jiāng is a bound form - it appears only in
豇豆 and its derivatives. The bean itself (Vigna unguiculata, also called cowpea or yard-long bean) is a Chinese kitchen staple, especially in Sichuan and Hunan cooking. Common compounds:
酸豇豆 (pickled / sour long beans, a famous Sichuan condiment),
干煸豇豆 (dry-fried long beans). The pods routinely grow 30-60cm long, hence the English name 'yard-long.'