The vegetative growth of cannabis plants can be one of the longest periods of cultivation when growing outdoors – or indoors – where in, for example the countries of southern Europe they enjoy a lengthy spring and summer. During the course of this growth phase, problems can arise that may lead to the death of the plant even before the flowering stage begins.

Let’s see what are the most frequent causes of death during the plants’ growth period and what we can do to avoid a premature, unhappy ending.

Overwatering cannabis plants

The most common cause, especially among beginner growers, is undoubtedly overwatering. The excess of irrigation particularly affects young plants in their first weeks of life (although it can also occur with larger plants).

The main reason is watering too much, too frequently. Small plants have a very limited resistance, partly because of their size. A plant with two cotyledons and three small roots can not cope with large amounts of water, simply because they can not process it, not having sufficient absorption capacity in the root zone.

If we overwater our plants, the substrate will be permanently wet, meaning the roots will not get enough oxygen and they can begin to rot, falling prey to various fungal pathogens that are capable of killing the plant in just a few days. Depending on their genetic inheritance, there are varieties that are are better or worse adapted to withstand an excess of irrigation, for example Afghani genetics such as Northern Lights; originating in a dry, arid climate, they can not tolerate continuous moisture in the root zone. On the other hand, cannabis plants with tropical ancestors will tend to have a greater resistance to root rot, as in the case of the world-famous Skunk.

To avoid a plant suffering from excessive watering during its cultivation, first we need to be careful not to swamp the substrate. If this happens, let it dry before watering again. Avoiding heavy substrates with too much clay will also help, remember that the roots also need oxygen!

A long period of wetness in the substrate means that the roots can’t transport food to the upper parts of the plant, which will feed from its nutrient reserves, causing the smaller leaves to lose their colour gradually, from a pretty green to a yellowish shade.

Once the leaves of the plant change to a yellow colour due to excessive watering, root rot will often come next, and with it the consequent death of the plant. Another symptom of possible overwatering is if the plant appears to be wilted (with droopy leaves) even though the substrate is moist.

The most harmful fungal pathogens for cannabis plants

Pythium, Fusarium and Phytophthora are the 3 most harmful moulds to cannabis plants, capable of killing them in a very short time and with almost no possibility of treatment. Prevention is always the best method to combat these fungal pathogens.

These fungi come into contact with the plants via the roots, usually due to root rot or any decomposing organic matter such as the fallen leaves of the plant that remain on the soil surface. For this reason it’s important to remove any dead leaves from the substrate where they could cause a fungal infection.

Another way to protect the roots against fungal attacks is through the use of microbials, with the fungus trichoderma. This type of microbial life protects the roots while optimising nutrient uptake to the plant much more effectively than in a lifeless substrate.

This beneficial fungus colonises the roots and leaves no room for harmful fungi to settle on them, and if they do, the trichoderma takes care of eliminating them. This type of microbe can be used any time from germination until harvest, although ideally we’d use it from the beginning and maintain it in good conditions, or keep adding it with each transplant. Using sugars and carbohydrates helps the micro-life to stay active throughout the grow.

Drought or under-watering

A lack of water, or drought stress is another of the principal factors that can seriously affect plants and cause premature death. The plants require water for their survival, and if they run out of it for a few days or even hours (depending on the cultivation system) they can die very easily.

Even if the plant manages to survive the drought period, it’s inevitable that part of the root system will be affected. When part of the root system dries out and dies, it is usually accompanied by some of the leaves and branches, which will weaken the plant and reduce it’s capacity to process nutrients, impacting negatively on development and translating into a smaller final harvest.

If this period of drought is repeated or is too prolonged, the plant may not be able to survive as well it could the last time, as its root system is now much less extensive than it was initially. Remember that you can use products to stimulate root growth, helping the plant to develop more roots and therefore feed more efficiently and more able to tolerate any periods of drought.

Lack of light in the grow

Although it is not very common in outdoor gardens, where fortunately the sun is always present, in indoor cultivation a lack of illumination could occur for various reasons: if we have a blackout, a bulb fails, the timer doesn’t work properly, the ballast stops running unexpectedly or if we have some other problem with the electricity supply or our installation.

In this case, the plants can last for a few days without light, although they will be greatly weakened and more likely to experience possible rotting of roots and leaves if the humidity in the grow space is high. Initially, the upper leaves will take on a yellowish color due to a lack of chlorophyll, as the plant can’t carry out photosynthesis properly.

Too much light in the grow

Lights that are too powerful or too close to the plant can dehydrate the leaves, curving the edges upward (indicating a lack of transpiration). When the stomata – which are small holes in the leaves where the plants breathe – close, the plants can not “breathe” properly, and if we add to this a very low humidity, they can dry out quickly and end up dying. Therefore, it is vital to carefully select the proper type of lighting for a cannabis grow.

