain the routine. Not having a good routine can give your plants stress (hermaphrodite) and it can even cause the process of putting them in the dark to be disturbed and cause them to slip back into the growing phase.
  • Watch out for mould. If you wrap your weed plants up tightly, the humidity in the air can rise quickly. Especially when you put bags over your plants you have to watch out for this. If your plants and buds stay wet for too long and the humidity is too high, you can suffer from bud rot. So it is not wise to put soaking wet weed plants in a bag. Some growers remove the bags when it is completely dark outside. But that does cost you extra time. So invest in a handy blackout tent.
  • Don't damage your plant. If you have to drag weed plants into a dark growing room every day, put a bag over them or place them in a tent, accidents can happen. Be careful not to damage the plants. Broken branches will not benefit the yield and will stress the plant. Cuts and wounds are breeding grounds for bud rot and pests. So be careful!

What month should I harvest outdoor weed 

If you don't put your plants in the dark, you can often only harvest outdoor weed in the course of October. If things go wrong, not until the beginning of November. So now the question is: "In what month can I harvest outdoor weed if I put the plants in the dark?" That depends on when you start putting them in the dark and how long the strain's flowering period lasts. If you start putting them in the dark in July with a strain that has a flowering period of around 10 weeks, you can harvest by the end of September. Fast flowering strains have a flowering period of less than 7 weeks. In this case you can already harvest outdoor weed in August. 

Whichever way you look at it, putting your weed plants in the dark will speed up and encourage them to enter the flowering phase. It just costs you a lot of time.