Root Rot

Root rot is a group of water mold pathogens, usually Pythium, colonizing roots that have been sitting in warm water with not enough oxygen. It is most common and most destructive in hydro, where a plant can go from healthy to dead in under a week.

What you are seeing#

Look at the roots, because the leaves lie. Healthy roots are white or cream, firm, and smell like clean earth or nothing at all. Rotting roots turn tan, then brown, then dark slimy brown. They feel slippery, and the outer sheath slides off between your fingers leaving a thin string behind.

The reservoir goes cloudy instead of clear. The smell is the giveaway: sour, swampy, sometimes sharply ammonia-like. A healthy reservoir has no real smell.

Above the medium you get drooping that never lifts no matter how you adjust watering, yellowing that starts low and climbs, and growth that simply stops.

Why it happens#

Three conditions together: warm water, low oxygen, and a pathogen that got in from somewhere.

In hydro, reservoir temperature is almost always the culprit. Water above 70F holds much less dissolved oxygen and is the ideal breeding temperature for Pythium. Add a light-leaking reservoir that grows algae, or an air pump too small for the volume, and it takes hold.

In soil and coco it is nearly always chronic overwatering. The medium never dries, the root zone goes anaerobic, and the rot follows. Dirty reused pots and standing water in saucers help it along.

How to fix it#

1. Dump the reservoir completely. Do not top it off.
2. Scrub the reservoir, air stones, lines and net pots with dilute bleach or hydrogen peroxide, then rinse thoroughly.
3. Rinse the roots and cut away anything brown and slimy with clean shears. Damaged root does not recover and it keeps feeding the problem.
4. Refill at half your normal nutrient strength, pH 5.8. A stressed root system cannot handle full feed.
5. Add a beneficial bacteria product containing Bacillus amyloliquefaciens at every reservoir change. This is the preventative that actually works long term.
6. Increase aeration and keep light out. More air stone surface than you think you need, an opaque reservoir, taped lid seams, no gaps around net pots.
7. In soil or coco: stop watering until the pot is genuinely light, run a fan across the medium surface, and water to 10 to 20 percent runoff instead of frequent small amounts. If more than half the root ball is brown and mushy, the plant is usually not worth saving.

Change the reservoir every 7 days from here, and check root color every time you do.

How to tell it apart#

Versus overwatering. Overwatering is the condition, root rot is the infection that follows. The test is your eyes and nose on the roots: white and earthy means you still only have overwatering and a dry-back fixes it. Brown, slimy and sour means drying out alone will not clear it.

Versus underwatering. Both wilt. Underwatering wilts a light pot and recovers within hours of watering. Root rot wilts a wet pot and does not recover after watering at all.

Versus fungus gnats. Gnat larvae also chew roots and stunt seedlings, but you will see dark flies on the medium surface and larvae in the top inch, and the roots will be chewed and ragged rather than slimy and brown.

Versus nitrogen deficiency. Nitrogen deficiency yellows from the bottom up with no wilting and normal root color. If leaves are yellowing and the plant also droops on a wet medium, look at the roots.

How long until it looks better#

Caught early, the plant stops getting worse within 2 to 3 days and you will see new white root tips within 5 to 7 days. White tips are the real sign of recovery, not anything happening in the leaves.

Yellowed leaves will not turn green again, and damaged roots do not heal, the plant builds new ones alongside them. Judge recovery on new root tips and new top growth over 10 to 14 days.

Be realistic. A plant that has lost most of its root mass in mid flower finishes small and light even if it survives, and it is often better to clean the system, start over, and hold the water temperature this time.