The 10 most common mistakes when growing cannabis

A hose watering a small cannabis plant in a black pot

As a long-standing grow shop employee, I have had to look at disappointed faces from behind the counter time and time again, as ambitious home growers have only managed to produce a few sorry buds instead of the giant harvest they had been expecting. Usually, these failed harvests were caused by a combination of inexperience and avoidable mistakes.

However, despite an abundance of books, blogs, and video tutorials, many a home grower wishes to find their “own” way – whatever that may be – to grow cannabis at home. Since the arrival of the indoor culture in the 1990s, the same mistakes are made again and again in trying to achieve that. To oppose the many persistent half-truths and mistakes with a few facts, we would like to present you with the 10 most common mistakes when growing cannabis.

1. The wrong manure

This is often cited as the main culprit. However: All manure that is available through professional grow shops contains enough nutrition, in principle, to be effective, even in small doses. Problems are usually self-inflicted and if you have manured according to specifications, usually the problem lies deeper. A grower ensures that nutrients are transported correctly by creating the right climate.

If the nutrients that are given while watering do not reach the leaves, the plant will no longer be supplied sufficiently. The transport problem can have many causes, but the most common ones are using too much manure and/or too much water.

Here are some preventive measures for nutrition transport problems: The use of an EC device to determine the right quantity of manure; calculation of the pH value; fitting a watering system with the exact dosage; and a tensiometer to determine the right time for watering. In principle, the medium should be damp but not too wet.

2. Identifying pests and mould too late

A close-up of three dark green cannabis leaves covered with a layer of white mould

A lot of growers recognise pests and mould only when their plants have already been seriously affected. At this stage, thunderbug, spider mite, soft-bodied mite, powdery mildew and downy mildew have already multiplied to the extent that it is impossible to fight them in a natural way. Only the use of specific insecticides or fungicides will do the trick, and then usually only for a short period of time.

In fact, these remedies should not be available to home growers at all because of their health risks. In a healthy microclimate, pests or diseases that are detected early enough are usually not a problem.

Unfortunately, inexperienced home gardeners often only recognise the damage when it is too late. Here, too late means when the population of thunderbugs, spider mites, white threads and red brown dots is already recognisable with the naked eye.

  • Brown rot and downy mildew are the result of an overly damp and warm climate. Brown rot attacks the flowers from within and has left many a grower in despair, as the extent of the damage only became apparent during the harvest. One can only recognise brown rot when examining the plant extremely closely. The affected flowers will lose their green colour and will turn a greyish green at first.

Usually, when you discover brown rot, most of the inside of the flowers will have already been destroyed. The affected plant should be isolated and the affected parts cut out in order to at least be able to enjoy the flowers that haven’t been affected.

  • Mildew can be cured relatively easy, provided it is discovered early enough. For this, you must inspect the bottoms of the branches for white mould regularly, as mildew usually starts there, before spreading to the stems and leaves.

In the event of downy mildew it helps to spray it with a mix of water and milk at an early stage, or another strongly alkaline mix. In addition, the grow box must be meticulously cleaned.

Powdery mildew, on the other hand, is a virus that the plant will not be able to get rid of. The damage can be restricted by using Bordeaux mixture.

Here are some preventive measures for pests and fungi: Optimal air humidity will make it hard for fungi and pests to spread. Also, inspect your plants for decolouration of leaves, pests’ eggs at the lower stems and the grow medium daily. Fungi damage usually starts at the bottom of the stems or where stems and medium meet. Of course, your growing room should be clean and tidy. Here, you will find more information about pests and pest control.

3. Wrong position of the lamps

This is one of the most fatal mistakes, because sodium vapour lamps lose effectiveness quickly as their distance increases. The poor effectiveness in terms of depth is even stronger with the new LED lamps.

As optimal placement in terms of distance depends on numerous factors, it would be useless to set ground rules for lamp distance. The lamp itself, the reflector, the size of the room, the size of the patch and the power of the lamps all play a major role. As the light intensity of all lighting sources exponentially decreases with every centimetre of extra distance, you will only optimally benefit from your expensive lighting when it is placed at the perfect distance.

If you hang the lamps too close to the plants tops they are likely to damage the plants because of the intensive heat – the top leaves will dry out and die.

