Agave Problems: Rot, Snout Weevil, Scale and Spine Injuries

The short list of what actually goes wrong

Agaves have very few problems, which is part of why people grow them, but the few they have move fast. Root and crown rot account for most losses and are usually the gardener's doing. The pests are a handful of familiar ones plus one genuinely serious insect. Everything else on this list is cosmetic.

Rot: catch it at the base or not at all

Rot starts in one of two places. Root rot begins below ground in compost that stays wet, works up into the stem, and shows as a base that has gone soft and slightly sunken. Crown rot begins in the center where water has collected in the funnel of furled leaves, and shows as central leaves that come away with almost no resistance and smell sour.

Neither is treatable with anything you spray on. The response is surgical: remove the affected tissue, get everything dry, and replant only what is firm. Speed matters. A plant caught within a few days of the base going soft usually lives; one left a fortnight usually does not.

1. Stop watering immediately Do not water again until the plant has been cut back and has visibly recovered. Move a potted plant somewhere warm, airy and out of the rain.
2. Lift it and look at the roots Knock the plant out of its pot, or dig it up. Firm pale roots and a firm white stem mean you have time. Brown, hollow, stringy roots and a squashy stem mean cut now.
3. Cut back to clean tissue With a sharp knife wiped with alcohol between cuts, slice up through the stem until the cut face is uniformly white and firm with no brown flecks or rings. Brown specks in the cut face mean the rot is above your cut. Go higher.
4. Strip the affected leaves Pull off any leaf that is soft at its base, wearing gloves, and bin the material rather than composting it. Cut leaves stay sharp and irritant for a long time, so bag them.
5. Dry the wound properly Stand the plant on its side or upright out of soil, in a warm airy shaded place, for one to two weeks until the cut is dry and hard. This step is not optional and cannot be shortened.
6. Replant dry, and wait Pot into fresh, mostly mineral mix in a clean pot. Do not water for a further two weeks. New roots form into dry mix perfectly well.

If water is sitting in the center of a rosette in cool weather, tip the pot to drain it or blot it out that day. That one habit prevents most crown rot.

Snout weevil, scale and mealybugs

The agave snout weevil is the one pest worth learning to recognize. The adult is a dark beetle with a curved snout; it chews into the base of the rosette, and its grubs eat out the core. The visible sign is dramatic and late: a large healthy plant suddenly collapses, the leaves splaying flat, and the whole rosette lifts away from a hollowed, foul-smelling base. There is no rescue at that stage. Remove and destroy the plant and the soil immediately around it, because the grubs pupate in that soil and will move to the next plant.

Scale and mealybugs are slower and manageable. Both feed on sap and both hide: scale as hard limpet-like bumps along the leaf undersides and in the axils, mealybugs as white cotton wool in the tight center and sometimes on the roots. Neither kills a strong plant quickly, but both weaken it and leave sticky honeydew that turns black with sooty mold.

Inspect the leaf axils on the shaded side of the plant every time you water. Scale and mealybugs are far easier to deal with at ten insects than at a thousand, and the axils are where they start.

The hazards the plant poses to you

This is not a minor caveat. The terminal spine on an agave leaf is hard, needle-sharp and rigid enough to go straight through skin, and on a mature plant those spines sit at chest and eye height. Puncture wounds from them are deep, close over quickly and are prone to infection, and eye injuries from bending over a rosette are a genuine and recorded risk, not a theoretical one.

The sap is the second hazard. It contains irritant compounds that provoke a burning rash and small blisters in many people, and the reaction is often delayed by several hours, so you feel nothing while you work and find the damage that evening. Sunlight can make the reaction worse. Anyone who removes a large agave without covering their arms tends to learn this once.

1. Blunt the tips on anything near a path Snip about a sixteenth of an inch (2 mm) off each terminal spine with pruners, wearing eye protection. It is barely visible and it removes the puncture risk completely.
2. Never work above a rosette Reach in from outside rather than leaning over the top. Wear safety glasses whenever your face is within reach of the leaves.
3. Cover skin when cutting Long sleeves, thick gloves and long trousers for any pruning or removal, and wash exposed skin with soap and cold water straight afterwards, before any reaction appears.
4. Treat punctures seriously Wash a spine wound thoroughly and watch it. A puncture that stays painful, swells or reddens over the following days needs medical attention, particularly on a hand or near a joint.

Keep young children and pets away from mature rosettes entirely. A spine at adult chest height is at a child's eye height, and no amount of care makes that safe.

When to give up on a plant

Agaves are worth some effort but not unlimited effort, and a plant kept on when it is beyond saving is a reservoir of pests and rot organisms for everything nearby. The judgement is simple: if the growing point is gone, the plant is gone, no matter how many healthy-looking leaves remain.

What to write down

Records turn a one-off disaster into something you can prevent next year. Most agave problems are seasonal and repeat in the same conditions, so a short note at the time is worth far more than a memory a year later.

1. The date rot first appeared, and the weather before it Nearly always a wet spell, a late watering or a pot left in the rain. Written down, the cause is obvious the following autumn.
2. The date you first saw each pest, each year Scale and mealybugs return at roughly the same point in the season. Knowing that date lets you inspect a fortnight earlier next time.
3. The low temperature a plant survived, and whether it was dry This is the single most useful number you can own, because published hardiness figures assume dry roots and your garden may not deliver that.