Root-rot is the Achilles heel of DWC. It can spread quickly and kill a plant in days. When it comes to root-rot an ounce of prevention is worth a pound of cure. There are two schools of thought on DWC: Sterile vs Beneficial Bacteria. I’ve come to prefer a hybrid of the two. Practicing sterile growing techniques while adding beneficial microbes as a preventative measure is the way to go.

Signs of root rot:

• Slimy roots – roots that feel slimy are the best indicator of root issues.
• Leaves curling – looking like an overwatered soil plant.
• Brown roots – do not confuse with roots that are slightly brown from your MicroGro nutrient
• Musky smell – if the water reservoir has a moldy, cellar-like smell this can be an early indicator of root rot. Do a water change and reservoir change immediately
• Stress signs – as in the picture above the plant with root issues has yellow leaves and purple buds.
• Rust Spots – Occasionally early signs of root rot with show up as rust coloured spots on the leaves.

Prevention of root rot:

• Keep water temperatures below 68F. If you are running between 68-72F you are in the danger zone. I would not bother with DWC if you cannot keep temperatures below 72F, your dissolved oxygen levels will be too low to prevent pathogen growth. Paint your container white or use a windshield sunshade to lower temperatures.
• Replace clogged air stones. I have had plants die in days from a pinched airline. You want a vigorous boil in your reservoir. Lower dissolved oxygen promotes pathogen growth. Use heavy-duty airstones that can be boiled and cleaned multiple times
• Eliminate Light Leaks. This is extremely important. Water reservoir lids should be painted with multiple layers of paint. The stock yellow lids can be used in veg but do not provide enough protection from light leaks with LED boards. Hydroton stones use should be small and well packed in deep net pots. No light in the water reservoir. Green algae is often an early indicator of light leaks.
• Do not use hot water from your tap. Pathogens can thrive in hot water tanks which are rarely cleaned. It is the same reason that hot water is not recommended for drinking/cooking and often tastes different from cold water.
• Sanitize Everything prior to use. I use star-san (an acid-based, no-rinse, food-safe sanitizer commonly used in homebrewing) to sterilize my tank after cleaning. No-rinse sanitizers do not need to be rinsed off. They are neutralized once diluted with water.
• Perform full water changes more frequently. You do not need to drain and clean your tank every water change (top ups are much easier). but full water changes are important. Small bits of roots that fragment can decay in the water reservoir and cause issues. If you are running on the high end of the temperature range you should be doing full changes more frequently.
• Soak Hydroton before using. I no longer recommend reusing hydroton but if you’re in a pinch you can boil for an hour to sterilize used hydroton.
• Avoid Rockwool – I believe rockwool can act as a substrate that promotes pathogen growth. Follow my DIY Cloner guide to take clones without using rockwool.
• Autoclave Airstones. Airstones are one of the worst offenders when it comes to holding pathogens deep within their porous structure. To sterilize airstones I use an Instapot to pressure cook them for 1 hour.

How to Fix Root Rot in DWC

• Cut off slimy/dead roots – if the roots are barely hanging onto the plant and can be pulled off get rid of them. These roots provide food for the bacteria and should be removed.
• Rinse roots with Hydrogen Peroxide – Use a 3% solution of peroxide to rinse the roots. Perform a full water change and sterilize your tank before putting the clean roots back in the reservoir.
• Perform a full water change. Top off with fresh water.
• Add Beneficial Microbe Tea – to the nutrient solution
• Know when to Cut your losses – If you have one sick plant it can spread quickly to other plants. This is the main disadvantage of rDWC systems that connect multiple tanks together.

Rotten roots will never fully recover, but you can stop them from getting worse. Ideally you will have healthy new white roots taking over the diseased ones, but this won’t happen late in flower. If new roots do come in you can cut off old dead roots as the plant no longer relies on them. Know when to cut your losses, especially with shared water reservoirs. If you do have an infection, clean and sanitize everything well before your next grow. I like to soak everything in oxiclean before sanitizing with star-san. Beneficial microbes help and the tea below has helped me saved some plants that looked pretty far gone.

