Native Hawaiian Garden (7722)

Hau kuahiwi (Hibiscadelphus distans & H. hualalaiensis) – Catering to the birds

What do the following endemic Hawaiian plants all have in common: about 125 species of lobelioids (Campanulaceae), Geranium arboreum, Scaevola glabra, and eight Hibiscadelphus species? If you said, “They all have curved tubular flowers,” you would be right! Prior to humans arriving, Hawai‘i had few large effective insect pollinators like butterflies, moths, and bees. Therefore, many Hawaiian plants evolved smaller drab-colored flowers pollinated by small insects, while others evolved larger brightly colored flowers pollinated by native birds. What happened next was amazing! As the endemic group of birds collectively called Hawaiian honeycreepers underwent an adaptive radiation in the Islands, part of the group coevolved with the above-mentioned endemic plants; short straight beaks became longer with a downward curve, while flowers became tubular and curved. Some honeycreepers evolved insanely long curved beaks while others evolved only slightly longer beaks. Biologists think this variation was evolution perfectly matching each bird species with a small number of plant species, thereby reducing competition for food (i.e., nectar). Coevolution can improve the efficacy of something like pollination or food-gathering, but it can also leave one or more of the coevolved species dangerously dependent on the other. So, while Hawai‘i’s native plant communities have experienced a multitude of detrimental threats (e.g., habitat loss, introduced herbivores and pests) over the past few hundred years, it is likely the extinction and enormous decline in Hawaiian honeycreepers has placed many of their coevolved endemic plants in even further jeopardy. (However, see my notes below on Collecting Seeds.)

Habitat & Appearance: There are three presumed extinct and five extant species of Hibiscadelphus, all endemic to the Hawaiian Islands. The five remaining species are extremely rare and listed by the State and Federal governments as critically endangered.

Hibiscadelphus distans is endemic to dry forests on Kaua‘i. Today, there are only two known naturally occurring small populations (about 200 plants combined), both in the Lower Koai‘e Canyon area of the Pu‘u Ka Pele Forest Reserve in Waimea Canyon. H. distans is a shrub or small tree up to 16 ft tall with smooth gray bark and heart-shaped serrated fuzzy leaves. Its flowers never open, but remain folded together in a slightly-curved tubular form with a bright green corolla that turns dull red with age. Its fruit is a dehiscent capsule, divided into five sections with each section containing two small (5 mm) fuzzy seeds.

Hibiscadelphus hualalaiensis is endemic to dry and mesic forests on the lava-field slopes of Hualālai, Hawai‘i. The last known wild plant died in 1992. However, it survives in cultivation at various botanical gardens and has been planted out in several restoration sites on the Big Island. H. hualalaiensis looks very much like H. distans. However, it is a bit larger at maturity (up to 23 ft tall) with smaller bracts at the base of its flower and a yellowish-green corolla that turns purple rather than red as it ages.

Hibiscadelphus distans (Photograph courtesy of David Eickhoff CC BY-NC-SA 2.0). Hibiscadelphus hualalaiensis with immature fruits. H. hualalaiensis seeds and seedling.

Hau kuahiwi in Hawaiian Culture: The name hau kuahiwi translates as “hau of the mountains.” Hau (Hibiscus tiliaceus) is a tree that is possibly indigenous to Hawai‘i or was introduced by Polynesians before Western contact. I could not find any other Hawaiian references or uses for hau kuahiwi.

Collecting Seeds: I have never seen a wild hau kuahiwi. Hibiscadelphus hualalaiensis is presumably extinct in the wild (since 1992), and there are only a couple hundred wild H. distans on Kaua‘i. So, I cannot tell you when these two species naturally flower and fruit. Likewise, the number of flowering and fruiting hau kuahiwi I have seen in cultivation is limited. However, among these, flowering was most common during the winter and spring, with fruits maturing in spring and summer.

Collect fruits when they are tan to brown and dry. Mature capsules will split open revealing the fuzzy seeds inside. Sterilize the seeds by soaking them in a 10% bleach solution for 15-30 minutes. Many of the seeds will float in the solution because of the air trapped in the hairs. Therefore, stir the solution every 2-3 minutes to maximize contact and penetration of the bleach. Between stirrings, I force seed submersion with a small folded paper towel. After the bleach, rinse the seeds in clean water and let them air dry. Stored in a refrigerator, hau kuahiwi seeds remain viable for at least five years; in one batch of seeds I collected and stored 20 years ago, four out of 16 sprouted.

Growing from Seed: Hau kuahiwi seeds and seedlings are extremely susceptible to fungal or bacterial infections resulting in death. This is how I have achieved some success.

If you have not yet sterilized the seeds, do so now (see previous section for my bleach solution protocol). If you sterilized the seeds and then placed them in storage for a long time (i.e., months or years), I recommend you sterilize them again; it cannot hurt, and it may help. After sterilizing the seeds and rinsing them, soak them in a shallow container of clean water overnight. Initially, all or most of the seeds will float. However, after several hours some or all of the seeds will sink. A few seeds may even crack, exposing the yellow-white embryo. In the morning, gently squeeze each seed between your fingers. (Wash your hands thoroughly with soap and water before touching the seeds.) If the seed easily collapses, discard it; often it is the still-floating seeds that collapse. After the overnight soaking, if a seed did not crack on its own, carefully scarify the seed with a piece of sandpaper. Sand the seed until you are through the dark seedcoat to the lighter reddish coat below; try not to sand all the way to the embryo to avoid possible microbial rot.

