For those who live in Honshu, Shikoku and Kyushu, tsuyu officially began last week. Although in some years little or no rain falls in this rainy season, I personally always hope the heavens open to give some respite from the relentless heat.

Two plants to look forward to in bloom at this time of year are the hana-shobu (Japanese water iris; Iris ensata) and the ajisai (hydrangea; Hydrangea macrophylla), both of which depend on the rains in June to flower well.

Almost anywhere in these islands, you won't have to travel far to see either plant, because, like the sakura (Japanese cherry), people here have a long history associated with the cultivation of these lovely plants. Though hydrangeas are certainly showy and eye-catching, whether encountered in the wild or in gardens, it's the more singularly exotic and extensive iris family that I'm drawn to myself.

The best time to view the Japanese water iris is after June 15 -- and I recommend visiting an iris garden in the rain.

The genus Iris, in which there are estimated to be more than 300 species, is native to a wide area of the northern hemisphere, though only eight occur naturally in Japan, of which just three grow in bogs or ponds. Because there are so many species, botanists have divided the genus into 27 sections, or subgenera.

The sumptuous hana-shobu (Japanese water iris) that we see cultivated in water gardens around the country is an aquatic or water-marginal herbaceous perennial that is the result of careful selection and hybridization with other Japanese and Eurasian irises. No-hana-shobu (Iris ensata var. spontanea) is the wild Japanese water iris that growers have used to create better, more colorful iris plants. No-hana-shobu grows naturally in ponds in throughout Manchuria, northern China, the Korean Peninsula and Japan.

Whatever their individual characteristics, however, all iris are rhizomatous (tuberous perennials). Their leaves are strap-shaped (linear) and the flower has six petals, of which the three broad outer ones that hang down are known as falls, in contrast to the three inner ones that are erect. Each flower has three stamens and the style is branched inside a "tube" immediately above the falls. The falls on the no-hana-shobu are 7 cm long and the crest is light-yellow colored.

Basically there are four main strains of hana-shobu in cultivation, and from these umpteen cultivars, or strains (kei in Japanese), have been derived, most notably the Edo, Higo, Ise and Ito.

During the middle of the Edo Period (1603-1867), horticulture flourished. In Edo, present-day Tokyo, a strain of iris known as Edo-kei was developed with petals larger than those of the native species. Next came the Higo-kei from present-day Kumamoto Prefecture, with even bigger petals and, in some varieties, double flowers. The Kii Peninsula in present-day Wakayama Prefecture produced the Ise-kei, while from the Izu Peninsula in present-day Shizuoka Prefecture came the Ito-kei.

More recently, gardeners have used the ki-shobu (yellow-flag iris; I. pseudoacorus), a perennial native of a huge swathe of territory extending from North Africa through Europe and across to Siberia. After the ki-shobu was introduced to Japan in 1896, breeders began crossing this species with cultivars of hana-shobu to expand the range of colors.

However, hybridization was not confined to Japan, and growers in both the United States and Germany were busily doing the same. Many of the best of these colorful hybrids were then imported to Japan and are now grown side by side with old Japanese selected cultivars. The growing conditions in Japan were very favorable for ki-shobu, and it is now completely naturalized all over the country. Furthermore, its wide dispersal has also been helped by the fact that its seeds float.

Two other native species you will see cultivated in iris gardens are ayame (blue-flag iris; I. Sanguinea) and kaki-tsubata (rabbit-ear iris; I. laevigata). Ayame will tolerate somewhat drier conditions than its water-loving cousin, hana-shobu, though it, too, is a perennial with long strap-shaped leaves 30-60 cm long and with a flowering stem of approximately the same length.

Between May and July, two or three bluish-purple flowers are borne on each stem, with falls that are around 6 cm long and slightly crested at the narrow part with a yellow-white pattern. The colorful crest is like a runway to visiting insects, guiding them to the nectar at the base of the petals. The crest also houses the modified male stamens, and insects in search of the nectar unknowingly get coated with pollen.

The perennial kakitsubata, which grows at the edge of ponds throughout Japan, has flower stems between 40-80 cm high, with two or three white-crested flowers borne at the top of each.

If you, like me, are an iris-lover, you can easily create your own iris garden by growing hana-shobu in the ground or in pots or other containers in a sunny position in rich soil containing plenty of humus such as leaf mold. The best time to divide the plants is in early autumn.

One iris garden I went to recently was the Eitakuji Japanese Iris Garden in the countryside of Sanda City in Hyogo Prefecture. The 3-hectare iris park around this 600-year-old Buddhist temple was opened in 1975, and now boasts some 3 million plants, representing 650 cultivars and species from all over Japan. In early spring the same garden is also well known for its peony collection, to which fine displays of mizu-basho (Lysichiton camtschatcense) and shiba-zakura (Phlox douglasii) have more recently been added.

Just inside the entrance gate there is a good display of giboushi (hosta, or plantain lily), and a particularly interesting plant I saw there was suji-giboushi (striped hosta; Hosta undulata). The botanical name undulata, which means "undulating" or "wavy," refers to the leaf-edge of this variety. It was the very first variegated hosta cultivated in Europe and was introduced to Japan in 1829 by the famous German botanist Philipp Franz von Siebold (1796-1866). Conveniently for home gardeners, this attractive hosta is inexpensive and easy to cultivate.