Overwatering cannabis plants

Water is essential to life; without water, nothing thrives, but like everything else, too much of a good thing can be harmful. Today we’re going to explain how overwatering affects cannabis plants during their life and different stages.

Cannabis grown indoors
Cannabis grown indoors

Cannabis plants need sun, nutrients, water, and substrate to survive. We can divide the plant’s life into different phases: germination, seedling, vegging, and flowering plants.

Overwatering and seed germination

The paper towel method

Cannabis seeds need moisture, heat, and oxygen to germinate. If these three elements fall apart, seed germination can be jeopardised and might not be completed.

The quintessential method when germinating cannabis consists in placing the seeds between two paper towels or two plates, preventing the moisture from escaping too quickly, and allowing the seed to enjoy a stable environmental humidity.

Properly germinated and nice coloured seed
Properly germinated and nice coloured seed

When you exceed the water dose at the time of germination, it’s very likely that your seeds will rot quickly. If, in addition, it’s a warm time of the year, this fungal infection can be even faster.

Stagnant water is synonymous with fungi, so you need to make sure that any excess water is thoroughly drained before placing your seeds between the paper towels. It is also highly recommended to change the paper towels every 2 days when the seeds take a little longer to sprout, as fungi can emerge more easily and infect both the seeds and the germination medium.

If your seeds don’t germinate all at the same time, but little by little, remove those that have already germinated to prevent them from rotting. You will know they’re rotting because the tip of the root will take on a black, brown or dark yellow hue.

rotten seed excess of water
Rotten seeds due to an excess of water

At the beginning, it’s also possible to use low doses of hydrogen peroxide to prevent the development of fungi, but it's always best to avoid this and use instead beneficial fungi such as Trichoderma, which, on top of preventing the risk of fungi like Fusarium, it also contributes to rapid root growth.

Germination with jiffy pellets

There are other germination methods apart from the paper towel technique, such as directly germinating in peat moss pellets. In this case, you’ll need to take into account several factors to avoid any errors, as it's not as simple as just placing the seed in the jiffy.

First, you’ll have to let the jiffy’s fill up with clean water to prevent contaminants such as fungi or viruses from, for example, wells, streams, and other places.

Once the jiffy pellets are soaked and have expanded to their maximum size after being completely hydrated, you’ll need to squeeze them a bit with your hands to get rid of the excess of water that might be trapped inside them.

Seeds germinating in jiffy pellets
Seeds germinating in jiffy pellets

If the pellet stays compact after the squeeze, you’ll have to decompress it, keeping the moisture inside the jiffy but without it being too compacted. This way, when the roots emerge, they can easily grow through the pellets, and the plants can begin to grow above the surface.

Remember not to push the seed too deep inside the jiffy, and simply place it on the top and cover it with a little loose soil from the same pellet. This way, the plants will pop their heads in about 4-6 days, at which time they should be transplanted.

Small plants and overwatering

Once the germination phase is over, the plants will enter the seedling (small plant) stage.

At this point, you must be very careful, as it’s one of the most challenging moments in the plants’ life, since they’re very small and frail. Any serious error could lead to premature death.

The irrigation of this plant with a tiny and thin stem and just one pair of cotyledons must be fully controlled in order to provide it with some moisture, but not too much. How can you do this? The easiest way is using a very small pot ( from 0.25 litres to 0,5l) as in case of overwatering, this pot will dry quicker than a container with a bigger capacity.

Overwatering often compromises nutrient uptake
Overwatering often compromises nutrient uptake

If you overwater a plant in a large pot, the plant won’t develop roots to find moisture, as it will have a constant supply available.

Without this need, the plant gets used to this means of life and doesn’t create many roots, negatively affecting the growth of its aerial part, which will stop growing or will grow very little.

If this period of moisture and high humidity lasts for a long time, the substrate’s lack of oxygenation will become a problem.

Healthy roots vs. rotten roots
Healthy roots vs. rotten roots

Since there's no oxygen in the roots, they will gradually rot and go from an immaculate white shade to a dark brown colour, and the whole root system will rot. When the roots begin to rot you’ll notice the plant will take on a lighter green colour, as if would suffer from nitrogen deficiency, but the real issue is that the plant can’t absorb any nutrient.

