Slow Cannabis Plant Growth And What You Can Do About It
When your cannabis plants grow slowly or stop growing altogether, there is always a reason. It could be a problem with nutrients, an environmental factor, or something else entirely. Let’s explore the reasons your cannabis plants or seedlings may experience slow or stunted growth.
“Why are my plants growing so slow?”. Sometimes, marijuana plant problems occur out of the blue. Your baby may not have shown any signs of an issue, but now you notice that development has halted and have no idea why. Here are some possible factors behind the slowed growth of your cannabis seedling or plant.
18 REASONS FOR SLOW OR STUNTED CANNABIS GROWTH
1. SEEDS ARE OLD OR LOW-QUALITY
Old seeds don’t just take longer to germinate (if they germinate at all); plants grown from aged seeds can also sometimes grow at a reduced pace. Likewise, good genetics are essential for healthy and vigorous growth from seed to harvest. A random bagseed will not perform nearly as well as quality seeds obtained from a reputable seedbank.
2. CLONE STRESS
Sometimes cuttings don’t root well, which hampers their growth. To prevent this from happening, apply a little bit of rooting hormone immediately after taking your cuttings.
Also, make sure your environment promotes root growth. The medium should be humid (but not too moist) with a pH level of about 6.0. Keep your cuttings at a temperature of around 22ºC. If they get too cold, they won’t root at all, and if it’s too hot, the roots will die.
3. ROOT HEALTH
When your plant’s roots can’t receive enough oxygen, metabolic functions slow down. In some cases, a lack of oxygen may stop their growth altogether. One common reason for this is overwatering or using substrates with poor drainage.
What to do about it? Create a light and airy growing medium with good drainage. You can improve poor-draining soil by adding some perlite.
The root zone for your cannabis plants should never get much hotter or colder than room temperature. Likewise, physical damage to the roots, mould, or bacteria can severely affect the growth of your plants. Always use non-transparent planters so light doesn’t reach the roots, as this is bad as well.
4. CANNABIS PLANTS STRETCH TOO MUCH
Stretching among seedlings can be particularly problematic. Multiple factors can induce this response, but the most likely culprit is a lack of light.
If your seedlings are spindly, increase light intensity or bring the lights closer. Prop them up with dowels as an aid during recovery. As a last resort, you can (carefully) replant them deeper into a new pot.
5. PLANTS ARE NOT GETTING ENOUGH LIGHT
Although requirements can vary from strain to strain, light is nonetheless a critical factor for the development of all cannabis plants. A lack of “good” light can absolutely lead to slowed growth. If you grow indoors and suspect that your plants aren’t getting enough light, try to decrease the distance between your lamps and the tops of plants. If you grow outdoors in pots, move your plants to a sunnier spot.
6. PLANTS ARE GETTING TOO MUCH LIGHT
Any type of stress on your cannabis plants, including many hours of exposure to direct sunlight without rest, can also halt or slow down growth. If you grow indoors and suspect light exposure to be the source of stress, decrease the intensity or move lamps further away from the canopy if possible. Know that seedlings are particularly sensitive to intense light! If you grow outdoors and you’re able to, move your plants into a spot where the light is diffused, such as around a shade tree.
7. INCORRECT LIGHT SPECTRUM
How fast and how vigorously plants grow are influenced by the spectrum of light they receive. Make sure you use the correct type of light according to each stage of growth. For healthy vegetative growth, you want a cooler light with more blue in its spectrum, a so-called “vegging light”. Lights with a warmer, more reddish spectrum are used for the flowering phase.
8. LIGHT STRESS: DARK CYCLE INTERRUPTION
Light is essential for all plants to grow. Any changes in light intensity or exposure will have an effect on growth. Flowering cannabis is especially susceptible to interruptions in the dark cycle. A light leak in your tent, stray light from a street lamp, and even a red light from a camera can disrupt flowering, and in a worst case scenario, can turn plants hermaphroditic. For that reason, it is very important to maintain complete darkness during the lights-off hours.
Exposing weed plants to irregular light hours can cause a hormone imbalance that confuses their internal clock. Your plants could flower prematurely, or they could revert back to the vegetative stage. If this happens, growth and yields will greatly suffer. For that reason, make sure to keep your light cycle consistent.
The above suggestions predominantly apply to photoperiod strains, as autoflowering cannabis flowers based on age rather than light exposure.
