A blackened stem at soil level
Blackleg. The stem darkens and softens where it meets the compost, and the plant is lost from below unless a cutting is taken from above the damage today.
This one is worth acting on today rather than at the weekend. A week of waiting changes what is left to save.
What it usually is
1 Rot at the collar from wet compost
| How to tell | The stem is black or dark brown at the point it enters the compost, softer than the stem above it, and the discolouration is creeping upwards week by week. The compost is damp and may smell sour. |
|---|---|
| The fix | Take a cutting from firm tissue well above the blackened part today, let the cut dry for a few hours, and root it in fresh barely damp compost. The parent is usually not recoverable, because the damage is between the roots and everything above them. |
2 A cutting rotting before it rooted
| How to tell | A cutting in water or compost that has blackened at the cut end rather than callused, often after being kept warm and wet with no air movement. |
|---|---|
| The fix | Cut back to clean white tissue, let the new cut dry for a few hours until it seals, and start again in fresh compost that is damp rather than wet. Change the water every few days for cuttings in water, and use a clean jar. |
3 Cold and wet together
| How to tell | The plant spent a cold night in wet compost, on a windowsill or in an unheated room, and blackened within days. |
|---|---|
| The fix | Move it somewhere warmer and let the compost dry. Cold plus wet is what turns ordinary damp compost into rot, and either alone is survivable. |
Through the year. Almost entirely an autumn and winter problem, and above all in an unheated room. Warmth alone rarely does it; cold and wet together do it reliably.
Blackleg is the name gardeners give to rot at the collar, the point where stem meets compost, and it is one of the few plant problems where speed genuinely changes the outcome. The tissue there is the plant’s only route between roots and leaves. Once it has gone soft and black, nothing above will be supplied, and no amount of care for the roots or the leaves will reconnect them.
So the response is triage rather than treatment. Look for firm, sound stem above the blackened section. If there is enough of it, cut there with clean scissors, take that piece as a cutting, and accept the loss of the base. Many houseplants root readily from such a cutting, and a great many plants that people describe as having died of rot could have been saved as a cutting a week earlier.
The cause is nearly always the same combination: compost that stays wet, air that does not move, and a room that is cool. Fungi and bacteria that live harmlessly in every bag of compost become aggressive in those conditions, and a small wound at the base, from a knock or from a leaf that rotted against the stem, gives them their way in.
Prevention is dull and effective. Water at the compost rather than over the crown of the plant, so water does not sit against the stem. Keep fallen leaves off the compost surface. Let the top of the pot dry between waterings in winter. Give the plants enough space that air moves between them. And keep tropical plants out of an unheated room in the cold months, because a plant at ten degrees in wet compost is in exactly the state this thrives in.
For cuttings specifically: let a cut surface dry for a few hours before it goes into compost, use fresh compost rather than reused, and change the water every few days for cuttings rooted in a jar. The black end of a cutting in cloudy water is the commonest form of this in the house.
Entries that name it
What the journal says about this symptom on each plant.
| Basil | Cold, wet conditions or fungal disease. Keep above 10 degrees, water in the morning at soil level, and improve air flow. |
|---|---|
| Bird's nest fern | Crown rot from water sitting in the nest at the centre. Water around the edge of the pot instead, and improve the air movement. |
| Blackberry | Cane blight, a fungus entering through a pruning cut or another wound, blackening and girdling the cane above the point of entry. Cut out and destroy affected canes right down to the ground. Prune with clean tools and avoid wounding canes unnecessarily, since the fungus needs a wound to get in at all. |
| Garden privet | Honey fungus, which privet is one of the more susceptible garden shrubs to catching. There is no chemical cure. Dig out and destroy every trace of infected root and stump, by burning or landfill, rather than replanting into the same soil. |
| Geranium | Blackleg, a rot at the base caused by cold wet compost. There is no cure for an affected plant. Take healthy cuttings from the top, root them in gritty compost, and water far less. |
| Staghorn fern | Rot from a mount that never dries between soaks. Soak less often and let it drain properly. It should feel light before the next soak. |
Questions from the margin
- What is blackleg on a plant?
- Rot at the collar, where the stem meets the compost. That point is the plant's only route between roots and leaves, so once it has gone soft and black nothing above it will be supplied, however well you treat the rest.
- Can I save a plant with a black stem base?
- Only by taking a cutting from firm tissue well above the damage, today. Let the cut dry for a few hours so it seals, then root it in fresh barely damp compost. The base itself is not recoverable.
Checked against RHS, Blackleg on cuttings; Penn State Extension, Root and crown rots.