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Watering and flushing marijuana plants

Like all plants, cannabis requires water in order to perform its basic functions. Water delivers nutrients throughout the plant, and without it, the plant can’t survive. But giving a marijuana plant the proper amount of water may be more difficult than you think. 

There isn’t an exact science for watering a weeds plant—you can’t observe the roots in most cases and you don’t have a way of knowing exactly what’s happening in the soil. 

Also, a plant is constantly growing and its climate will likely fluctuate, so the amount of water it needs will constantly change.

So here are some tried-and-true tips to keep your weed plants healthy and properly hydrated.

How often should you water marijuana plants?

A common mistake first-time growers make is to overwater marijuana plants. A healthy cycle of wet and dry is necessary for the roots of a plant to grow out and reach deeper into the pot. Roots pull in oxygen as soil dries and when soil is too wet, the plant essentially can’t breathe.

To see if a cannabis plant needs watering, stick a finger down 1-2 inches into the soil. If it’s wet, hold off. If it’s dry, it’s time to water.

You can also pick up a pot and feel its weight to determine if it needs water. This will take some experience—be sure to lift up your pots after watering to get a feel for how heavy they are when full of water. This will give you a sense of what a light, dry, plant feels like.

An under-watered marijuana plant looks droopy and weak, with yellow or brown leaves. There is no strength in the leaves and they look lifeless. Leaves of an overwatered plant look slightly similar in that they droop, except the leaves will be dark green and the leaf tips will curl.

Note the intervals at which you water your plants, and even write it down in a log. Take notes, make calculations, and get your marijuana plants on a watering schedule. Setting a cycle where plants need to be watered every two to three days is ideal.

Keep in mind that as plants get bigger, they will need more water and need to be watered more frequently. When growing weed outdoors, you’ll need to water more often as the weather gets hotter.

The sooner you find the sweet spot between too wet and too dry, the sooner you’ll see your garden flourish.

How much should you water marijuana plants?

The amount of water your marijuana plants need will depend on a number of factors, including:

  • Size
  • Outside temperature
  • Overall health
  • Stage of growth

You want to soak the pot and have runoff through the drainage holes in the bottom of the pot. Water should pool up on the surface of the soil when you’re watering, but it shouldn’t sit on the surface after you move on to the next plant.

If a weed plant is very dry, water will run straight through the pot and quickly come out the drainage holes. If this happens, water the plant a little bit and come back to it after 10-15 minutes and water it again, and maybe even a third time. This will allow the soil to slowly absorb water incrementally, until all soil is thoroughly wet.

Roots are constantly on the hunt for water as they grow and move farther away from the main taproot. As a plant gets bigger, so should the watering radius—the area around the stalk of the plant that you water. Doing this will help guide roots to the edges of the pot as they seek available nutrients in soil.

But watering too far away from where roots currently are can create standing water, which can lead to root rot, mold, and pest issues.

Is your container the right size?

To properly water a cannabis plant, it needs to be in the correct size container. If a pot is too big, the plant’s roots can’t drink water where they don’t reach. If the roots aren’t absorbing water, water will sit in soil and take a long time to evaporate, which can promote root rot and unwanted insects and fungus.

Conversely, if a container is too small, the roots won’t be able to stretch out, which will ultimately stunt the growth of a plant. You will also need to water the plant all the time, which will add labor.

Ideally, cannabis plants should start in a small pot and progress to bigger and bigger pots as they outgrow each container. For example, you can start a seedling or clone in a 4″ or 1-gallon pot, then move on to a 2-gallon, 5-gallon, 10-gallon, and so on.

Plants are ready to transplant when a healthy root structure encompasses most of the soil and the roots aren’t bound. When transplanting, take time to look at the quality of the roots: bright white roots with a strong, thick structure is a sign plants are getting watered correctly.

What is flushing?

flushing cannabis plants

Flushing is an important part of the marijuana growing process, when you stop giving a marijuana plant nutrients and give it straight water. It’s primarily done to flush out any nutrients that may have built up in a plant during its life, and it’s usually done a week or so before you chop the plant down at harvest. 