Overwatering is one of the most common mistakes made by new cannabis growers. It’s like suffocating your plants, and one of the main reasons behind slow growth, nutrient deficiencies, root rot, fungus, and many other problems. Don’t water too often and do not water on a fixed schedule. It is better water less frequently so that the soil can dry out between waterings. A good way to test whether you should water or not is to lift up the pot itself. If it feels quite light, it is time to water again.
10. NOT ENOUGH NUTRIENTS
Although not as common as overfeeding cannabis plants, an insufficient amount of nutrients for healthy growth can well be the reason for slow growth. Know that the nutrients found in most commercial potting mixes will only last for 3–4 weeks; afterwards, you will have to administer some more quality nutrients. Check the label of your nutrient products for the recommended dosage for healthy growth. Also know that your plant’s nutrient requirements are closely linked to the light intensity your plants are exposed to. Plants under intensive lights grow faster and will require more nutrients than plants under fluorescent lights, for example.
11. CALCIUM DEFICIENCY
Calcium is among those vital elements that your plant needs for healthy development. A lack of calcium can manifest in the following symptoms:
- Fresh growth is slow, twisted, and curled
- Young shoots are discoloured and turn purple or yellow
- Overall plant growth is slow and lacks vigour and vitality
- You can avoid a calcium deficiency by adding dolomitic lime to your soil or growing medium
Address a calcium deficiency immediately with commercial CalMag products that contain liquid calcium. You can add these products to your nutrient solution or administer them as a foliar spray.
Be aware that some growing media, like coco, increase the risk for a calcium deficiency. If you grow in coco, you should use special coco nutrients and/or regularly add CalMag to your nutrient regimen.
12. INCORRECT PH LEVEL
Incorrect pH level of your nutrient solution is among the most common reasons for cannabis growing problems, including slow growth. The reason for this is that cannabis thrives only in a relatively small window of suitable pH values. If the pH is off, the plants are unable to take in nutrients, even if they are present.
Make sure to dial in the correct pH level depending on your growing method. If you grow in soil, make sure the pH level is from 6.5 to 7.0. If you grow in hydro, an optimal pH level is 5.6 to 5.8. For soilless grows, such as coco, a pH level of 6.0 to 6.3 is optimal.
13. TEMPERATURES ARE TOO LOW OR TOO HIGH
Cannabis likes it warm to grow healthy, and does best at daytime temperatures between 25–30°C. Temperatures lower than that will slow down your plant’s metabolism, resulting in slower growth. But excessive temperatures are not optimal either. At very high temperatures, heat stress can also slow down or even halt plant growth altogether. If you grow indoors, adjust your temperature to a comfortable level. You can also provide some cooling with fans that blow a mild stream of air over your plants. This can also help prevent hot air pockets from forming inside your grow room.
14. PLANTING POTS ARE TOO BIG
Cannabis growers often start their seedlings in small cups. Later on, when the plants have reached an adequate size, they will “pot-up” to larger containers.
If you start your cannabis plants in containers that are too big, there is a high risk that you’ll overwater them. The issue is that seedlings cannot absorb all the moisture that is held in a large container, unlike mature cannabis, which can “drink” much more. Furthermore, a large pot will also take much longer to dry out.
To avoid the problems that come with too much soil and moisture, start seedlings in smaller containers until they’re growing vigorously. Once they have a set of 5–6 real leaves (not counting the cotyledons), then transfer them to a larger container, at least twice the current size.
If your seedling is already in a big container and you don’t want to or can’t move it into a smaller cup, water only a small area around the seedling.
• What Is The Right Size Pot For Your Cannabis Plant?
Use this rough guide to determine what size pot you should use for your cannabis plant:
- Plant height 30cm: 7.5–11l container
- Plant height 60cm: 11–19l container
- Plant height 90cm: 18–26l container
- Plant height 120cm: 22–37l container
- Plant height 150cm: 30–37l+ container
15. STRESS CAUSED BY PESTS / DISEASES
Insects, pests, and disease can cause damage and compromise a plant’s immune system. In a best case scenario, your plant may survive, but you will have poor yields. In the worst case, your plants could die.
Insects may feed on the leaves, affecting a plant’s ability to retain water and transpire. Other pests may damage the roots or cause additional problems. Any time your plant is sick or infested with insects, it will spend most of its energy defending itself and recovering from damage, which will slow down growth.
