Root rot in containers: diagnosis and recovery

Healthy roots are white or cream, firm, and smell of soil. Rotted roots are brown or black, soft enough to squash between two fingers, and smell sour. If the outer layer slips off leaving a thin thread, that root is dead. Cut away everything rotted, repot into fresh dry mix, and do not water for several days.

Root rot kills more container plants than every insect pest combined, and it is nearly always caused by the owner rather than by bad luck. The pathogens involved, mostly Pythium and Phytophthora, are already present in most growing media. They only become destructive when the mix stays wet long enough for oxygen to run out around the roots.

That is the important part. Root rot is a drainage problem that a fungus takes advantage of. Treating the fungus without fixing the drainage just delays the next round.

Confirming it is root rot

The symptoms above ground are unhelpfully similar to drought: wilting, yellowing, dropping leaves, stalled growth. A plant with dead roots cannot take up water, so it wilts exactly like a thirsty plant. Many gardeners respond by watering more, which is why root rot often accelerates once it is noticed.

The only reliable diagnosis is to look at the roots. Slide the plant out of the pot and check.

The rescue sequence

Work quickly once you have confirmed it. A plant with half its roots gone can usually be saved. A plant with almost none cannot.

1.    Take the plant out of the pot and remove as much of the old mix as you can. Rinse the root ball gently under slow-running water so you can see what you are dealing with.

2.    Cut away every soft, brown or blackened root with clean scissors. Cut back into firm white tissue. Be ruthless here; leaving rotted tissue behind restarts the problem.

3.    Trim the top of the plant back by roughly the same proportion as the roots you removed. A reduced root system cannot support a full canopy.

4.    Let the remaining roots air-dry for one to two hours in shade. This alone kills a large share of the surface pathogen.

5.    Repot into fresh, dry, free-draining mix in a clean pot. Do not reuse the old mix, and wash the pot if you reuse it.

6.    Water lightly once, then leave it alone. The next watering should not come until the top few centimetres are dry.

Keep the plant out of direct sun for the first two weeks. A damaged root system cannot supply a plant that is transpiring hard.

Treating the pathogen

Once the plant is in fresh mix, biological treatment does more good than a chemical drench in a container, because you are trying to recolonise the root zone rather than sterilise it.

Trichoderma is the standard choice. It colonises the root surface and physically crowds out Pythium and Fusarium. A Trichoderma viride powder mixed into the fresh potting medium at planting, or Trichoderma harzianum as a soil drench, both work for this.

For a bacterial approach, Striker (Pseudomonas fluorescens) colonises the root zone quickly and is particularly useful as a preventive drench on seedlings and cuttings. BAXIL (Bacillus subtilis) covers a broader range of soil and foliar pathogens.

Apply these to moist, not soaked, medium, and keep the treated pot out of strong sunlight for a day. These are live organisms and they need reasonable conditions to establish.

A seaweed extract at low rate once new growth appears helps push root development. Use it after the plant shows signs of recovery, not during the first fortnight.

Why it happened, and stopping it happening again

1. The pot

No drainage holes, or holes blocked by a compacted layer, is the most common single cause. A decorative outer pot holding standing water does the same thing. Empty saucers within thirty minutes of watering.

Oversized pots are the second cause. A small plant in a large pot cannot use the water in that volume of mix, so the outer region stays wet for weeks. Pot up gradually rather than in one jump.

2. The mix

Garden soil in a container compacts and holds water. Old potting mix breaks down over two or three years and loses its air spaces. Both create the low-oxygen conditions that root rot needs. A mix with perlite, coarse sand or bark keeps air pockets open. Iowa State University Extension covers the mix and watering side of this well.

Well-decomposed organic matter in the mix helps too, because it supports competing microbes. Enriched vermicompost at around 20 percent of the mix is a reasonable proportion for most container plants.

3. The watering habit

Watering on a schedule rather than by checking the mix causes most cases. Plants use dramatically different amounts of water in different weather. Check with a finger, or lift the pot and judge by weight.

Watering little and often is worse than watering thoroughly and less frequently. Shallow watering keeps the surface wet and the lower root zone dry, which is the worst combination.

How to tell whether the rescue worked

Give it three to four weeks. Signs it is recovering: firm new growth at the tip, no further leaf drop, and the mix drying at a normal rate because roots are drinking again.

Signs it is not: continued wilting despite moist mix, blackening moving up the stem, and mix that stays wet for a week. At that point the crown is affected and the plant is unlikely to come back.

If you have several plants and one has advanced rot, keep it away from the others while you decide. Water splashing between pots spreads these pathogens easily.

Michigan State University Extension has a useful description of how Pythium behaves in container production, including why it appears after periods of overwatering. Their note is here.