Why cactus cuttings rot, and how to avoid it

When a cactus cutting fails, the usual cause is rot. Part of the stem turns soft and dark, and the damage spreads. Most often it starts at the cut end, but it can start anywhere the skin has been broken or damaged. Rot comes from a small set of conditions, and most of them are in your control.

This page explains what those conditions are, how to tell rot from changes that are normal, and what to do if rot has started. It is written for cuttings whose cut end is still sealing, or that are rooting.

The short version

Rot needs moisture, and usually a way in. It often starts at a cut end that sits in damp mix before it has sealed or rooted, but it can start at any wound.
Let the cut end callus, meaning dry and seal over, before it touches soil.
Root the cutting in dry, gritty mix, and plant it only deep enough to stand.
Keep it warm. A cold cutting roots slowly or not at all, so it stays at risk for longer.
Safest: no water until it has rooted.
If any part turns soft and dark, act the same day. Cut back to clean tissue, let the new cut callus, and start again in fresh, dry mix.

Is it rot?

Rot has a recognizable set of signs:

the tissue is soft or mushy, and stays dented when you press it
it is dark brown or black, and often looks wet or glassy
it often smells sour or rotten
it usually starts at the cut end or at soil level and spreads upward, but it can start at any wound or damaged spot higher up

These are not rot:

A callus. The cut end is dry, hard and pale, from cream to tan.
Wrinkling or slight shrinking before the cutting roots. It is using its stored water. The stem is still firm.
Dry, firm, tan or brown marks that do not spread. These are old scars.
A pale or yellow patch on the side facing the sun. This is sunburn. Damaged skin is a way in for rot, so keep water off the patch and keep an eye on it.

If you are not sure, press it. Rot is soft and gives under your finger. Other blemishes that are not rot will be firm and dry.

Why it happens

A rooted cactus takes up water through its roots. A cutting has no roots, so it has no way to take water in. It does not need any yet. It is living on the water it stored before it was cut, and that store is more than enough to grow roots with.

The cut end is a wound. Left dry and in moving air, it seals over into a firm, dry layer called a callus, which protects the soft tissue inside from disease in the soil.

Rot always needs moisture, and it usually needs a way into the plant. A cut end that has not sealed, sitting in damp mix, provides both. The cut end is the obvious way in, but not the only one. The top cut on a mid-section cutting, a bruise or scrape from shipping, a spine puncture, and a patch damaged by sunburn or frost are all openings too. Tissue that stays wet for a long time can rot even without a wound. The cut end is still the main risk, which is why nearly all the advice for rooting a cutting comes down to keeping the cut end dry until it has sealed and rooted.

What sets it up

Planting before the callus is firm. A cut end that is still damp, tacky or soft anywhere is still an open wound. Thick cuttings and humid air both slow callusing, and a large cut can take much longer than you expect.

A mix that stays wet. Peat-heavy potting soil, or anything sold as moisture retaining, holds water against the cut end for too long. A gritty mix lets air in and dries quickly.

Cold. In cool conditions a cutting roots slowly or not at all, so it sits unrooted for longer, and cool mix is slow to dry.

Planting deep. A cutting only needs to go deep enough to stand firmly. Burying more of the stem does not help it root. It only puts more of the stem in damp mix.

A pot that is too big. A large pot holds a lot of damp mix around a cutting that has no roots to use the water.

Rain. A cutting rooted in the ground or left outdoors can be soaked before it has roots. Rain or overhead watering can also leave water sitting on an upper cut end or in a damaged spot.

Watering too early or too often. Water that the cutting cannot use sits against the cut end.

Your conditions change the risk

If your climate is hot and dry, mix dries quickly and the risk is lower. You have more room for error.

If your climate is cool, humid or rainy, the same pot and the same mix stay wet for much longer. Use a grittier mix, keep the cutting under cover, and keep it dry until it roots.

If the cutting is in a pot, you control the mix, the water and where it sits. If it is in the ground, you do not control the rain, so choose a spot that drains quickly and cover the cutting when rain is forecast.

Watering while it roots

The safest choice is no water until the cutting has rooted. Keeping it dry is the surest way to avoid rot, and the cutting does not need the water.

Some growers water lightly while cuttings root. It can help, but it is not necessary, and it only makes sense if your conditions are warm and dry enough, and your mix gritty enough, for the mix to dry out fully again soon afterward. If you are not sure, skip it. How to root a cactus cutting covers this in step 7.

If rot starts: what to do today

On an unrooted cutting, do not count on rot stopping by itself. The fix is to remove it.

Take the cutting out of the mix and brush the base clean so you can see it.
Cut above the rot with a clean, sharp blade. Cut high enough to remove all of it.
Check the new cut face. It should be firm and evenly colored all the way across, with no dark, soft or discolored spots. If you see any, clean your knife and cut again, a little higher.
Clean the blade between cuts, so you do not carry rot into clean tissue. Wipe off any sap and dirt, then wipe the blade with 70% isopropyl alcohol so it stays wet for at least 30 seconds. Let it air dry.
Let the new cut callus. Stand the cutting somewhere dry and airy, in bright shade out of direct sun. This is the step that matters most. A large cut can take weeks to seal, and sometimes longer. Some growers dust the fresh cut with sulfur powder. It is not necessary. Keeping the cut dry, in moving air, is what matters.
Start again in fresh, dry mix. Throw out the old mix, and wash the pot before you reuse it.
Do not water it. Treat it as a new cutting.

If the rot is on the side or the top

A small soft spot on the side can be cut out. Use a clean blade, remove everything soft or discolored until only firm, evenly colored tissue is left, and keep the hollow dry until it seals. Sulfur powder can be dusted on here too.

If the rot is at the top, cut below it and keep the clean lower section. If a spot is large or goes deep into the stem, treat it like rot at the base: cut through the stem above and below the rot, and keep any piece that is clean.

In every case, keep water off the wound while it dries.

When a cutting is too far gone

There is no fixed minimum size for a cutting to root. A shorter piece holds less stored water, so it has less margin while it roots, but a piece that is firm and calluses cleanly is worth trying.

A cutting is past saving when no firm, clean tissue is left once the rot is cut away.

Next steps

How to root a cactus cutting: the full method, step by step.
After your cactus arrives: how to check a callus, and how to store a cutting until you are ready to plant it.