Hydroponic Cannabis Cultivation Is Best Done Indoors

Growing cannabis does not have to be a difficult process, in fact we love pointing out that growing cannabis outdoors with minimal investment can have amazing results. In regions with long, warm summers and lots of sunshine, you can wonder whether setting up an indoor grow room is actually worth the effort. In the spirit of keeping it simple, growing cannabis outdoors usually means soil is the way to go.

Indoor cannabis cultivation is a bit of a different beast, here all sorts of efficiency questions come into play. Because your “season” is much shorter and the fact that you’re in complete control of the growing environment, it makes sense to make this process as efficient as possible. Growing cannabis in a hydroponic system means faster veg times and can make it very easy to produce high quality buds. It’s no wonder that “hydro” cannabis, or simply “dro”, has a reputation for being extremely potent.

Hydroponic cannabis cultivation means that you’re growing in an inert medium, that is to say a sterile, non-living medium that doesn’t contain any nutrition whatsoever. While soil contains nutrition, bacteria and fungi that promote plant growth, hydroponic grow media contain absolutely nothing. Growing cannabis in a hydroponic system means you’ll have to provide the nutrients (and possibly beneficial bacteria and fungi) in your feeding solutions. Fortunately there are many cannabis nutrition companies that provide excellent products to make this process less confusing.

1 – Medium

Growing cannabis in a hydroponic system can be very simple depending on which medium you use. The main difference between media lies in how often they have to be watered. Soil grows are easy because you don’t have to water or feed very often, whereas more intricate commercial hydroponic systems will feed as often as possible.

One of the more complicated hydroponic setups is the “nutrient film technique”, or NFT. These plants will grow far beyond the small amount of medium they’re in and need to be watered up to 2-8 times per day.

The most common media of choice are coco and rockwool, other popular choices include hydroton clay pellets, perlite and mapito. The same general rules apply for all of these media. There are also several options to grow without any medium at all, but we’ll leave those for a future article.

If you’ve never used a hydroponic system we recommend starting out in coco coir as it is similar to soil in many respects and very straightforward to use. Most nutrient companies also sell products that are specifically designed for this medium, allowing you to skip a lot of guesswork.

2 – Watering System

You should consider a number of things in your hydroponic system design, the main thing being how you intend to water all your plants. With the exception of Deep Water Culture (DWC), hydroponically grown cannabis plants will require much more frequent waterings and feedings than soil grown plants. Most hydroponic growers use (digital) watering systems that automatically feed plants every day or multiple times per day. Does your plant count warrant using a reservoir and drip system, or is your setup manageable enough that you can hand water?

If you only have a small setup with a few plants it is usually easier to mix your nutrient solution every time you water. This will prevent your nutrients from going stale, saves space and means you won’t have to buy any extra equipment.

This grow is small enough to hand water right now, but this drip feed hydroponic system won’t be a luxury in a month or two.

In bigger setups you’ll usually have to resort to an automated watering system connected to a reservoir with nutrient solution. The most reliable ways to deliver these nutrients are top feeding through a drip line or bottom feeding through a flood and drain setup. Make sure your reservoir is always properly aerated by using an air pump with one or more air stones. A simple aquarium pump will add plenty of oxygen, keep the water moving and prevent algae, mold and harmful bacteria from growing in your reservoir.

3 – Mixing Nutrients For A Hydroponic System

Mixing your nutrient solution is a simple process, but has to be done accurately. In a normal workflow you would fill up your reservoir or watering can with (preferably reverse osmosis) water, add nutrients and possible additives and finally adjust the pH.

The ingredients for a basic nutrient solution are: water and hydroponic nutrients. We’re using the Jungle Juice line by Advanced Nutrients as an example here, but there are many similar products by other companies as well. You’ll almost always have to adjust the pH down to around 5.8 using a pH- solution.

Nutrient concentrations are very dependent on which strain you are growing, but the general rule is that you’d “rather be under than over”. Overfeeding can shock a cannabis for days if not weeks and basically cripple a plant, while plants can go without nutrients for days without any health problems. Having a bit of faith in your nutrient companies of choice is one thing, but you should always be aware that your tap water can be very different then theirs.

underfed cannabis plant vs nutrition burn cannabis plant

Left you see a unhealthy small drooping plant, yellow-ish color all over then in the middle a vigorous looking cannabis plant, healthy green color and on the right tense” dark green plant with “taco-ing” leaves with burnt tips that curl down

You might get decent results following the exact instructions on the bottle, but it’s much better to actually understand what you’re doing. Start off slow by using 25-50% of the recommended dose and work your way up. You’ll rarely run into deficiencies, but these will be quite clear if they do start to occur. If your plants start showing burnt leaf tips, you’re adding too much nutrients. There’s no need to rush, a lot of feeding regimes don’t reach their maximum dosage until a good 4 weeks into flowering. We recommend getting an EC (or ppm) meter so that you can accurately compare nutrient concentrations between different feeding solutions.

