How Big Can a Weed Plant Get? Growing Monster Plants Indoors and Out

How big a cannabis plant gets comes down to space, container size, strain and how long it stays in veg. With a big container and a head start indoors, a plant can reach 1.4 to 1.8 m (about 4.5 to 6 ft) by the end of veg, and once it moves outdoors it can double or triple that size. Most people picture giant weed plants as an outdoor thing, and it's true that the biggest plants come from wide-open grow spaces, but that doesn't mean indoor growers are stuck harvesting medium-sized plants. To grow giant cannabis in a grow room or tent, get ready to make a few changes.

How do you grow giant weed plants indoors?

Figuring out how to grow giant cannabis indoors can be tricky for some, because a lot of us don't measure our space properly from the start. The rush and excitement of setting everything up can work against you, so take this step slow. Here's what to keep in mind to pull off giant cannabis plants.

Giant cannabis needs space:

A typical 1 × 1 × 2 m (about 3 × 3 × 6.5 ft) grow space might hold up to a dozen clones in small containers, and that same area can later be filled with 1 to 4 cannabis “trees” in big containers. Before you pick a strain that really stretches, make sure you've got the room for it.

Use big containers:

For plants like these, skip the in-between container sizes and start with 20 liters (about 5 gallons), and if they're air-pots, even better. Giant plants need plenty of room for their roots, and whatever containers you use need good drainage.

The right strain for big cannabis plants

As for strains, if conditions allow, almost any strain can grow into a giant, but for top-notch buds you can go with Moby Dick, White Widow or Gorilla Glue. Do your own research on which strains you like and what kind of yield each one gives. Shop around different seed banks.

The best growing medium for giant cannabis

When it comes to growing medium, organic is the way to go, but you can also go hydro, and if you do, “hempy buckets” are ideal. You'll need a 20-liter (5-gallon) bucket with a small drainage hole, filled with a mix of 70% perlite and 30% vermiculite.

If you want to learn more about hydroponics, check out our online course (in Spanish, from Argentina) by clicking here

If you're planning an organic grow, the most important thing is more soil and really big containers, like we said above. 

How to water giant cannabis plants

A giant cannabis plant will need more water, but don't overdo it. The amount of water should be 10 to 20% of the container's volume. If you're using a 30-liter (8-gallon) container, give it 3 to 6 liters (roughly 0.8 to 1.6 gallons) a day, and make sure drainage is working properly. Don't forget your pH! In regular soil, aim for a pH between 6.0 and 6.5. If you're going organic or hydro, keep it a bit lower, around 5.8 to 6.3.

Lighting for giant cannabis plants

When it comes to lights, you can stick with your usual setup, but read each light's manual carefully and respect the recommended hanging heights. Remember that roots are everything for these plants, so don't overload them with nutrients. Go with high-quality basic organic nutrients plus one or two synthetic bloom boosters for the best results.

Training techniques like LST (low-stress training), topping, FIMming, lollipopping and the SCROG (screen of green) method are all highly recommended. For the best results, combine several of them based on the shape and size of your grow space.

Finally, if you take a cutting from a flowering plant and root it, you'll end up with a born monster on your hands, but this only works with photoperiod strains. Monster cropping is a technique where you re-veg a clone taken during flowering. It's not the prettiest plant, but these little monsters grow into dense bushes and are the best-kept secret of giant-plant growers.

Planning ahead: how to grow the biggest weed plant possible

As with any plant, the prep work directly affects how long your cannabis can grow, and that drives its final size and yield. So get ahead of things and head off any problems early, so every stage goes as smoothly as possible. To grow giant cannabis plants, ideally you'll germinate in early February (Northern Hemisphere timing; in the Southern Hemisphere, shift it six months to early August).

Starting indoors:

If you start indoors, you'll need to set up a dedicated space for healthy growth. The goal is a vigorous, healthy root mass, transplanting step by step and keeping an eye on the timing. You can use different lighting setups, fluorescents included, on a light schedule of about 18 hours on and 6 hours off to push vegetative growth and end up with giant cannabis plants.

By the end of the indoor growth phase, the plants should average 1.4 to 1.8 meters (about 4.5 to 6 feet) tall. That's your sign it's time to move them into a permanent container, preferably outdoors. In the Northern Hemisphere that usually happens in late May or early June (late November or early December south of the equator), and they'll keep growing for two more full months.

Growing giant cannabis outdoors

Outdoors, by the time the season kicks off, your plant should already have sprouted. From there you've got about 3 months to push its growth as far as it'll go. If you see roots poking out of the bottom of the container, that's the sign it's run out of room to keep growing, so transplant it carefully.

You'll also need to know the right fertilizer doses to grow a big cannabis plant, always adjusting them to the strain you're growing.

Outdoors, cannabis plants can double or triple in size. To get there, prep the soil before moving them outside, and keep the plants in the dark for at least 48 hours beforehand to reduce stress. Then let them acclimate gradually to the heat and intensity of the sun by setting them near a bright window for two or three days.