How to Plan an Indoor Cannabis Grow: Genetics, Lights and Airflow

To plan an indoor cannabis grow, nail down four things before you buy anything: genetics that fit your space (compact indicas or indica-leaning hybrids), a solid grow light, ventilation with an exhaust fan and fresh-air intake, and a tent or room sized for your setup. Planning ahead matters because outdoor temperatures can make or break a grow, and in the hottest months many outdoor growers simply have to stop.

That's why so many growers move indoors. It's not complicated, but plenty of people get stuck on how to start, how long an indoor weed plant lives or how to grow inside at all. If it's your first time, keep a few basic concepts in mind, and we'll walk you through them: lighting, ventilation, the best genetics for indoor and more.

What do you need to grow indoors? Seeds or clones?

Seeds or clones? Honestly, either works. What matters for a home indoor grow is choosing a strain that fits your tent or grow space. Indicas and indica/sativa hybrids will always be easier to manage than sativas or sativa-dominant hybrids, which can grow huge and turn an indoor setup into an out-of-control jungle. Sativas also usually have a much longer flowering period, sometimes up to 3-4 months.

Clones have one big advantage: you can know how they'll behave before you even start growing them indoors. If you know the person who gave you the cutting, they can tell you how tall the plant gets, how it does in containers, how long it takes to flower, how much it yields, what the buds taste like, what kind of high it gives and so on.

Clones are also much easier to manage, since you can flip them to flower at whatever size they are. Seeds, on the other hand, can be a bit of a lottery: even within the same strain, one phenotype might yield more than another, take longer to flower or come out more potent.

Choosing your grow lights

Next up for any indoor grow is the right lighting system. Not every lamp works for growing (halogen bulbs, for example, don't cut it). Today there are plenty of effective options, from classic high-pressure sodium (HPS) lamps made for flowering and metal halide (MH) for veg, to compact fluorescents (CFLs), LED grow lights and LEC. Either way, lighting is one of the pillars of growing weed indoors, so it's worth making a smart investment: both plant growth and a solid harvest depend on it.

LED, HPS or LEC: which grow lights are best?

LEDs run cool, use little power and last a long time. The catch is that a good LED panel is usually expensive and takes a while to pay for itself. HPS and metal halide lamps are the cheapest up front and pay off quickly. On the downside, they're power-hungry, their bulbs have a short lifespan (2-3 grows before they start losing efficiency) and they put out a lot of heat. Somewhere in the middle are the newer LEC lamps, similar to HPS and MH: pricier, but they use less power and last much longer.

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Setting up ventilation

Cannabis uses a lot of CO2 in an indoor grow. Outdoors, plants have unlimited CO2; indoors, they burn through it fast. That's why ventilation is another pillar of growing, and you need steady fresh-air exchange. In some cases, you'll also need a humidifier for your grow.

You also need to get rid of the heat your lights put out. That's what an exhaust fan is for, and depending on the size of your grow tent or room, you may need an intake fan too, which is basically an exhaust fan working in reverse: it pulls air in instead of pushing it out. In small spaces like a grow tent, passive intake can be enough, meaning air coming in through any gap or vent. Hot air rises, so the exhaust fan always goes up high in your tent or grow space. The intake goes down low, ideally on the opposite side from the exhaust. One or more oscillating fans to move the air around are a good idea too. They also help strengthen your plants and prevent light burn.

Once again, there are grow tents in just about every size, from tiny 40 x 40 cm (16 x 16 in) units to huge 600 x 600 cm (20 x 20 ft) setups and beyond. It's all about finding one that fits your needs and the space you have. And that covers the essentials for growing cannabis indoors.

How many weeks of flowering indoors?

How long flowering lasts in an indoor cannabis grow depends on the strain you're growing and the kind of harvest you're after. In general, though, indoor flowering usually runs 6 to 10 weeks.

Keep in mind this isn't everything you need to know about indoor growing. These are some of the most important things to think about when you start, but you'll also need to consider the size of your space, which container size makes sense, which growing medium to use, your watering system, your fertilizers and everything else that'll help you grow some seriously impressive plants.