A flush can also be done to clear plants and start fresh if they have a nutrient imbalance, such as nutrient lockout, when your plants are unable to absorb new nutrients. 

How to flush weed plants 

Flushing marijuana plants before harvest

The final flush should occur a week or so before you cut the weed plant down for harvesting. Water your plants with the same amount as you normally would, but only with water. This will force the plant to use the nutrients stored within itself—if its nutrient reserves are not used or broken down, it could affect the quality of your harvested buds’ smoke and flavor.

By looking at the trichomes on marijuana plants, you’ll be able to tell when the plants are ready for a flush—begin when they start turning milky.

Different growing mediums require different flushing timeframes before harvest:

  • Soil: 7-10 days
  • Rockwool and coco: 7 days
  • Hydroponics: 5-7 days

If growing in amended organic soil, it is not recommended to flush plants. This is because the soil already holds all the nutrients the plant needs to thrive, and by flooding the soil you can wash away and damage the complex ecosystem that you’ve worked hard to develop. 

When to stop watering before harvest

Water your marijuana plants as normal when in the flushing phase—don’t let them get too dry or too wet. You especially don’t want to harvest dry or wilting weed plants; they should be nice and healthy when you cut them down.

Flushing marijuana plants for nutrient imbalance or lockout

Giving weed plants too many nutrients, improper pH levels, and other stresses on plants can result in a nutrient imbalance or lockout. Whether it’s salt buildup from nutrients or incorrect pH levels that prevent absorption, a nutrient lockout results in a buildup of nutrients in soil and the plant can’t access them.

Flushing plants will remove this excess buildup and help restore the soil’s pH balance. This will allow plants to resume absorbing nutrients to grow at a healthy, successful rate.

To perform this type of flush, excessively water your plants with water at a pH level between 6.0-6.8 for soil and 5.5-6.5 for hydroponics. Fully saturate your pots, and repeat 15 minutes later. The flush should clear any blockage and make room for new nutrients.

How to source the best water for marijuana plants

In order to raise healthy, strong cannabis plants, you’ll need to pay close attention to the type of water you’re providing your crop.

There are two common misconceptions when it comes to sourcing water for a cannabis garden:

  • All water is the same
  • Water deemed safe for consumption will also be adequate for your plants

Water can contain a number of contaminants, some of which are safe to be used in a garden and some that can have serious consequences for a plant’s health. Every grower should know where to source clean water and how to treat contaminated water to make it suitable for a garden.

Know Your pH and PPM

An important term to understand when talking about water quality and distinguishing between water types is pH, or potential hydrogen, which is used to measure the acidity and alkalinity of a given fluid. pH measurement occurs on a scale of 0 (most acidic) to 14 (most alkaline).

Examples of highly acidic fluids include battery acid, lemon juice, and vinegar, while highly basic fluids include household ammonia, milk of magnesia, and bleach. Distilled water is neutral with a pH of 7.

Depending on the grow medium you’re using, cannabis prefers its water to be in the 6-7 pH range, which is optimal for nutrient uptake.

Another important term to know is ppm, or parts per million. This measures the presence of dissolved solids in water. Because most water isn’t pure H2O, ppm gives an accurate measurement of the percentage of contaminants in a given water source.

Contaminants found in water sources can include:

  • Chemical: chlorine, chloramine, magnesium, calcium, salts, nitrogen
  • Physical: rocks, sand, sediment, organic material
  • Biological: bacteria, mycotoxins, viruses, parasites
  • Radiological: uranium, cesium

Many water sources naturally have contaminants. Streams, ponds, and lakes can contain a range of biological contaminants like bacteria and parasites.

Other water sources, such as treated municipal water, or tap water, is often treated with some amount of chemicals like chlorine, calcium, and magnesium, in order to get rid of the possibility of biological growth.

Water that contains higher quantities of minerals such as calcium or magnesium is called hard water. This type of water has a higher ppm due to the extra dissolved solids in it. Water with less minerals and a lower ppm is called soft water.

