Growing Cannabis is generally a resilient plant, and most strains can tolerate mistakes with feeding and watering to some extent. However, consistently delivering water and nutrients in the wrong concentrations or quantities can cause deep-rooted problems that can prove detrimental—or even fatal—to your crop.
Cannabis plants need abundant water, particularly in the flowering stage. As a general rule, plants should never be deprived of water for too long at any stage of their life cycle, as this can severely slow or even halt growth entirely. As well as this, depriving plants of water deprives them of nutrients, which not only slows growth but can lead to deficiencies.
Moderation is key
However, over-watering is a mistake many new growers make. Over-watering can lead to pythium (root rot), botrytis (grey mould) and powdery mildew, as well as causing nutrient toxicity and anaerobic (oxygenless) soil conditions in extreme cases. These issues can be disastrous, and are often extremely difficult for newbies to correct.
Most growers agree that soil should be allowed to dry out slightly between feeds—this lessens the risk of rot, mould and nutrient burn, and may also encourage the roots to grow denser. However, plants should always be watered before the roots start to dry out.
Finding the right balance is crucial, and it varies according to the strain (tropical sativas may be remarkably thirsty, while many indica strains abhor damp) and the size and growth stage of the plant—as well as other factors such as choice of medium.
When should cannabis plants be watered?
There is some controversy over the best time of day to water cannabis plants. Most growers believe that cannabis should be watered at night time, but some argue that they should be watered earlier in the day.
Those that believe cannabis plants should be watered at night argue that in a natural environment, precipitation does not fall when the sun is shining, due to cloud cover. Many also state that if water droplets resting on the leaves are exposed to heat and bright light, they may act as “magnifying glasses” and burn the leaves.
It appears that this is just a myth, although leaf burn can occur through prolonged contact with some fertilisers. In any case, possible issues can be avoided altogether if care is taken to simply direct the water towards the soil and not the leaves!
On the other hand, watering earlier in the daytime allows the plant to utilise the available nutrients more effectively, as many of the fundamental processes of plant growth occur in sunlight. Leaving soil damp throughout the cooler hours of night can promote mould growth.
If growing outdoors, watering in the morning rather than the warmer afternoon appears to be optimal, as the rate of evaporation is slower. If growing indoors, watering at around the time lights are switched on may be preferable—while temperatures rise quickly with “hot” lights such as HPS, it usually takes some time for them to peak.
How often should cannabis plants be watered?
At first, young plants will require small amounts of water at frequent intervals—up to twice per day if room temperatures are high and relative humidity is low. As they mature, they may be fed less frequently with greater quantities. Typically, large flowering plants must be fed at least once every 2-3 days.
The correct time to water plants should become apparent upon testing the moisture level of the growing medium. It should feel dry on top, but still somewhat damp below the surface. If it remains dry to the touch at depths of more than 5cm or so, plants are probably too dry.
This may all vary widely according to the drainage properties of the medium. For example, coco typically requires daily watering at first, then twice-daily as the plants increase in size; this is due to the fact that it retains water extremely well, so giving too much at once can saturate the medium and reduce airflow.
With experience, a visual check or simply testing the weight of the container will aid in determining when to feed, as a dry container obviously weighs substantially less than one full of water.
A step-by-step guide to watering cannabis plants
First, prepare your equipment. You will need a source of water, a pH meter, pH down (this could be any strong acid), your chosen nutrients, and a watering can, hose or similar.
Water should be around room temperature—hot or cold water can shock and stress plants severely and sometimes irreparably. Most agree that water should be left to stand for 24 hours before use as this allows pH to be more accurately determined.
pH should be between 6.0-6.5 if growing in soil; if growing in soilless medium this value may be somewhat lower (coco prefers the 5.5-6.0 range). Use pH down to reduce pH if necessary, making sure to mix well. Incorrect pH can result in nutrient burn and various deficiencies, as roots are unable to process nutrients as effectively.
Add nutrients according to the manufacturer’s instructions, and mix thoroughly. Nutrient concentrations may be adjusted according to the strain and the stage of growth of the plant—such as when plants are young, or just after being transplanted.
Using a watering can, hose or similar, water the plants until runoff is visible in the trays beneath the pots. Many growers aim for around 10-20% of total water draining off, as this is believed to prevent nutrient build-up—but this is disputed.
Nutrient build-up occurs due to oversupply of nutrients (or reduced uptake by the roots due to overwatering or pH issues), and is fixable by flushing with pure water and not by adding more nutrient solution.
This basic guide should give novice growers all they need to successfully water and feed cannabis plants. However, adjustments to the basic principles may need to be made according to local climate, choice of strain, type of medium, and several other factors.