Beneficial Microbe Tea Recipe:

• 2.5 Gallons (10L) de-chlorinated Water (Carbon Filtered or Campden tablets)
• Beneficial Bacteria (30mL Hydroguard OR 30 mL Miicrobial Mass OR 40mL Voodoo juice)
• 15g Mycorrhizal Inoculant (GreatWhite OR DynoMyco OR Mykos)
• 1 hand full of Earth Worm Castings
• 15mL Molasses
• Cheesecloth or Muslin Bag
• 1 Gallon Sprayer (optional for foliar feeding)

The beneficial microbe tea is brewed for 36-4 hours and is typically used at a rate of 1 cup per gallon (70mL/L). This recipe produces enough tea to inoculate 40 gallons of nutrient solution. After the initial dose a booster dose of 1 tbs per gallon (5mL/L) should be added after 5 and 10 days.

Beneficial Bacteria

Beneficial bacteria’s main benefit is to out outcompete other pathogens for resources in your reservoir. No matter how good your sterile practices, pathogens will eventually find their way into your reservoir. Keeping your reservoir at the appropriate temps slows down bacterial growth, but it is impossible to maintain a completely sterile environment. BUT if you have an established colony of beneficial bacteria then pathogens and cyanobacteria won’t stand a chance. You have a small army fighting your battles.

If you can find Miicrobial Mass I recommend it. In addition to the above mechanism for outcompeting pathogens, Miicrobial Mass increases the solubility of phosphorous which is essential for flowering plants. For inorganic phosphorus that is complexed to other minerals, the bacteria release the inorganic phosphorus from this complex by producing organic acids. These acids decrease the pH locally, which in turn solubilises the inorganic phosphorus and therefore it turns into a form that the plant can absorb.

Mycorrhizal Inoculant

A mycorrhiza is a mutual symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant’s rhizosphere, its root system. Mycorrhizae play important roles in plant nutrition, root health and water chemistry.

Mycorrhizal fungi form a mutualistic relationship with the roots of most plant species. In such a relationship, both the plants themselves and those parts of the roots that host the fungi, are said to be mycorrhizal. Relatively few of the mycorrhizal relationships between plant species and fungi have been examined to date, but 95% of the plant families investigated are predominantly mycorrhizal either in the sense that most of their species associate beneficially with mycorrhizae, or are absolutely dependent on mycorrhizae. The Orchidaceae are notorious as a family in which the absence of the correct mycorrhizae is fatal even to germinating seeds.

The mechanisms by which mycorrhizae increase absorption include some that are physical and some that are chemical. Physically, most mycorrhizal mycelia are much smaller in diameter than the smallest root or root hair, and thus can explore soil material that roots and root hairs cannot reach, and provide a larger surface area for absorption. Chemically, the cell membrane chemistry of fungi differs from that of plants. For example, they may secrete organic acids that dissolve or chelate many ions, or release them from minerals by ion exchange.

My experience with mycorrhizal inoculants in DWC has shown it causes healthy white roots that grow to a much larger size. I suspect that the fungi stimulate the roots and cause them to grow much larger than they otherwise would in a deep water culture environment.

Worm Castings

Worm castings are worm poop. Castings contain rich microbiological colonies that help fight root-borne plant diseases and repel insects. Worm castings are water-soluble allowing plants to quickly and easily absorb essential nutrients and trace minerals. Nutrients are readily available to plant material over a greater length of time and will not burn even the most delicate plants. Analysis of earthworm castings reveals that they are rich in iron, sulfur, calcium, nitrogen, phosphorus and potassium (NPK rating: 5.5.3)

Molasses

The purpose of the molasses is to feed the beneficial bacteria and fungi. Molasses has more nutrients than refined sugar, which should be avoided if possible. Get unsulphured molasses if available.

How to Brew the Tea

Brewing the tea is fairly simple. Put the worm castings in a cheesecloth, filter bag or hairnet and tie off the end. Mix all ingredients together in a bucket, starting with the water and molasses. Add 1-2 airstones and let it brew for 36-48 hours. It should start to foam up within a few minutes. Take note of the smell, it should be an earthy, mossy smell. If it ever starts to smell foul throw the tea away.

After 36-48 hours of brewing any unused portions should be refrigerated for up to 7-10 days.

Can you run a Perpetual Tea?

Unfortunately running a perpetual tea that you keep feeding molasses does not work out well. One bacteria within the solution typically takes over and dominates the others. Diversity is important for these teas and thus storing over 10 days is not advisable.