Plant Doctor Archive

Root rot in potted plants

SOME of my containerised shrubs and palms aren't doing well and I think they might have root rot. What should I do?

ROOT rot in potted plants is generally caused by a fungus disease with the formidable name of Phytophthora. The first sign a plant may be infected is usually poor growth followed by leaf drop and dieback of some shoots. Sometimes only part of a plant will die off while the rest remains apparently healthy. With potted plants a sure sign is wilting, even when the mix is moist.

The only way to tell for sure is take the plant out of its pot and check the roots. Healthy roots are usually firm and brown with whitish or yellow tips. Diseased roots break away easily in your hand and are mushy or dark brown overall.

Root rot in containerised plants is almost always caused by the mix being too wet, either from overwatering or lack of good drainage. Many plants can recover from root rot, so unless yours are too far gone, don't despair.

Make sure the containers have plenty of large drainage holes and when the pot is sitting on the ground the holes aren't blocked - you may have to raise them up a little so they drain freely. Saucers can contribute to this problem. If you leave them full of water they can encourage root rot, especially in winter.

Before repotting your plants cut away all dead and dying roots - with luck there will be some relatively healthy ones left. You could soak the roots in a solution of Kiwicare No Root Rot, available from garden centres. Repot using a good quality potting mix. Given the choice, I'd buy one with the beneficial fungus Trichoderma in it which can provide protection from soil-borne diseases, or you can buy the same fungus as Trichopel, which you water into the mix to help prevent root rot. Prune off any dead above-ground shoots. Then it's just a case of careful watering, though if you've used a good mix and got the drainage right, excess water should simply run right through with no adverse effect.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.