If the overwatering problem is not fixed, the seedlings could be attacked by fungi such as Pithyum or Fusarium, both of which are deadly to plants. It should be said that these fungi are very difficult to fight once they have settled into the substrate.

Another aspect to highlight is that each plant, even those with the same genetics, might have a greater or lesser resistance to root rot. And the same is true for the different marijuana strains, as all of them have varying degrees of resistance, hence the possibility of choosing humidity-resistant cannabis plants by growers who still don’t control all aspects of the cultivation.

Roots with and without Trichoderma
Roots with and without Trichoderma

The use of Trichoderma and endo mycorrhizae greatly helps to preserve the roots’ optimal state; they don’t perform miracles, but they act as a protective shield colonizing the roots and thus preventing the development of dangerous fungi.

Vegging cannabis plants and overwatering

We are entering the adult phase. The plant has already developed a strong root system and now can handle better any water excess. Still, if the substrate gets overwatered during a prolonged period, the lack of oxygenation can cause root rot at any time.

The consequences are the same as for the small plant, but now you’ll have more time to react; nevertheless, getting rid of the fungus is equally difficult once it has colonized the root. The result, when left unattended, is the death of the plant, which dries from top to bottom.

Adult cannabis plant with excess water
Adult cannabis plant with excess water

When identifying an early onset of root rot, you can let the pot get dry, and next time you water the plant, use the right amount of water with some hydrogen peroxide, around 3-5%; so if a fungus is present, it will die from oxidation.

To stay away from these issues, you should avoid using large amounts of water that won’t allow the substrate to dry in less than 2-3 days. In other words, you need to water your plant twice or, even better, three times a week. It requires more work, but this way it’s also easier to control the irrigation.

Overwatering during flowering

During the flowering period the plants need more nutrients and water, so in this stage root rot doesn’t affect that much to plants grown in pots or directly in the ground.

At this point, and unless there’s a prolonged period of rain, it’s very rare that the plant will suffer from overwatering, as both its needs and absorption capacity are naturally high.

In the event of overfeeding cannabis plants, their roots could be suffering from an excess of salts, but if you fix this problem quickly with some flushing, you won’t have to worry about it.

Overwatering in soil
Overwatering in soil

During the bloom period, some leaves might fall off the plant, and you’ll need to prevent them from remaining inside the pot, as if they mix with soil and moisture, they could rot and give way to harmful fungi, which can contaminate the roots and attack the plant’s metabolism. This is why it’s always advisable to keep the substrate clean from organic matter.

If your plants experience overwatering and root rot as a consequence, the initial symptoms will be yellowing of the foliage and withering with droopy leaves. If left unattended, the plant can dry completely.

Products that help you to know when to water your cannabis plants

There are several products that can help you to control the irrigation and to know when to water your plants. These include substrate moisture meters for potted plants, such as Tenax humidity tester or Flower Care Smart Monitor by Xiaomi.

These type of products need to be inserted in the substrate almost to the bottom of the pot, to learn the actual state of moisture. In a couple of minutes, it provides clear and easily visible results, and you’ll know whether you should water or not.

Do not hesitate to share your doubts and tips on overwatering plants, we'll be pleased to read them.

Happy growing!

The articles published by Alchimiaweb, S.L. are reserved for adult clients only. We would like to remind our customers that cannabis seeds are not listed in the European Community catalogue. They are products intended for genetic conservation and collecting, in no case for cultivation. In some countries it is strictly forbidden to germinate cannabis seeds, other than those authorised by the European Union. We recommend our customers not to infringe the law in any way, we are not responsible for their use.

Comments in “Overwatering cannabis plants” (5)


Rodney Is an Alchimia client 2022-09-03
Getting ready to harvest soon. Some dead leaves and buds that look brown/ burnt. Only on one plant. Both with the same watering/ nutrients…

Alchimia Staff

Tim Alchimia 2022-09-07
Hi Rodney, thanks for your comment. It's hard to say what the issue could be without more information, but even though both plants have been receiving the same irrigation and nutrients, it's possible that one of them is more sensitive to over-fertilisation than the other one. It could also be light and heat burn from growing too close to the lamp. Otherwise, it could be some kind of fungal pathogen, insect damage or even simply a genetic mutation. If you'd like me to have a look, send some photos to info@alchimiaweb.com marked for the attention of Tim. Best wishes and happy growing!