If your plants are infested, you’ll want to treat them immediately with appropriate measures. Even better, you can use preventative methods (e.g. neem oil, slug barriers, etc.) to minimise the risk for pest infestations. During all stages of growth, ensure that you regularly check for symptoms of pest infestations, including under the leaves.
16. STRESS CAUSED BY TISSUE DAMAGE
Physical damage, such as broken branches, can significantly slow your plant’s growth. Any damage will make the plant redirect valuable resources to repair wounds—resources that could be better spent on growing or flower production.
If you’re growing outdoors, situate your plants in an area sheltered from strong winds and heavy rains, and use chicken wire and stakes to maintain support.
Seedlings and young cannabis plants are especially vulnerable. Allow your seedlings to mature indoors for some weeks before setting them outside.
17. STRESS FROM CANNABIS TRAINING TECHNIQUES
Tissue damage from high-stress plant training techniques always causes some delay in plant development. But when you’re pruning excessively or too frequently, your plant may ultimately spend more energy repairing itself than growing.
If you plan on pruning, don’t overdo it. Be aware that each pruning can delay the development of your plant for days, if not weeks.
If you’re using other plant training techniques such as topping, make sure you start as early as possible. If you’re growing autoflowers, don’t use any plant training techniques that involve tissue damage, such as pruning and cutting.
18. AGE STRESS
Older cannabis plants have different nutritional requirements than young plants. Their tissues become hard and woody, they’re less vigorous, and they’re unable to take in as many nutrients.
Because of this, you’ll want to adjust your feeding regimen accordingly. Otherwise you risk overfeeding, which in turn results in stunted growth, deficiencies, and disease. Keep this in mind if you’re keeping mother plants around for a long time.Why is my cannabis plant growing slowly or not at all? Find out the answer to this question and see what you can do to solve it.
Heavy Pot Smoking Linked With Stunted Growth
— Study adds to evidence for developmental abnormalities with teen marijuana use
by Jeff Minerd, Contributing Writer, MedPage Today May 20, 2015
This article is a collaboration between MedPage Today and:
Boys who smoked marijuana heavily were significantly smaller than their peers by the end of their adolescent growth phase, according to a study conducted in Pakistan.
Study participants who were described as “marijuana addicts” during boyhood were on average 4.6 inches shorter and 4 kg lighter at age 20 than nonsmokers, reported lead investigator Syed Shakeel Raza Rizvi, PhD, and colleagues at the PMAS-Arid Agriculture University Rawalpindi, in a presentation at the European Congress of Endocrinology in Dublin.
The investigators compared height; weight; and levels of luteinizing hormone, testosterone, growth hormone, and cortisol in 220 marijuana-addicted and 217 nonpot-smoking boys.
Plasma concentrations of luteinizing hormone, testosterone, and cortisol were significantly higher in the marijuana addicted boys, but levels of growth hormone were significantly lower.
Chronic pot smoking may trigger a stress response that stimulates the onset of puberty but lowers growth hormone levels, the researchers suggested, on the basis of a separate set of laboratory experiments.
To determine the acute effect of smoking marijuana on cortisol levels, the researchers measured salivary cortisol in 10 “drug addict volunteers” of unstated age before, during, and after smoking a 0.25 g marijuana cigarette. “Acute administration of marijuana induced significant increases in the salivary concentrations of cortisol,” the researchers reported.
“The mechanism underlying the stimulation of reproductive function by marijuana in pubertal boys needs to be examined in detail. Furthermore, the suppressive effect of cortisol on growth directly through inhibition of growth at the cellular level by depletion of nutrients or indirectly through inhibition of growth hormone secretion needs to be further studied,” Rizvi said via email to MedPage Today.
“In addition, it is to be ascertained whether cortisol itself or some other factor like nutritional status is attributable to decline in growth rate in pubertal marijuana addicted boys. We are examining these aspects currently,” Rizvi said.
In the U.S., marijuana is the most popular drug among young people, with 11.7% of 8 th graders and 35.1% of 12 th graders reporting having used it within the past year, according to a 2014 survey from the National Institute on Drug Abuse (NIDA).
More than 6% of 8th graders and 21% of 12th graders reported using marijuana within the past month, the survey found, and about 1% and 6% of 8th and 12th graders, respectively, said they used the drug daily.
No funding sources were reported for this study.
Dr. Rizvi reported no relevant financial relationships with industry.
European Congress of Endocrinology
Proceedings of the National Academy of Sciences
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