Bidens amplectens possesses some of the largest flowers among the 22 many (about 20 species) endemic Hawaiian koʻokoʻlau. As with most Bidens spp., the leaves can be used to brew hot teas and tonics, and the flowers can be added to salads. Hawaiian Bidens spp. have barbless seeds achenes (single-seeded fruits), unlike the introduced Bidens species. Most readers will not know the difference between and awn and a barb. Therefore, this sentence is confusing since several koʻokoʻlau have awns. Take a look at how I worded this topic in Special Features and Information below.

Bidens amplectens has one of the largest flowers of all the native Hawaiian Bidens species; held between fingers for scale (photograph courtesy of Damien Wallace CC BY 4.0). Two wild B. amplectens: one on a steep Wai‘anae mountainside overlooking the north shore of O‘ahu (photograph courtesy of Gerry Carr CC BY-NC 4.0); another growing out in the open in a compact shape; koʻokoʻolau growing in more shaded areas tend to be more lanky (photograph courtesy of Gerry Carr CC BY-NC 4.0).

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Growth Requirements

General

    Bidens amplectens exhibits the standard morphology of many of the native Hawaiian Bidens species; it is a small to medium-sized herb with pinnately compound leaves and yellow daisy-like flower heads. What makes it exceptional is that it has perhaps the largest flower heads of all the native species and it is among the easiest species to grow. However, because cultivated koʻokoʻolau typically only live a few years, use your plant(s) as a landscape accent rather than trying to create a permanent hedge or groundcover, or you can keep it/them in a container (recommended) for easier pest and disease treatment. 

    Place your ko‘oko‘olau the sunniest place available. However, Bidens amplectens can survive in more shaded sites but will likely produce fewer flowers. Also, select a location with a consistent (but not strong) breeze. Combined, these will help prevent infections of powdery mildew. While wild B. amplectens are sometimes found in relatively dry sites, avoid any type of water stress with your cultivated ko‘oko‘olau; always keep the soil or media surrounding its roots moist but also make certain the soil or media drains away excess water easily. B. amplectens doesn't seem to be picky about soil or media type and will grow in organic-, cinder-, or clay-dominant soils. Similarly, various mixes of sphagnum peat moss and black cinders or perlite all seem to work well for potted plants. 

    Cultivated ko‘oko‘olau grow fast but typically do not live very long, only lasting two to three years at best. You can expect your planted Bidens amplectens to be about two feet (61 cm) or more tall and flowering in a year or less. Containerized ko‘oko‘olau usually become stunted unless progressively transferred to larger and larger pots. In-the-ground ko‘oko‘olau normally do not require fertilizer. If potted, apply a granular controlled-release fertilizer every six months or use an organic or inorganic foliar fertilzer at ¼ the recommended strength every month or so. Avoid overfertilizing, particularly with a fertilizer rich in nitrogen, since this can lead to large, floppy leaves and fewer flowers. 

    Because cultivated ko‘oko‘olau are typically short-lived, you will need to replace them every one to three years if you wish to keep them long-term. The easiest way to do this is by rooting cuttings of your existing plant. However, propagating ko‘oko‘olau from cuttings doesn't entirely "set back the clock" on these short-lived perennials, and these cloned plants often lack the vigor of ko‘oko‘olau propagated from seeds. Also, unlike seeds, you cannot easily store away a hundred ko‘oko‘olau clones in your refrigerator. 

    Propagating ko‘oko‘olau from seed comes with its own set of problems. The first is the unusual breeding system of many Hawaiian Bidens described above (see Description above). In order to get viable seed, you need to make sure your plant is producing perfect (bisexual) florets and not just pistillate (female) florets. This requires close examination of the flower head. Alternatively (and maybe easier), you can maintain several flowering B. amplectens at the same time, increasing the odds that at least one of the plants has perfect florets. 

    The second problem is Bidens amplectens can successfully hybridize with other native Bidens species [Ganders & Nagata 1984; Plants of Hawai‘i]. This makes collecting seeds of known genetic identity a challenge. Fortunately, a study by Knope et al. [2013] showed that B. amplectens cannot be successfully pollinated by the introduced, naturalized, and common B. pilosa and B. alba. Unfortunately, Knope et al. [2013] did not test the other two introduced and naturalized Bidens species, B. cynapiifolia (found on all the main Hawaiian Islands except Ni‘ihau and Kaho‘olawe) and B. gardneri (currently found on Moloka‘i) [Flora of the Hawaiian Islands]. Therefore, before your ko‘oko‘olau dies, you need to decide if you are a purist and want to ensure the genetic identity of any seeds you collect or if you are okay with the possibility that the seeds you collect will have hybrid genetics (most likely native x native; much less likely alien x native). Of course, if you are only growing B. amplectens and there are no other native or alien Bidens in your landscape or nearby — honeybees typically fly one to two miles from their hive — the chances that your B. amplectens is producing genetically-pure seeds are more likely. However, the only way to be absolutely sure about the genetics is to bag any young flower heads you plan to collect achenes from before they open and then pollinate them by hand. Past research [Ganders & Nagata 1984] has shown that hand-pollinating between flowers results in more viable seeds than hand-pollinating within flowers (i.e., self-pollination). Refer to Pritchardia martii for other details on how to collect genetically-certain seeds.

