Young ponytail radish with greens
A small young radish, thumb to finger length, used together with its long leaves; the root is firm and crunchy, the greens pleasantly bitter.
Chonggak-mu, also called altari-mu, is a small Korean radish variety harvested young, while root and leaves are still tender; the name ponytail radish comes from its tuft of long leaves. The root is denser and crunchier than a large daikon, juicy and mildly peppery, and it keeps its bite even after long fermentation, which is why it is usually pickled whole or just halved. The leaves taste grassy and slightly bitter and are used along with the root rather than trimmed off. Look for it fresh in Korean or well-stocked Asian grocers, mainly in autumn and early winter, and occasionally at farmers' markets from growers of Asian varieties; seeds are available by mail order and the variety grows well in a home vegetable bed.
Substitute:
There is no true substitute, because the shape, the firmness and the greens used alongside belong together. The closest option is a firm, slender daikon cut into thick batons or chunks, with radish or daikon greens added; the pieces soften faster during fermentation and the dish loses the whole roots with their leaf stems. Small red radishes alone are too small and too watery and soon go limp.