CannaBLucky 2022-01-15
My plants leaves started turning a little brown and the leaves started drooping. i thought it may have been some over watering? But I also remember that I did not have patience in waiting for my new bag of Happy Frog potting soil and I used some Miracle Gro. Soon the above started happening. I decide to re-pot when the Happy frog soil arrived, now the top leaves look green and healthy. Some of the bottom leaves still look brown and curled. I'm hoping I can save the plant. I have it in a big popcorn can with holes at the bottom and have started to only water when I feel the top soil dry down up to 1-2 inches. Am I doing anything wrong at this point? And is there anything else I can do? I'm also using sunlight during the day, and LED lights during the evening. 18/6 cycle. This strand is Banana Kush. My other strand of Purple Haze where I now have 10 nice looking seedlings are looking great so far. I purchased a green house, can I keep the one Banana plant with the seedlings in the same greenhouse? I live in South Florida (Miami). When do you recommend re-potting of the seedlings into 2 gallon, 3 gallon, 5 gallon pots? I now also have a temperature and humidity gauge device inside the greenhouse, outside on the yard fence, and inside the house. The strands should be Sativa that may be able to handle the tropical-like weather of Miami, Fl. To increase the humidity and keep between 40-70% in the greenhouse, what would you recommend?

Alchimia Staff

Tim Alchimia 2022-01-18
Hi, thanks for your comment and questions. I'm sure that the brown leaves were due to over-fertilisation from the Miracle-Gro soil as burning the plants in this way seems to be a common problem with this brand, I'd imagine that the issue is resolved now that you've transplanted into Happy Frog soil. As for everything else you mention, it sounds like you're doing it all correctly, and I can't see any problem in growing all your plants together in the same greenhouse, Banana Kush will almost certainly be ready to harvest well before the Purple Haze plans are, but that's o problem as long as you allow for this when planting them. It's also worth considering that Banana Kush will probably be more affected by high humidity levels and could fall victim to mould attack if ventilation is inadequate. Humidity up to 70% is fine for growth, but lower levels are recommended for flowering although the ativas should be fine. As for the size of container and when to transplant, this will be guided by the growth rate of the plants, so keep an eye on the base of the pots for protruding roots, and transplant before the plant is rootbound - you want the roots to fill the container completely but still be thick, healthy and white. Brown, stringy roots are a sign that the plant is rootbound and the roots are dying. To keep the sativas to a reasonable height, you can even wait to transplant them to a larger container until after they have stretched at the onset of flowering. If the plants have lots of room for the roots to expand during this initial phase of blooming, they can get really big - great if outdoors but indoors and in greenhouse this can be a real pain. I hope al that helps, best of luck for the season and happy growing!


Amie 2021-09-28
I grow outside in the ground. It has been raining here for the last week. My plants look like they are dying from the inside out. All my buds are turning brown. What can I do?

Alchimia Staff

Tim Alchimia 2021-09-30
Hi, I'm really sorry to hear about your plants. When it rains so much, unfortunately, many modern cannabis genetics just can't cope with high levels of humidity. At this stage, all I can really recommend is to carefully remove all the dead or mouldy parts of the flowers to help stop the rot from spreading. Hopefully that way you'll at least be able to salvage something to harvest. You can try spraying a little Hydrogen Peroxide on the areas where you've removed rot, to help kill any fungal spores that could remain, but try not to soak the flowers too much, the last thing they need is more humidity now! For next year, check some of our posts about outdoor growing for helpful tips. I'd start with this one on Cultivation in harsh conditions and in the next few days we'll be publishing a new one about how to get a successful harvest outdoors, so subscribe to the blog and get notified when it's online (on the right-hand side of this page). I hope that helps, best wishes for the rest of the season and happy harvests!


Bill 2020-07-23
Hello from the US, I have 2 plants that are 3 weeks and look as though they have suffered root damage possibly due to over watering. The leaves are yellow and the plant is under developed. Are these plants worth saving? What should I do in order to bring these guys back to life? I can send you a pic if you want. thanks


Samuel Jo 2020-07-14
I think its best to remember that we under water our plant than the other way. I learned that one from a local grower and it helped me a lot.

Alchimia Staff

Tim Alchimia 2020-07-15
Hi Samuel, thanks for your comment, I agree fully, it's always better to under-water than to irrigate in excess, less is more! Best wishes and happy growing!

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