At Spliff Seeds we’ve seen a lot of soil growers making their way into hydroponics with mixed results. The ones that kill all their plants in a few weeks usually make the same mistake: they’re playing fast and loose with their pH.

Most nutrient problems are linked to pH, just by looking at this picture you can be 90% sure the pH of the nutrient solution was off by quite a bit.

Growing hydroponic cannabis isn’t very complicated, but you do have to follow certain guidelines for nutrition that you might be able to ignore in soil. Because soil has a natural pH buffering capacity you can get away with not measuring your feeding solutions, but doing the same in a hydroponic setup will usually mess up a crop quite quickly. Growing hydroponic cannabis means that you have to measure anything that you give to your plants!

It is virtually impossible to be successful at growing hydroponic cannabis without constantly measuring the pH of your nutrient solutions. A pH meter should probably be your first investment if you intend to start growing hydroponic cannabis. If you’re only using a single reservoir using a “pH meter kit” will suffice, but we recommend getting a digital pH meter as this will save you a lot of hassle.

Keep your pH between 5.2 and 6.2, ideally at 5.8. Higher or lower than these values will kill your plants, or at least cause a lot of lockout and deficiency issues that can easily be avoided.

The grower in this picture has made the wise decision to measure his reservoir before watering his plants. Watering with this pH 6.5 solution would do a lot of damage to his plants.

5 – Watering In A Hydroponic System

Once you’ve decided how to water, how much nutrients to give your plants and adjusted the pH of your nutrient solution, you’ll have to determine your optimal watering frequency. The idea behind growing hydroponic cannabis is that your medium dries out rapidly so that the roots have access to oxygen. This also allows you to give each cannabis plant a new dose of nutrients much more often than is possible in soil.

Growing cannabis this way means you’re constantly looking for a balance between too wet and too dry. How fast your medium dries depends on the type of medium, but is also very dependant on the environment and the size of the cannabis plant. Media with high water retention like coco coir will require fewer waterings than low-retention media like clay pellets or perlite. Different cannabis plant stages require different amounts of water, so prepare accordingly.

A flood table like this is filled up with nutrient solution and then drained in order to feed every cannabis plant in the table at once.

Just like with nutrient concentrations though, you’re better off going under than over. An underwatered cannabis plant can usually recover in a few hours if it is watered again, but there’s not much that can be done to fix overwatering. Use your best judgement and make sure your medium is at least partially dried out before watering. Multiple waterings per day might become essential once you’re halfway through flowering, but will probably kill seedlings. We recommend hand watering during the beginning stages of a grow for this same reason.

6 – Cannabis Plant Structure

If you’ve ever picked up a bag of coco coir or perlite the first thing you probably noticed is the weight. Most hydroponic grow media are much lighter than soil, which is great for optimal aeration and water retention, but not so much for stability. Because the media don’t provide much of a “stable base” and allow for such massive growth, plants can become top heavy quite quickly.

This enormous Amnesiac plant is being held up by two layers of trellis. Plants this big simply weigh too much to stay upright in a 25 liter pot.

It is always a good idea to provide some support for your plants towards late flowering, but even more so in hydroponic media. Make sure you prepare your stakes, trellises, yoyo’s or whatever else you might like to use before the flowering phase really kicks in, or your cannabis plant might topple over. You can use additives in your nutrient solution to strengthen branches slightly, but these won’t necessarily make a world of difference.

The average cannabis plant will also produce more foliage and side branches when grown in hydro media compared to soil, make sure you are prepared for this too. Expect explosive growth during veg and the first three weeks of flower and consider pruning/defoliating your cannabis plant to make optimal use of your grow room.

7 – Shorter Flushing Phase Than In Soil

Once you’re finally closing in on harvest time you should start thinking about flushing your medium to remove any leftover nutrients. Growing cannabis in soil means your medium will retain a lot of nutrients regardless, making two week flush phases quite common. Growing hydroponic cannabis allows you to flush for much shorter periods without any adverse effects.

Because you’re flushing out nutrients at a much faster rate, you can still get beautiful autumn colors within a week.

A week of flushing will usually be plenty for most hydroponic media, even the ones that retain small amounts of nutrients. We like to adjust the pH to prevent a possible shock effect, others argue that pH- contains nutrients and like to flush with pure RO water. There is a permanent discussion about whether or not you should adjust the pH of flush water, so do what works for you and don’t get distracted by details.

This is really all you need to know

We hope that this article has made the basics of growing cannabis in a hydroponic system a bit clearer. Most growers will start using more complicated techniques and feeding regimes as they become more experience, but hydroponics can be very simple if you’re not going for maximum efficiency. That being said, there’s an endless amount of details to work into your hydroponic system that can all help improve your yields. Just accept the fact that it’ll take at least a few grows before you’ll pull a real monster crop.