Where to source water

Cannabis homegrowers have several options available at various price points when sourcing water for a garden, each with its pros and cons.

Factors to keep in mind when looking at water sourcing options include:

  • Total cost upfront vs. cost accrued over time
  • Availability of water
  • Overall water quality
  • How difficult it is to scale or increase the amount of water needed
  • Labor needed to bring in water
  • Environmental impact

The options below represent the most practical water sourcing methods available to the average homegrower, but keep in mind that other methods are available.

Unfiltered tap water

Contrary to popular belief, using unfiltered tap water on cannabis is not a death sentence for plants. This type of water varies greatly depending on the municipality and its water-treatment protocols.

Some cities use incredibly hard water with high levels of contaminants such as chlorine, calcium, and magnesium. While water with a low ppm concentration of these chemicals won’t necessarily kill a plant, it can have a negative impact on the biological activity in organic soil.

One trick to rid water of chlorine is to let your water sit out for 24-48 hours. Doing so will allow ample time for the chemical to evaporate, making tap water usable for growing.

Pros

Tap water is inexpensive, meaning it’s easy to scale. Also, there’s little labor involved in using tap water after the ppm and pH are adjusted.

Cons

This option may not be available for growers living in cities with heavily treated water systems. Organic growers will also find that chemicals in treated water may have a negative impact on biological life in soil.

Water collection systems

You can create a system to collect rainwater or gray water. These systems work very well under the right circumstances and can be both inexpensive and environmentally friendly.

Pros

Water collection systems such as rainwater catches are a great way to sustainably source water for a garden. These systems can last for long periods of time with little maintenance and can be scaled for any size garden. Systems like this are especially useful in climates with dry periods where water saving is encouraged.

Gray water recycling is a great way to reuse unwanted water. Using catching and filtration systems, you can recycle water that has already been used on a property.

Cons

Unfortunately, many jurisdictions have ordinances that either completely prohibit or set strict limits on the collection of rainwater and the reuse of gray water. Proponents of these restrictions argue that there are health and safety concerns.

Although setting up a simple water-catching system can be inexpensive, there is still some start-up capital required. Water that has been collected either by rain or by reuse will also need to be filtered and stored properly, requiring filter systems and specially graded storage containers built to withstand the elements without risk of contamination or breaking.

Bottled water

This water is a great pure, uncontaminated source that’s relatively inexpensive for a small-scale garden. Most grocery stores and shopping centers have bottled distilled water and many companies offer water delivery services at reasonable prices.

Pros

This water is affordable in low quantities and easy to source. It’s also safe for plants and doesn’t need any extra filtration.

Cons

There is also a certain amount of labor involved in retrieving the water.

Bottled water also has a big negative impact on the environment, in the resources needed to create containers for the water and resources needed to transport the water, such as fuel. Trash is also a consideration with water containers.

Reverse osmosis (RO) systems

For large-scale marijuana growers with less financial restrictions, water filtration systems are the go-to option for an unlimited supply of clean water. There are several effective filtration systems available, though reverse osmosis (RO) systems seem to be the most popular for cannabis cultivators.

These systems work by pushing water molecules through a semi-permeable membrane, filtering out most contaminates. There are many varieties of RO systems that vary greatly in price.

Pros

Using an RO system will ensure absolute filtration and decontamination, making it a safe method for cleaning large quantities of water for a grow operation. After initial installation costs, this system will supply a virtually endless supply of clean water for a grow.

Cons

The initial cost upfront for even the most basic RO system can be hundreds of dollars, with more advanced systems stretching into the thousands. With such a high barrier-to-entry, small-scale growers may find that this system is a pipe dream.

RO systems are also known to waste quite a bit of water, making them high on the list for negative environmental impact. RO systems continue to draw and filter water for a period of time after use, thus wasting water. By installing a permeate pump, you can reduce the amount of water wasted.


Patrick Bennett and Trevor Hennings contributed to this article.

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