Tucked away inside an extinct volcano, Koko Crater Botanical Garden is probably the least-known of the city of Honolulu's five botanical gardens.

Geologically, Koko Crater stands as the largest tuff cone on O'ahu. Tuff is basically made up of fine, loose, rock fragments that were fused by intense heat when they were first formed.

This unique East Honolulu garden — established as a botanical garden in 1958 — is still in its early developmental stages. About 60 acres of the crater floor and lower, inner slopes of the 200-acre site are planted. Due to the low rainfall — an annual average of less than 26 inches — only plants that are adapted to dry conditions are planted here. Some are rare and endangered.

Visitors may park outside the garden and walk the loop trail, which is about 2 miles along mostly flat terrain studded with some large rocks. Tours can take an hour or two to complete.

The outer part of the crater holds the plumeria and bougainvillea collections. The bougainvilleas are in flower now, but the plumerias are leafless and dormant. The best time to see the 80 or so different kinds of plumeria in full, glorious, color usually begins in May and continues into early summer.

The inner part of the crater is arranged geographically, with plants from the Americas, Hawai'i, Madagascar and Africa making up the garden's four major plant collections.

In the Hawaiian section, look for the natural stand of native wiliwili (Erythrina sandwicensis) trees. These special trees are protected by state law, under Exceptional Trees Act 105. They are in fair condition, having so far survived the Erythrina gall wasp infestation that has nearly wiped out the wiliwili trees elsewhere in the Islands.

In the African section, you won't be able to miss the dead-rat tree (Adansonia digitata). Hanging by long stems, the oblong, brownish-green, velvety fruits look like rats hanging by their tails from the tree.

Close to the dead-rat tree are the popular desert rose (Adenium obesum) plants. They grow near the walking path as well as on the lower slope of part of the crater wall. In full bloom now, the light-pink to deep-red flowers add splashes of color to the crater.

There are also interesting collections of palms, cactuses and other dryland plants for gardeners looking for water-thrifty plant ideas.

While at the garden, you may see the two hard-working staff members. Robin Sunio has cared for the plant collections at Koko Crater Botanical Garden for 24 years. Rodney Choy Foo has been there about three years. This dynamic duo keeps weeds at bay, trails passable and plants thriving.

The walking trails are unpaved, so wear sturdy hiking shoes when you explore the garden. Also recommended: sunscreen, and bring plenty of drinking water. Enjoy your adventure!