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When to Switch From Vegetative Growth to Flowering?

Bjorn Dawson October 19, 2020

When to switch from veg to flower?

Making the switch from vegetative growth to flowering is a pivotal moment as a grower. Too early and your yield will be lower and your plant might suffer health problems. Too late and you could end up with overgrowth or burned buds.

There are a lot of factors that will affect your decision to make the switch, from plant age to strain, and unless you’re growing an auto-flowering plant - you have complete control over when to make the switch happen. 

Our guide will help you learn how and when to make the switch for your individual grow set up.

Cannabis Growth Cycle

Before we get into the various conditions that will help you know when to make the switch from vegetative growth to flowering, here’s a quick reminder of the basic growth cycle of your plant with (extremely!) rough timescales:

  • Germination/Seed: 1 - 2 weeks
  • Seedling: 2 - 3 weeks
  • Vegetative: 2 - 8 weeks
  • Flowering: 6 - 10 weeks
  • Harvesting: time to reap your rewards!

As we’ve mentioned above, strains will have a huge effect on these timeframes (which we’ll explain further later on) so it’s absolutely worth making sure you do your research on your chosen strain before you grow. 

How to Make the Switch From Vegetative Growth to Flowering

It’s all about your light schedules. When your plant is in its vegetative state, it will need more light to flourish, a minimum of 18 hours a day during this growth period. Most growers will use an 18-6 light schedule, although some choose a 24-0 light schedule.

When you’ve decided it's time to make the switch from vegetative growth to flowering, cut the lights to force your plant into flowering. A 12-12 light schedule will do the trick.

But how do you know when it’s time? We’ve broken down all the factors you need to bear in mind below.

Strains

We briefly touched on this above but there are real genetic differences between indica and sativa and they both behave very differently during flowering.

Indica strains are known to generally produce bushy, short plants comparatively to sativa. They’ll only gain around a quarter to half of their height during flowering, so to maximize your yield it's best to extend your vegetative period. Sativa, on the other hand, is known for being taller and they grow a lot more in height throughout flowering, with some sativa strains even doubling in height from the start of flowering to harvest.

If you’ve picked a hybrid strain, do your research or ask another experienced grower for their advice. 

Clones or Seeds

If you don’t give your plant enough time to establish a solid root system, you’re far more likely to have issues and complications in the flowering stage.

Clones can shoot up in height quickly, so you might need to switch based on this alone. Seedlings can be flowered earlier if your growth method requires it, but they’ll still need 2 to 3 weeks before making the switch. 

Age Is Just a Number

Some growers will swear by a full two months for vegetative growth, but this isn’t always true. Especially for clones, as soon as they have the root system mentioned above, you can switch to flowering if that’s what your growing method dictates. However, it's worth remembering that mistakes in the flowering stage are far more difficult to recover from, so ensure you have a healthy plant before the switch.

A lot of where a plant's age will factor into your decision is how it fits in with your growing method and whether you are looking to maximize your yield or speed up your growth cycle. Plants can be kept in vegetative growth for up to 60 days if you’re looking to create the largest yield possible.

Fairly often though, grow space limits this anyway, so it's often advisable to switch to flowering before your plant has grown too large for your space. 

Plant Height

Only you know how much space you have to grow. The longer plants are kept in a vegetative state, the taller they’ll become. If you leave them too long, you’re going to end up with an overgrown plant for your space, which will lead to your canopy sitting too close to the lights and burning.

How close to let your plant get to the light will depend on your light fixtures. If you have high powered lights, you’ll need to keep your plant's canopy further away from them. A general rule of thumb is to never have your plants closer than 30cm to your light fixtures. 

Growing Methods

Whether you call it a growing method or training your plant, all this will affect when to make the switch from vegetative growth to flowering. Generally, high-stress methods mean your plant will need more time to establish itself in vegetative growth, while low-stress methods mean you can make the switch to flowering earlier. 

If you’re growing multiple plants and using the sea of green method, you want your plants to flower as early as possible so they only produce one large bud. As you’ll generally be growing several indica strains in close-knit quarters here, aim to make the switch to flowering when your plants have reached a height of approximately 15cm to 30cm depending on your grow space. As this is an amplified version of 12-12 from seed, similar rules apply for this method.

With the screen of green method, where you lay a mesh screen horizontally above your plants, you need to leave your plants in vegetative growth until they’ve grown through the screen. This will depend on your individual grow space set up, but screens are generally between 30cm to 60cm above the base of the plants. To reach the screen, your plants will need to stay in vegetative growth for several more weeks than with the sea of green method.

High-Stress Methods

If you’re super cropping to produce larger yields, leave your plant in a vegetative state for longer as it will keep the height of your plant in check. Similarly, if you’re topping by removing the plant's main stem as a seedling to grow more colas, this high-stress method demands you leave your plant in the vegetative growth period for much longer than other methods to ensure you have a healthy, established plant for flowering. 

For those lollipopping, by removing the lower growth of the plant to focus energy on the canopy of the plant, your switch will be based on height and grow space. For sativa strains as they’ll continue shooting up when flowering, around 30cm to 45cm is common. For shorter indica strains, leaving them in vegetative growth up to 100cm is common.

Outdoor Grows

If you’re growing outdoors, typically your plant will flower of its own accord. Usually in line with the seasons, when days become shorter than 12 hours. Some climates don’t allow for this natural light cycle, so you might need to force your plant to switch from vegetative growth. The same rules apply here as indoors, all you need to do is reduce exposure to light to no more than 12 hours a day. 

Sometimes plants will struggle to make the switch even if the natural outdoor light has shortened to 12 hours. If this is the case for your plant, it might be due to man-made lights like garden lights or street lights.

Autoflowering Cannabis

If you’re growing an auto-flowering strain, your plant will start flowering regardless of the light schedule you put it on. This makes auto-flowering plants ideal for outdoor grows as long as the temperature remains hot enough. Generally speaking, auto-flowering plants will switch from vegetative growth to flowering after around 3 to 4 weeks. Flowering stretches for auto-flowering plants will vary depending on the strain.

While you won’t need to worry about light cycles as much or when to make the switch, your plant will still yield more if it receives the recommended amount of light relative to its growth period. So ensuring your plant receives the full 18 hours of light in vegetative growth will help ensure you have a strong and healthy plant with a good yield later on.

The Flowering Stretch

For most strains, the flowering period will be somewhere between 7 to 10 weeks, though there are some up to 12 weeks. Usually, you have a 2 to 3-week window on when to harvest for most plants, but leaving your plant longer will increase your yield as your plant bulks up.

Flowering Too Early

Although the vast majority of growers now use feminized seeds, it's always worth making sure you’ve sexed your plant as early as possible and dealt with the male plants accordingly.

If you’re using a high-stress growth method, flowering too early may also cause your plant to become a hermaphrodite so this is always something to keep an eye on when routinely checking plant health. If you notice a hermaphrodite plant, one with male and female flowers, it's often best to destroy it to limit its influence on other plants. If the plant only produces a few male flowers, these can be removed and the pollen sterilized with water.

Switching with Grobo

If you’re using a Grobo as your grow set up, then you’re in for a much easier time. As an automated grow box, Grobo uses deep water culture, a hydroponic technique, to grow the healthiest plants.

All you need to do is add your plant, pick your grow recipe from our hundreds of strains on our app, and your Grobo system will do the rest.

Find your perfect automated grow box on our site.

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