Measures: Use a thermometer at the height of the plant tops. Use a flexible hanging device to place the lamps as closely over the plants as possible without exposing the flower tops to a temperature over 28-29°C. Here, you will find more information about lighting equipment and reflectors.

A close-up of a cannabis plant with a bright light shining on it from behind

4. A little more is all right

No! All external influences should be limited to an absolute minimum. Watering, changing pots, picking leaves, changing the position of the pot, or even touching the leaves: these are all stress factors.

Beginners especially, never stop mothering and pampering their – usually – illegal tenants. Proper care of course is a prerequisite for a successful enterprise but examining seedlings meticulously several times a day or watering them three times a day will actually hamper development instead of stimulating proper root development.

Lack of experience also often leads to panic, for instance when beginners or those who are too lazy to read the manual detect harmless changes such as leaves that become yellow and die off at the lower and darker areas of a grow box, and instantly take unnecessary measures.

5. Root growth

When the flowering cycle is started, it’s irrelevant how big the plant is. A large and healthy root ball during the vegetative period is much more important for optimal development. In the first 3 to 4 weeks of flowering, the plants will grow very quickly without showing any recognisable defects, even when root development has been poor.

However, as soon as the flowers are supposed to get heavier after the fourth week, their need for phosphorus and, especially, potassium increases. If the roots have not developed well when the plant starts flowering, the consequences will only be visible after 3 or 4 weeks.

If the roots are not capable of transporting all the nutrients that the plant – which by now has grown considerably – needs, defects will start showing in spite of sufficient manuring. Here, you will find more about the development of healthy hemp roots.

6. Cold feet, overly low night temperatures and overly damp flowering room

These three factors are closely interconnected and can spoil your entire harvest. When humidity is too high – 60% or more – the flowers will hardly grow during the last weeks of the flowering cycle.

In a box that exhibits a comfortable temperature of 25 to 28°C at the height of the plant tops, it can easily be only 12°C or less at ground level. In those types of boxes, where plants have cold feet, climate will always be a problem as the metabolic process in the root area slows down. If less nutrients are processed by the metabolic process the soil will be damp all the time as a result of which humidity increases.

Furthermore, in boxes that are placed in unheated surroundings, the plants will have cold feet, and it will be cold more quickly during the dark phase. If the difference between the light and dark phases is more than 12 to 14°C the metabolic process will slow down even more.

Countermeasures: Fit the grow room with Styrofoam. Use black pots. If necessary, use heating during the dark phase. Here, you will find more about the climate in the grow room.

7. Too early, too mean, too impatient

Indoor: often, growers are too impatient to wait until their variety has really matured, and then to wait even longer until the drying process has progressed to the stage where the specific aroma of the variety has developed. Even perfectly grown cannabis can deteriorate into an average herb if the flowers are harvested too early. And letting them dry for too short a period or in the wrong fashion can make your cannabis unusable.

Artificial acceleration by means of heating or even hot oven air, which is used for commercially-grown cannabis, will result in poor pot with plenty of chlorophyll. It is particularly fatal to wrap buds that have not properly dried in plastic, as they need to dry out to 20% humidity or less. Here, you will find more about drying.

Outdoor: planting out small plants too early, planting out seedlings that haven’t been pre-grown, or using varieties that flower for too long –  these will all result in outdoor problems. Plants that are too weak or too small can be easily eaten by animals in spring or will be overgrown by the surrounding vegetation.

Varieties that flower for a long period have problems finishing in the short, northern European summer followed by a damp autumn, and often fall victim to brown rot or early frost.

Countermeasures: Properly pre-grow your plants before planting them out. Make sure you are aware of the flowering time of your variety and whether it is outdoor. Here, you will find more about this subject.

8. Higher, quicker, further

Not even the most expensive grow box with the best equipment in the world will guarantee horticultural success. On the contrary, if you use expensive technology the wrong way you will burn routs, leaves, and money. Watering maniacs will not become better gardeners because they have bought a high-end watering installation. With that sort of attitude, they are more likely to cause water damage by incorrectly assembling their expensive gear.

