Growth Requirements
General
Bidens hawaiensis is one of the few native Hawaiian Bidens species that has simple rather than compound leaves. It is also among the minority of koʻokoʻolau that still have prominent awns on their achenes, although its awns are not nearly as annoying as those of the introduced, naturalized, and common weed Bidens pilosa (Spanish needle). B. hawaiensis often grows in an aesthetic ball-like shape, both in cultivation and in the wild (see photographs above). However, it can also assume a more sprawling form (i.e., be wider than it is tall). 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 for easier pest and disease treatment.
Place your ko‘oko‘olau in the coolest but also the sunniest place available; however, Bidens hawaiensis can survive in more shaded sites. Also, select a location with a consistent (but not strong) breeze. Combined, these will help prevent infections of powdery mildew. While wild B. hawaiensis 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. I (Koebele) have only grown ko‘oko‘olau in rocky clay-dominant soil or a media mix of sphagnum peat moss, vermiculite, and black cinders or perlite (approximately one-third of each medium). However, I suspect B. hawaiensis would also grow well in more organic-dominant or cinder-dominant soils or media mixes.
Cultivated ko‘oko‘olau grow fast but typically do not live very long. You can expect your planted Bidens hawaiensis 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. I have never had my in-the-ground ko‘oko‘olau live past two years or my potted ko‘oko‘olau past three. However, my B. hawaiensis have usually fared better than some other species I have grown (e.g., B. torta). I never fertilized my planted ko‘oko‘olau; my potted ko‘oko‘olau received an application of granular controlled-release fertilizer every six months.
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. hawaiensis at the same time, increasing the odds that at least one of the plants has perfect florets.
The second problem is Bidens hawaiensis 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. hawaiensis (likely) cannot be successfully pollinated by the introduced, naturalized, and common B. pilosa and B. alba. (There's a bit of uncertainty here because Knope et al. [2013] list B. hawaiensis as one of the native species used in their study [Table 1] but do not list B. hawaiensis in their results of crosses [Table 2].) 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. hawaiensis 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. hawaiensis 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, 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. However, the Bidens hawaiensis I (Koebele) have grown have rarely been bothered by any of the above-mentioned pests (with spider mites being the most common) and diseases, making it one of the easier ko‘oko‘olau to grow. However, if you're not as lucky, go to the Pests & Diseases page for methods to control the above-mentioned pests and diseases.