Excess of fertilisers, nutrients or salts

Plants require food to be able to grow, but an excess of fertiliser could kill the plants if we don’t take quick action to remedy it.

Organic fertilisers don’t tend to over-fertilise plants because they release nutrients slowly. If using mineral fertilisers for cannabis, this possible problem must be taken into account if we want to avoid problems.

To find out if the substrate has an excess of mineral salts, we need to monitor the EC of the drainage water. If the runoff comes out with a similar EC to the inlet (the one used for irrigation) then everything is correct. However, if the EC of the drainage water is much higher than the inlet water, a root-flush should be performed to wash the excess salts out of the substrate. Using enzymes in each irrigation helps keep the substrate free of salts.

Insect infestation

Insect pests can be many and varied, and although not all are 100% fatal to the plant, many of them are capable of devastating our garden if they’re not treated properly. Of course, it’s always possible that the plant will survive the infestation, but a serious insect attack will almost always lead to weak plants and lower yields.

One of the most feared insect infestations is the stem-boring caterpillar, which eats into the central stem and hollows it out. The flow of the sap is obstructed, halting growth and after a few days the plant ends up wilting and dying. It can be very difficult to detect with the naked eye and to find it you’ll need to look for holes in the stem, usually beneath the nodes of the non-woody parts which have more tender vegetable tissue that’s easily edible by the caterpillar.

Red Spider Mites are also one of the most common and devastating pests if not treated in time. In addition, their eggs can remain dormant for a long time, ready to hatch and infect the crop again when the conditions are right for their development.

Snails and slugs can be very harmful when the plants are very small and don’t yet have a lot of leaves. If the plant only has 4 leaves and these small devourers eat them, it will not be able to transpire or capture solar energy for photosynthesis, which can seriously compromise its development.

Fungus gnats, nematodes, aphids, or any root-feeding insect can cause the plant to lose its vigour during the growth period, meaning that the flowering phase can be seriously affected.

Extreme temperatures: hot y cold in the grow

Extreme temperatures – both cold and hot – can be lethal to plants. In winter, the cold can be controlled using radiators indoors, and even in outdoor cultivation through the use of greenhouses.

Keep in mind that if the temperature drops below 12ºC, the plants may die. From 12 to 16ºC the plants growth is halted as they become dormant, as if they were hibernating, but essentially what they do is completely slow their metabolism down to keep alive while waiting for the climate to improve.

A temperature lower than 12ºC can kill the plant in case of prolonged exposure and it is not remedied. They can also die by freezing when the temperatures get as low as 0°C.

On the contrary, extreme heat is another factor that can cause the plant to die. Although it is less harmful than the cold, it also ends up severely affecting the development of the plant in all its stages of life, hindering its transpiration, nutrition and cellular development.

With high temperatures the roots can be “cooked”, for this reason when growing outdoors we recommend using white pots instead of black, and if possible to cover them from the direct impact of the sun using insulating material.

Dehydration is very common in high temperatures, since the humidity of the substrate will evaporate more quickly, the roots dry out, the stomata close and the plant cannot transpire and will not develop with the proper vigour and vitality.

To fight against extreme temperatures in an indoor grow, air conditioners or radiators can be used. In the case of an outdoor crop, you can use greenhouses against the cold and shading mesh to protect plants from the suffocating heat of summer.

Conclusion

These are the most common causes of death in young cannabis plants. Remember that depending on the case, this can happen in a very short time and if we don’t take action, the plant most definitely will end up dead.

It should be noted that, although these are the most common causes, they can occur with greater or lesser ease depending on the growing conditions that the plants have to endure. Of course, if a substrate is used that is not suitable for cannabis cultivation (such as field clay), problems with the roots may be more present than using a suitable substrate for cannabis.

A lack of ventilation in the cannabis grow space can facilitate the appearance of fungi due to stale air and excessive heat in the environment, as well as a lower evaporation of moisture from the substrate. In turn, this encourages the possible appearance of insects such as white flies or fungus gnats, in addition to promoting the appearance of fungal pathogens such as botrytis, pythium or fusarium.

In many cases, where problems discussed in this article arise in the grow, it is usually due to poor management of the environment or not taking into account the needs of the plants when selecting the appropriate substrate or fertiliser. One simple error in a cannabis grow can cause the appearance of other problems that are even more devastating, so for this reason it is important that all aspects of the crop are well under control before starting out on the grow adventure, and avoid nasty surprises such as the premature death of your plants.

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