Strelitzia nicolai, the White Bird of Paradise “Tree”

Strelitzia nicolai, the white bird-of-paradise “tree “ reaches 20’ (6 meters) in height in Florida but even higher in its native habitat- coastal forests and dune thickets of eastern South Africa. Based on local (central Florida) observations, the species can be grown where winter temperatures do not drop below around 26 F. (leaves will be killed), although even temperatures just below freezing will damage the leaves unless the plant is in a protected location.

Strelitzia nicolai "tree"

Strelitzia nicolai “tree”

S. nicolai is a spectacular landscape plant capable of forming clumps of stems with leaves 8 feet long. It is considered an herb rather than a tree because of the paucity of ligneous fibers in its stem, which is comprised of overlapping, tightly packed leaf sheaths similar to the banana. When mature the plant forms a large basal caudex that is sometimes considered as semi-ligneous. Another species in the genus is the better known, smaller orange bird of paradise (S. reginae) which is more cold sensitive, and therefore restricted to southern Florida, California and Hawaii in the United States.

The Strelitzia "boat" seen from below.

The Strelitzia “boat” seen from below.

The bird-of-paradise name refers to the flowering structures (inflorescences) which emerge from a large, thick boat-shaped bract. The bract is initially closed but then opens along the top providing space for the emergence of highly modified bisexual flowers . A series of six to eight flowers sequentially emerges from the bract. Each flower consists of 3 petals and 3 sepals, one of the former being conspicuously blue and arrow-shaped. The other petals and one of the sepals are white and relatively large . The remaining sepals are concealed within the large bract and therefore are not visible without cutting  it away.

The vertically oriented flower is mature. A younger flower is to the right.

The vertically oriented flower is mature. A younger flower is to the left.

The blue arrow-shaped petal holds within its elongated fold the sexual structures. As it ages, the flower and its blue petal arc upward, becoming vertical relative to the horizontal bract. On the 3rd day the flower, if unpollinated, dies. Within 48 hr, the next flower emerges from the bract, and so on for each flower until the last one has emerged. When mature the blue petal stands erect and the female structure (a thin, 3-part stigma about 4 cm long attached to a style over 9 cm long), projects beyond the tip of the petal. The stigma has a sticky surface, perfect for catching and holding pollen grains.

Three stamens have been pulled out from the left side of the boat bract . Note pollen on anthers.

Three stamens have been pulled out from the mature flower to the left.  Note pollen on anthers and aging flowers with dried  nectar on  to the right.

The male parts ( five linear, pollen- producing anthers attached to very narrow filaments), are entirely contained within the blue petal, but positioned just inside the petal’s opening. In the photo above, several of the stamens have been pulled out of the blue petal.  Pollen can be seen on the narrow anthers. Manually separating the lobes of the blue petal reveals that the anthers release pollen within the petal , although some pollen also appears on the exterior of the petal (photo below).  

Pollen has accumulated naturally on the exterior of the blue petal.

Pollen has accumulated naturally on the exterior of the blue petal.

A surprising amount of clear liquid (presumably nectar) collects where the bases of the petals and sepals overlap to form a cup within the large bract. In some cases there’s enough liquid to overflow the bract and stream down its exterior. The large volume of liquid cannot be explained by precipitation because there have been no recent rains. This liquid was very attractive to paper wasps, honeybees and flies. However I was unable to detect a sugary taste to the nectar such as is found in nectar from honeysuckles and other plants. Possibly the insect visitors have more proficient sugar receptors than I have. Sugar concentrations vary widely in plants and may be well under 10% (Nicholson and Thornburg). But amino acids, lipids and other organic compounds occur in nectar and these substances might explain the eagerness of the insect visitors. The wasps and bees were competing vigorously for access to the nectar with the wasps attacking the bees to chase them away. It did not appear that the insect visitors were collecting or feeding on pollen and the bees lacked pollen in their pollen baskets. Thus these visitors were not serving as vectors for transferring pollen to Strelitzia stigmas. Despite the presence of several other Strelitzias in the neighborhood, there appeared to be no fruit or seed development in our Strelitzia (in this or in prior years) as one would expect in the absence of cross-pollination.

In Africa, Strelitzias are pollinated by sunbirds- small, colorful, long-billed birds belonging to the Nectarinidae family (80 species in 15 genera.  As the family name implies, sunbirds take mostly nectar besides feeding insects and spiders to their young. Sunbirds have some habits similar to those of hummingbirds, and, in fact, convergent evolution between the New World hummingbirds and sunbirds has been suggested. Both groups access flowers to consume nectar. One difference is that hummingbirds hover while sunbirds must perch in order to take nectar. In a study of the pollination of S. nicolai by four species of sunbirds in South Africa, Frost and Frost (1981) found that the birds cued to the angle made between the mature flower and the bract as an indication of nectar flow rates. The sunbirds perched such that pollen was transferred to the stigma via the bird’s feet- a very different approach than hummingbirds! Seed set was high indicating the efficacy of sunbirds as pollinators. Other species visited S. nicolai flowers (bushbabies, monkeys and insects) but none were significant pollinators. The pollination of Strelitzias by hummingbirds in central Florida would run into seasonality problems even if there were no other limits. S. nicolai flowers in winter months when hummingbirds are in their Central American wintering grounds. There may be a better opportunity to evaluate pollination in southern Florida where some Ruby-throated Hummingbirds winter or in  California where several species of wintering hummingbirds and flowering Strelitzias may co-occur.

References used include:   Frost, S.K. and P.G.H. Frost 1981. Oecologia 49:379-384. Nicholson and Thornburg: