Each flower is about 1–1.5 cm (about 0.4–0.6 in) long, purple, and highly fragrant. The flowers are copious nectar producers and are visited by many species of insects, including bees, butterflies and moths. Flowering occurs in late summer and is followed by production of brown, hairy, flattened seed pods in October and November, each of which contains three to ten hard seeds. Seeds, however, are only produced on plants that are draped over vegetation, fences, and other objects. Only one or two viable seeds are produced in a cluster of seed pods.
Once established, these plants grow rapidly, extending as much as 20 m (60 ft) per season at a rate of about 30 cm (12 in) per day. This vigorous vine may extend 10–30 m (30–100 ft) in length, with basal stems 1–10 cm (1–4 inches) in diameter. Kudzu roots are fleshy, with massive taproots 10–20 cm (4–8 in) or more in diameter, reaching depths of up to in older patches, and weighing as much as 180 kg. As many as thirty stems may grow from a single root crown.
Kudzu grows well under a wide range of conditions and in most soil types. Preferred habitats are forest edges, abandoned fields, roadsides, and disturbed areas, where sunlight is abundant. Kudzu grows best where winters do not drop below −15 °C (5 °F), average summer temperatures are regularly above 27 °C (80 °F), and annual rainfall is 1000 mm (40 in) or more. This fast growing plant does not do as well in less temperate areas.
The non-woody parts of the plant are edible. The young leaves can be used for salad or cooked as a leaf vegetable, the flowers battered and fried (like squash flowers), and the starchy tuberous roots can be prepared as any root vegetable. The starchy roots are ground into a fine powder, known as kuzu, and used for varieties of Wagashi and herbal medicines. When added to water and heated, kudzu powder becomes clear and adds stickiness to the food. It is sometimes known as "Japanese arrowroot", due to the similar culinary effect it produces.
Its leaves are high in vitamins A and C, as well as calcium and protein. Its roots are rich in starch and its flowers are an excellent honey source.
The name Kudzu appeared first in Kojiki and Nihonshoki as a type of vine or Kazura used commonly by the people who lived in Kuzu, an area around present-day Yoshino, Nara prefecture. It is unclear whether the name was taken from the people or the name of the plant was applied to the people. Kudzu has been used for over 1300 years and it is speculated its use goes back even further. Records from the Nara and Heian era indicate that kudzu was collected and sent as a part of tax. Even today, "Yoshino Kudzu" has the best image of kudzu powder. The Kagoshima prefecture is the largest producer of kudzu products.
Kudzu also contains a number of useful isoflavones, including daidzein (an anti-inflammatory and antimicrobial agent), daidzin (a cancer preventive) and genistein (an antileukemic agent). Kudzu is a unique source of the isoflavone puerarin. Kudzu root compounds can affect neurotransmitters (including serotonin, GABA, and glutamate) and it has shown value in treating migraine and cluster headache.
Research in mice models suggests that Kudzu is beneficial for control of some post-menopausal symptoms such as hypertension and diabetes type II.
In traditional Chinese medicine, where it is known as gé gēn (葛根), kudzu is considered one of the 50 fundamental herbs. It is used to treat tinnitus, vertigo, and Wei syndrome (superficial heat close to the surface).
Kudzu was introduced from Japan into the United States in 1876 at the Philadelphia Centennial Exposition, where it was promoted as a forage crop and an ornamental plant. From 1935 to the early 1950s the Soil Conservation Service encouraged farmers in the southeastern United States to plant kudzu to reduce soil erosion as above, and the Civilian Conservation Corps planted it widely for many years.
However, it was subsequently discovered that the southeastern US has near-perfect conditions for kudzu to grow out of control — hot, humid summers, frequent rainfall, temperate winters with few hard freezes (kudzu cannot tolerate low freezing temperatures that bring the frost line down through its entire root system, a rare occurrence in this region), and no natural predators. As such, the once-promoted plant was named a pest weed by the United States Department of Agriculture in 1953.
Kudzu is now common throughout most of the Southeastern United States, and has been found as far northeast as Paterson, New Jersey, in 30 Illinois counties including as far north as Evanston, and as far south as Key West, Florida. It has also been found growing (rather inexplicably) in Clackamas County, Oregon in 2000. Kudzu has naturalized into about 20,000 to 30,000 square kilometers of land in the United States and costs around $500 million annually in lost cropland and control costs.
The spread of kudzu is mainly by vegetative expansion by stolons (runners) that root at the nodes to form new plants and by rhizomes. Kudzu will also spread by seeds, which are contained in pods and mature in the autumn, although this is rarer. One or two viable seeds are produced per cluster of pods. These hard-coated seeds may not germinate for several years, which can result in the re-appearance of the species years after it was thought eradicated at a site.
For successful long-term control of kudzu, it is not necessary to destroy the entire root system, which can be quite large and deep. It is only necessary to use some method to kill or remove the kudzu root crown and all rooting runners. The root crown is a fibrous knob of tissue that sits on top of the root (rhizome). Crowns form from vine nodes that root to the ground, and range from pea-size to basketball-size. The older the crown, the deeper they tend to be found in the ground because they are covered by sediment and plant debris over time. Nodes and crowns are the source of all kudzu vines, and roots cannot produce vines. If any portion of a root crown remains after attempted removal, the kudzu plant grows back.
Mechanical methods of control involve cutting off crowns from roots, usually just below ground level. This immediately kills the plant. Cutting off vines is not sufficient for an immediate kill. It is necessary to destroy all removed crown material: Buried crowns can regenerate into healthy kudzu. Transporting crowns in soil removed from a kudzu infestation is one common way that kudzu "miraculously" spreads and shows up in unexpected locations.
Close mowing every week, regular heavy grazing for many successive years, or repeated cultivation may be effective, as this serves to deplete root reserves. If done in the spring, cutting off vines must be repeated as regrowth appears to exhaust the plant's stored carbohydrate reserves. Cut kudzu can be fed to livestock, burned, or composted.
Late-season cutting should be followed up with immediate application of a systemic herbicide to the cut stems, to encourage transport of the herbicide into the root system. Repeated applications of several soil-active herbicides have been used effectively on large infestations in forestry situations.
Prescribed burning is also used on old extensive infestations in order to remove vegetative cover and promote seed germination for removal or treatment. It is usually done to prepare for treatment of the root crowns. Landscape equipment, such as a skid loader ("Bobcat"), can also remove biomass. While fire is not an effective way to kill kudzu, equipment such as skid loaders can remove crowns and thereby kill kudzu with minimal disturbance of soil.
Efforts are currently being organized by the U.S. Forest Service to search for biological control agents for kudzu. Several fungi are pathogenic to kudzu. Colletotrichum gloeosporioides is one tested example.
The city of Chattanooga, Tennessee has undertaken a trial program using goats and llamas that graze on the plant. The llamas serve double-duty as defense against predators due to their aggressive nature. Currently the goats are grazing along the Missionary Ridge area in the east of the city.