Sow your seeds using Method One with these modifications: (1) Rather than 100% vermiculite, use a 1:1 mix of new vermiculite and perlite. (2) Do not cover the seeds with a layer of moist sphagnum moss, but do add a quarter teaspoon of sulfur to the vermiculite-perlite mix. (On some trials, I have also added a quarter teaspoon of the fungicide Captan®, but I cannot definitively say this helped.) (3) Sow your seeds in a widely spaced pattern with no more than nine seeds per tray. Also, sow the seeds shallowly so you can easily view each seed through the clear container top.

Check your seeds every day. If a seed shows any signs of rot (e.g., watery secretions, discoloration) remove it and repeat the clean-hands squeeze test. A rotting seed will usually collapse, exude some fluid, or smell putrid. After examination, if the seed passes the test and seems okay, resow it in the tray. (If you wish to be extra careful, you can re-sterilize the seed.) It is very important to remove rotting seeds promptly because the fungus or bacteria causing the rot can quickly spread through the tray and infect other seeds.

Seeds can begin germinating in as little as a week with the emergence of the radicle (embryonic root), and continue to germinate for about two months. After three months, if any seeds have not yet germinated, repeat the clean-hands squeeze test, re-sterilize the seeds, and resow them in a newly-made tray of fresh vermiculite-perlite-sulfur mix. Unfortunately, even germinating seeds are vulnerable to infection, often attacking the tiny emerging root. I have not discovered any successful treatment for this, except when the infection begins later between the seedcoat and the cotyledons. In these cases, I manually (and very gently) removed the seedcoat, sometimes having to cut away a portion of the cotyledon. Infection and collapse of the hypocotyl can also happen; I have no successful treatments for this (however, refer to my page on Lama for possibilities).

Carefully transplant each hau kuahiwi seedling to an individual pot as soon as it sheds its seedcoat and expands its cotyledons; this will lessen its chances of late infection within the tray. I have used three seedling repotting media mixes; all were equally successful: 1:1:1 vermiculite, perlite, peat moss; 1:1:1 black cinder, perlite, peat moss; 3:1 black cinder, vermiculite.

Seedlings grow moderately quickly in their individual pots and should be six inches tall in about three months. They seem to benefit from a dilute fertilizer (e.g., Miracle-Gro®) in their waterings. Look out for aphids and other sap-sucking pests that can attack and kill these still-vulnerable young plants. Refer to Enemies in the Garden for methods to combat these pests. After about a year and 1-3 more repottings, your hau kuahiwi should be 1-2 feet tall and living in a ½ to 1-gallon pot. It is now ready for your garden.

Growing from Cuttings: I have only propagated Hibiscadelphus distans a couple of times from cuttings using a 1:1 mix of vermiculite and perlite, a 1:10 dilution of Dip’nGrow®, and Method One. Other horticulturists have successfully air-layered hau kuahiwi or rooted cuttings using Method Two. For me, the cuttings took 3-4 months to develop enough roots for a successful transfer out of the clear container and into a new pot with a 2:1 mix of black cinders and vermiculite. Even then, the cutting’s roots were not nearly as developed as a seedling of comparable size. Let your new hau kuahiwi double or triple in size in the nursery before planting it out in your garden.

Growth in the Garden: I found germinating seeds of Hibiscadelphus distans and H. hualalaiensis equally challenging. Likewise, growing seedlings of the two species in a nursery was similar and less difficult. In contrast, I have had more success with H. distans than H. hualalaiensis after planting the two in the ground. I think this disparity relates to the difference between the natural habitats of the two species. H. distans grows in the hotter Waimea Canyon of Kaua‘i (1,000 to 1,800 ft elevation) while H. hualalaiensis grows on the slightly cooler slopes of Hualālai, Hawai‘i (3,000 to 3,300 ft elevation). All of my planting sites for the two species have been at an elevation of 500 feet or less. Therefore, I would recommend H. distans for lowland native gardens and H. hualalaiensis for montane native gardens.

Both species grow best in full sun to partial shade. And, both are extremely drought-resistant, traits similar to ma‘o hau hele (Hibiscus brackenridgei). In fact, after your hau kuahiwi has an established root system (3 - 6 months), you should avoid watering your plant unless it exhibits obvious symptoms of water-stress such as wilting or extreme leaf loss. Hau kuahiwi does best with infrequent deep waterings followed by a drying out period, rather than frequent shallow waterings. Actually, watering too often is a good way to kill your hau kuahiwi.

While both species are recorded to grow into small trees, I have never grown or seen a hau kuahiwi taller than me. Perhaps, it takes a really long time to obtain treelike stature. All my H. hualalaiensis have, unfortunately, died within five years of planting, while my ten-year-plus H. distans are still about four feet tall. Initially, both species grow quite quickly when placed in the ground, becoming 2-3 feet tall within two years. This is also when, in my experience, they begin flowering and fruiting. I have never fertilized my planted hau kuahiwi, so I cannot comment on how fertilizer would affect their growth or health.

Diseases & Pests: The leaves of hau kuahiwi are often eaten by Chinese rose beetles, or occasionally infested by whiteflies. Protected by ants, mealybugs can also become a problem; they normally attack the stem-tips or flowers. Others have reported their hau kuahiwi being attacked by black twig borers. Refer to Enemies in the Garden for ways to combat these pests.

Ā ā Ē ē Ī ī Ō ō Ū ū