Deploying a 600w bulb instead of a 400w bulb with high-end reflector does not automatically result in a higher yield, but often in temperature problems. Quite often, high investment is followed by a few bad harvests after which the person in question doesn’t want to continue their hobby. They try and sell their equipment and often complain in the following way: “Grow shop X doesn’t buy used equipment, shop Y doesn’t pay enough, and I don’t trust eBay.”

Countermeasures: Make sure that you are setup is expandable from the start. Grow along with it instead of starting on a high note.

9. Wrong advice

After your initial purchase in the grow shop and the time-consuming assembly of box and equipment, you are tasked with correctly assembling two boxes and about 200 parts. Normally, a grow shop employee cannot express what’s on their mind as they are not allowed to even mention the word cannabis, let alone give advice on growing cannabis. Although the shop is protecting itself and its customers that way, some customers perceive this as unfriendliness, arrogance, and narrow-mindedness.

In other words, the friendly person behind the counter has sold the whole shebang with a guarantee for horticultural success. How does a pH meter work? What exactly is an Ec meter? Are these things explained in the book that must be somewhere hidden in the big box? When growing indoor, many details must be considered even when you are cultivating a very uncomplicated variety. Uncomplicated in this case means growing in soil, using a simple and comprehensive manure programme, and refraining from gadgets.

Anyway, beginners who like their new hobby or those who have outgrown their initial installation will increasingly buy more professional equipment and try other techniques as time goes by. Anyone who is unsure whether their new hobby will be a long-term thing or who breaks into sweat when having to water their plants is better advised to quit straight away.

Anyone who purchases without knowing in advance what will be in their tent or box cannot be advised properly, not even theoretically. Different varieties require different conditions, and the number of plants and the size of the pots will also seriously influence the growing conditions later. Anyone who knows in advance which variety and amount they will be going for, and whether they will be using seed or seedlings, will find their initial purchases easier to make.

Measures: Conduct research and read up in advance. This will make communication easier for both sides, will protect inexperienced growers from shabby offers, and will prevent money wasting.

10. Resistance to advice

This is exactly the opposite of the previous point. Despite the availability of good professional literature, fake news sometimes does enter the growers’ community. Some of it is short-lived, other fake news is surprisingly persistent. The reason for this is that there are hardly scientific data available with respect to growing cannabis, but all the more individual experiences.

Noise pollution, unpaid bills, electricity theft or unnecessary arguments with the neighbours often result in that unwanted knock on the door. This is not caused by some Higher Force, as many victims claim. However, it can be easily avoided by adhering to a few simple rules.

Special soil is called special soil for a reason and not only to line the pockets of Dutch agricultural producers. Flower soil will simply not do the trick.

You cannot light a full square metre with a 125w bulb, even when it’s a state-of-the-art LED.

You should not light the mother plants for 16 hours just to save electricity, even when someone has said so on a blog or your neighbour, who is also a grower, insists on it.

Your cultivation is your thing and your thing only. You don’t grow cannabis for the purpose self-affirmation among friends or to show on Facebook how cool you are. The exhaust air system must be mounted before the plants have all but died or your whole house stinks.

Measures: No Smell-no tell – Read up – Cannabis Culture trips (e.g. to the Hash, Marijuana and Hemp Museum in Amsterdam or Barcelona)

Growing cannabis is subject to a licence and otherwise strictly prohibited. The information about the growing techniques that are described here are based on overviews in several cannabis professional magazines, blogs, and online portals. They should not be used to illegally grow cannabis. This text is meant to inform, not to incite.

  • Disclaimer:
    Laws and regulations regarding cannabis cultivation differ from country to country. Sensi Seeds therefore strongly advises you to check your local laws and regulations. Do not act in conflict with the law.


3 thoughts on “The 10 most common mistakes when growing cannabis”

  1. Very good info for beginner growers. And they ARE very common mistakes. I wish I read this when I first started 7 years ago!
    This person took the time to gather information and put it up to help and these idiots are making stupid comments

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  • Michael_Knodt

    Michael Knodt

    Berlin-based Michael Knodt specialises in cannabis and hemp-related journalism. Over the years, he’s written extensively on the topic for various German and English magazines, including Vice Magazine. Michael is the face and moderator of DerMicha, the popular German YouTube channel on cannabis and its prohibition.
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