Flushing Cannabis Plants Explained

If you are new to growing cannabis, you may not be aware of flushing your plants. Flushing is a process that involves giving your plants plain water for the final 2 weeks of the flowering stage. In this article, I explain the benefits of flushing your weed plants, the different ways to flush, the correct pH level to use, the right time to begin flushing, as well as common mistakes to avoid.

What Does Flushing Mean?

During the entire grow cycle, your growing medium will have built up salts that are present in the nutrients. Undissolved salts can accumulate around the roots, and if the plants are not flushed, it can result in a harsh-tasting smoke with charcoal-black ash that will not burn well and will need to be constantly lit.

The art of flushing is to cut off all the nutrients and to provide only pH-adjusted water before harvesting. The reason is that giving only water will wash away any undissolved salts around the roots, clean out the growing medium, and also give your plants a chance to use up the final internal nutrients they are holding onto.

The Benefits of Flushing Cannabis

• Flushing will produce a smooth, flavor-filled smoking experience.
• The ash will burn with a soft white or light grey appearance.
• Flushing allows a grower to save their flowering nutrients.
• The plant will use up all of its internal nutrient reserves naturally.
• Flushed cannabis produces a clean-burning flower.
• The joint will not need to be lit constantly.

How to Flush Your Cannabis Plants

If you are growing cannabis using soil, coco coir for hydroponics, cutting off the nutrients and using only plain water is essential. There are two ways to flush your cannabis plants, which are covered below.

Using the same amount of water 

This method is how I flush my plants, and all you need to do is use the same amount of water as usual. So if you have been giving your plants 1 gallon of nutrient solution every day during the flowering period, continue to use 1 gallon. Of course, avoid overwatering the plants and keep everything as consistent as possible.

Increasing the volume of water

The other method to flush is to increase the water. I am not a big fan of this method as oftentimes the growing medium struggles to dry out in time; however, a larger volume of water can work well.

In this instance, if you are used to feeding your plants 0.5 gallon of nutrient solution, then you can increase the volume of water to 1 gallon. I recommend only watering when the growing medium is beginning to dry out.

What pH Should the Water Be?

The pH of the water should be between 5.5 and 6.5 for the best results. You can use tap water, bottled water, or reverse osmosis water; however, be prepared to adjust the pH levels using a pH Up or Down product and check with a digital pH pen before watering your plants.

When Is the Right Time to Start Flushing?

Once your cannabis plants begin flowering, you will be focused on feeding the plants different NPK ratios as the weeks progress. The rule of thumb is to wait until the final 2 weeks of the flowering stage, and to cut off any NPK and use only plain water. Hydroponic systems only need to be flushed for 7 days.

Mistakes to Avoid When Flushing Your Plants

Flushing may sound straightforward, yet there are ways to make mistakes during this pinnacle time before harvest. Below are some common mistakes to avoid when flushing your cannabis crop.

Continuing to use PK boosters

It may sound counterproductive to the idea of cutting off boosters and high potassium and phosphorus feeds during the final 2 weeks. However, your plants will have a chance to use up any P and K stored inside the plants, and this will help give them one final push over the finishing line. Avoid feeding your plants and nutrients for the last 14 days, especially flowering boosters.

Not checking the pH levels.

Making sure that the pH levels of the water you will be giving your plants when flushing is crucial. pH plays a significant role in nutrient uptake, affecting how plants access nutrients. My tip here is to use a digital pH pen and check that you are within the optimal range of 5.5 – 6.5.

Ignoring the 14-day time frame

Giving your plants only water for the last 2 weeks is the ideal time frame. Avoid waiting until the previous week to flush your plants, as this can still cause the buds to burn with black ash and not burn as smoothly as a 2-week flush. Flushing for 3 weeks won’t hurt the plants, but from my experience, it is unnecessary.

My Final Thoughts on Flushing

There are many benefits to harvesting a well-flushed crop. Not only will your buds have a super smooth and lip-licking flavor profile, but you won’t need to worry about relighting your joint every 30 seconds, enduring coughing fits, and filling your ashtray with a chunky, dark charcoal black ash quality.

I love to use bottled water for flushing my plants, as the EC level is much lower than tap water, and it is a convenient solution for growers who do not use a reverse osmosis pump. Good luck flushing your weed plants and enjoying the compliments from your friends on how smooth and tasty your high-grade buds are.

Feature photo: A flushed Ice Cream Cake ready to harvest grown by Samir. Photo credit: Stoney Tark.