Gadenin日本語Get the appThe sap is the first hazard, and it is yours not the plant's
Cut any stem and it immediately weeps a thick yellow sap. It sets into a varnish-like film, stains skin and fabric a stubborn yellow-brown, and irritates skin on contact. On some people it produces a rash that appears hours later and is distinctly worse where the skin was then exposed to sunlight, so a light contact on a bright day can do more than the same contact indoors.
Treat it the way you would treat any plant with an irritant sap: gloves and long sleeves whenever you are cutting more than a stalk or two, no wiping your face with the back of a gloved hand, and a wash with soap and water immediately if any reaches skin. Keep an affected patch covered or out of the sun for the rest of the day. Do not strim or shred a large plant on a sunny day; that turns a manageable sap into a spray.
The same sap glues up pruners and knives. Wipe blades while the sap is wet, and wash them properly at the end of the session. Dried sap on a pivot stiffens a pair of snips permanently and picks up rust underneath.
Anyone who has reacted to other members of the carrot family, such as parsnip or wild parsnip foliage, should be especially careful with ashitaba sap and should not handle cut stems with bare hands at all.
Insects: what actually turns up
Aphids are the constant. They cluster on the newest furled growth in the center of the crown and on the underside of young leaflets, where they are easy to miss until the leaf opens distorted. A weekly look into the center of the plant catches them at the point where a strong jet of water still solves the problem. Left for a fortnight they coat the new growth in honeydew and sooty mold follows.
Swallowtail caterpillars feed on carrot-family foliage and will find ashitaba. They are large, boldly banded and unmistakable, and a couple of them can strip a stalk in days. On a mature plant the damage is cosmetic and most gardeners simply move them; on a young plant, pick them off. Leaf miners tunnel pale winding lines or blotches through the leaflets, which looks alarming but costs the plant almost nothing: pick off and bin the marked leaflets and do not spray.
Slugs and snails do the real damage, and they do it at the worst moment. Newly emerging spring shoots and the regrowth after a cut are exactly what they want, and a crown can be grazed to nothing in two wet nights. Clear debris from around the base, check at dusk during the first fortnight of spring growth, and keep the surface clear of the cut stalks you have just harvested. Spider mites show up as fine stippling and webbing on plants that are hot and dry, which is itself a sign that the plant is in the wrong place.
| Problem | Earliest sign | What to do | When to give up on the plant |
|---|---|---|---|
| Aphids | Colonies in the furled center, sticky newest leaves | Jet of water, repeat every few days; soap spray if persistent | Never; aphids alone do not kill an established crown |
| Slugs and snails | Shoots grazed flat overnight in spring | Clear debris, check at dusk, protect regrowth after every cut | Never, but a crown grazed twice in a season may not return |
| Swallowtail caterpillars | Whole leaflets gone, large banded caterpillars present | Move or remove them by hand; no spraying needed | Never; damage is cosmetic on a mature plant |
| Leaf miners | Pale winding tunnels or blotches inside the leaflet | Pick off marked leaflets and bin them | Never; the plant outgrows it |
| Spider mites | Fine pale stippling, webbing in leaf joints | Raise humidity, move to shade, hose the undersides | Rarely; treat the position rather than the mite |
| Crown and root rot | Stalks soft and brown at the base, whole plant wilting in wet soil | Stop watering, cut affected stalks out, improve drainage | If the crown is soft when pressed, the plant is finished |
| Rust | Orange-brown pustules on the underside of older leaflets | Remove affected leaves, thin the plant for airflow | Rarely fatal; expect to lose the affected leaves |
| Powdery mildew | White dusty film on older leaves late in the season | Improve airflow, water at the root, remove worst leaves | Never; it arrives at the end of the season anyway |
Rot: almost always something you caused
The leaf stalks are hollow. A stalk snapped off in wind or torn off in harvest leaves a ragged open tube pointing at the sky, which fills with rain and rots downward into the crown. That is the single most common route by which an otherwise healthy ashitaba is lost, and it is entirely avoidable. Cut cleanly, cut at a slight angle so water runs off, and remove any stalk that has snapped rather than leaving the stub standing.
The second route is a permanently damp collar. Mulch heaped against the base, fallen leaves left piled around the crown, or soil washed up over it keeps the collar wet and the tissue soft. Keep a clear ring of a couple of inches (5 cm) around the base at all times. In winter, mulch generously around the crown and not on it.
Once rot has reached the crown there is no cure. Press the crown with a thumb: firm means the plant is worth saving, and you should cut out the affected stalks, stop watering and improve the drainage. Soft and yielding, with a sour smell, means it is over. Lift and dispose of it, and do not plant another ashitaba or another carrot-family crop in the same spot for a couple of years.
Disorders that are not disease
Bleached pale patches on the leaves facing the afternoon sun are scorch, not disease. They appear over a single hot day and do not spread to new growth. Shade the plant and the new leaves will come through normally; the marked ones never recover and can be removed.
Yellowing that starts at the bottom of the plant and works up, on soil that is genuinely wet, is waterlogging. Yellowing that starts on the newest leaves with the veins still green is a nutrient problem, usually iron unavailable in soil that is too alkaline for the plant. Sudden collapse of a whole stalk with the leaf still green almost always means the stalk was broken at the base and you have not spotted it.
A sharp drop in leaf quality with the stalks turning fibrous within a fortnight, on a plant that had been growing well, is not a pest at all. That is the plant starting to run to flower, and since ashitaba dies after flowering it needs dealing with immediately rather than being diagnosed as a disease.
Note the date and location of the first aphid colony, the first slug damage and any rot, along with which crown it was on. Problems repeat in the same spot: a crown that rotted once marks ground that drains badly, and that is worth knowing before you plant there again.