Fixing Root Rot and Algae in Beginner Hydroponics

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Sooner or later, almost every hydroponic grower lifts a lid and finds something they did not want to see: green slime, brown roots, or a smell that is not lettuce. Do not quit. Root rot and algae are the two most common beginner problems, and both have clear causes. Once you understand them, they are mostly preventable.

What healthy roots look like

Healthy hydroponic roots are white to cream colored, firm, and smell like clean, damp earth or nothing at all. Some nutrient brands and some plants tint roots slightly, so get used to what your roots normally look like early in the grow. Take a photo when things look good so you have something to compare against.

Signs of root rot

Roots turning tan, brown, or gray
Slimy texture, roots that pull apart easily
A sour or rotten smell
Plants wilting even though they are sitting in water
Slower growth and yellowing leaves

Wilting while sitting in water sounds strange, but it makes sense. Damaged roots cannot take up water well, so the plant acts thirsty.

What usually causes root rot

Root rot is usually caused by disease organisms that take advantage of stressed roots. In home hydroponics, the stress usually comes from a short list:

Warm water. UF/IFAS recommends nutrient solution between about 65°F and 80°F and good oxygenation for lettuce. Warm water holds less oxygen.
Not enough oxygen. In DWC, a failed air pump. In Kratky, overfilling the container and drowning the air roots.
Light reaching the water, which feeds algae that compete for oxygen.
Dirty equipment carried over from a previous grow.

Signs of algae

Algae shows up as green film on surfaces, green or cloudy water, or slime on the top of growing media. It needs light to grow. If you have algae, light is reaching your water somewhere. University of Hawaii material on small hydroponic systems recommends shading all parts of the system exposed to sunlight for exactly this reason.

Preventing algae

Use opaque containers, or wrap clear jars completely.
Cover any holes not holding a net pot.
Keep the top of rockwool or pebbles dry where possible; a layer of clay pebbles on top helps.
Do not leave spare nutrient solution in clear bottles on a sunny counter.
Clean between grows.

A little algae on the outside of a net pot is not an emergency. Heavy algae in the reservoir is worth fixing because it competes with roots and can foul the water.

Preventing root rot

Keep water cool. See keeping reservoirs cool in a Texas summer.
In DWC, run an air pump and air stone continuously, and add a check valve.
In Kratky, never refill to the original level once roots have formed. Leave the air gap.
Use a bigger reservoir for bigger plants so the water stays more stable.
Clean and rinse containers, net pots, and air stones between grows.
Start with healthy seedlings.

What to do if you find root rot

Act quickly, but calmly:

Cool the water and add air. Move the system somewhere cooler. In DWC, check that the pump is running and bubbles are strong.
Change the solution. Dump the old water, rinse the container, and refill with fresh nutrient solution at a mild strength.
Trim the worst roots. With clean scissors, carefully snip away roots that are mushy and falling apart. Leave firm ones.
Watch for new white roots. New, bright roots growing in are the best sign of recovery.
Know when to start over. If most roots are mush, compost the plant, clean everything thoroughly, and restart. A fresh seedling often grows faster than a sick plant recovers.

You will see many products sold to fix root problems. I am not recommending any particular treatment here. If you choose to use one, follow the label exactly and check that it is labeled for use on edible plants.

What to do about algae

Find and block the light leak.
Wipe away algae from surfaces you can reach.
If the water is green or cloudy, change it and rinse the container.
Add a top layer of clay pebbles over exposed media.

Cleaning between grows

When a grow finishes, take everything apart. Rinse containers and net pots, scrub off residue, and let them dry fully. Clay pebbles can be rinsed and reused. Rockwool generally is not reused. Air stones clog over time and are inexpensive to replace. A clean start prevents a lot of trouble.

Root rot vs. normal root color

Not every color change is rot. Some nutrients can stain roots. The difference: rotten roots are slimy, mushy, and smell bad. Stained but healthy roots are firm and smell fine. When in doubt, trust the feel and the smell.

Keep a simple log

Note the date, water temperature, pH, and how roots look. Patterns show up fast. Most growers find that their worst root weeks line up with their hottest weeks. Get pH and EC basics in beginner pH and EC/PPM explained.

Where to go from here

The tools that make prevention easier, like opaque containers, air pumps, check valves, and thermometers, are listed on my Gear page. The full book, The Soil-Free Farming Revolution, has a troubleshooting chapter with more photos and a quick-fix chart.