Pests and Diseases

    Ko‘oko‘olau in general are attacked by spider mites (most common), aphids, lace bugs, mealybugs, scale insects, and spittlebugs. Snails and slugs can also sometimes become a problem. Ko‘oko‘olau are also prone to powdery mildew infections; look for white patches on the leaves. Bidens amplectens is infrequently plagued by any of the above-mentioned pests and diseases, making it one of the easiest ko‘oko‘olau to grow.  Refer to the Pests & Diseases page for methods to control all the above-mentioned pests and diseases.

    If you plan to harvest leaves from your koʻokoʻolau to make tea, be sure you have not used any systemic or otherwise long-lasting pesticides or fungicides in the past that could potentially still be present in or on the leaves. Rather, use only short-term and less toxic chemicals like insecticidal soap and baking soda, and be sure to wash the leaves before making your tea.   

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Special Features and Information

General

    Asteraceae is a very large family of mostly herbaceous plants with about 32,000 species within 1,900 genera with a subpolar to tropical worldwide distribution. Nearly all Asteraceae have what appears to be a single daisy-type flower that is actually a composite of several to many much smaller flowers called florets [WikipediA:Asteraceae]. 

    There are about 250 native and non-native Asteraceae in Hawai‘i, with a little more than half being native. About 20 of these are endemic Bidens species [Flora of the Hawaiian Islands].

    Many of the endemic Hawaiian Bidens species, including B. amplectens, have adopted an unusual breeding strategy (see Description above) [Wagner et al. 1990]. Additionally, many have abandoned the method of seed dispersal most common among their non-native congenerics. Most non-native Bidens, like the introduced, naturalized, and now common B. pilosa (Spanish needle), have harpoon-like awns (horn-like projections) on one end of their achenes (single-seeded fruits) that firmly attach to bird feathers, mammal fur, and people’s socks, thereby aiding in the long-distance dispersal of their seeds [WikipediA;Bidens; Britannica]. In contrast, many Hawaiian Bidens have no awns at all, or the awns are reduced or without the harpoon-like barbs. Instead, some ko‘oko‘olau have winged achenes that are thought to help with dispersal by the wind. Others have corkscrew or irregularly twisted achenes, where it's anyone's guess why they have assumed these shapes (see the photograph below). However, generally, botanists think that these alternatives to harpoon-like awns are a way for the Hawaiian species to limit, not extend, their seed dispersal [Carlquist 1980].

    Pollination studies have demonstrated that native Hawaiian Bidens species will produce vigorous hybrids when crossed with other native Hawaiian Bidens species [Ganders & Nagata 1984]. Genetically pure Bidens amplectens are restricted to the windward cliffs and crests in its range. However, it does hybridize and intergrade with B. torta from near Ka‘ena Point to at least the head of Mākua Valley on the summit ridges of the Wai‘anae Mountains [Wagner et al. 1990].

Hawaiian Name

    Bidens cosmoides is the only native Hawaiian Bidens with the species-specific Hawaiian name Po‘olā nui, while the genus is collectively referred to as Koʻokoʻolau or Koʻolau, with Kōʻokoʻolau being a variant spelling [Wagner et al. 1990; Pukui & Elbert 1986]. According to Andrews [1922], Kokolau is another name for Koʻokoʻolau. Koʻokoʻolau, Koʻolau, Kōʻokoʻolau, and Kokolau may all be compound Hawaiian words. However, because their component syllables (e.g., Koʻokoʻo, Lau) have many disparate meanings, it is impossible for us to determine an underlying meaning for the names.

Etymology

    Bidens is derived from the Latin bi, meaning "two," and dens, meaning "tooth" or "teeth," referring to the two awns on the achenes (single-seeded fruits) [Wagner et al. 1990]. However, it's important to note that some Bidens species have no awns (e.g., Hawaiian endemics) while others, like B. pilosa, can sometimes have more than two awns [Flora of Australia].

    The specific epithet amplectens means "stem-clasping (leaf base)," originating from the Latin amplector, amplecti, amplexus meaning "to embrace or encircle" [Gledhill 2008].