Hillia triflora and the bird pollination syndrome

Hillia triflora flowers in the typical cluster of three at the end of a branch

Hillia triflora is a somewhat obscure epiphytic shrub in the coffee family, Rubiaceae. I didn’t know it existed until I ran across a small cutting for sale on eBay a few years ago, and after a quick google search, I made certain that I would be the high bidder when the auction ended. It’s a hummingbird-pollinated plant, and as I have noted before, this gardener is attracted to the same floral features that attract hummingbirds.

H. triflora comes from central America, with specimens collected from southern Mexico to Colombia, and although it usually grows at higher elevations, plants have been found at only 100 m above sea level in Costa Rica [1]. As might be expected from this wide geographical and altitudinal range, the plant seems to be quite tolerant of both summer heat outdoors and cool winter nights in the greenhouse. There are two subspecies with overlapping range, H. triflora subsp. triflora, and H. triflora subsp. pittieri; judging by the equally sized leaves on flowering shoots and narrow, uninflated flowers, my plant belongs to the nominate subspecies [1].

The growth habit of this plant is very interesting. It forms an open shrub with flexible, infrequently branched stems bearing opposite, semi-succulent leaves. Each stem terminates with a protective sheath. Over the course of a couple of months, the sheath slowly swells and eventually splits, revealing that it was formed by a pair of bracts tightly pressed together. Emerging from the sheath is either a new stem segment with a pair of leaves and new sheath, or a cluster of (usually three) flower buds. At the base of the flower buds is one or more new sheaths, so the stem sometimes branches after flowering.

Sheath at the end of a branch. At this stage in its development, it is difficult to see that it is composed of two bracts.

The roots of H. triflora are fibrous, without obvious adaptations to epiphytic life, and plants in habitat are occasionally found growing terrestrially in leaf mould or rotting wood [1]. Consequently, I added some coarse peat to the mix of orchiata bark and perlite in which I potted my plant. I water when the mix is almost dry, and so far the plant seems happy. It grows with my vireyas under shade cloth in summer and goes back in the greenhouse when temperatures drop below 50 F (10 C) at night.

The flowers of H. triflora are very similar to those of the unrelated coral honeysuckle, Lonicera sempervirens (Caprifoliaceae), that grows wild in the woods around my house–it’s a remarkable case of convergent evolution driven by birds. I would guess that the hummingbird responsible for pollinating H. triflora is roughly the same size as the ruby-throated hummingbirds (Archilochus colubris) which pollinate L. sempervirens. I suppose it is even possible that A. colubris is a pollinator of H. triflora, given that the birds over-winter in central America and H. triflora has flowers in November/December as well as March-May [1].

Lonicera sempervirens flowering at the edge of our woods today

Lonicera sempervirens (top) and Hillia triflora (bottom)

I have previously featured a variety of hummingbird pollinated flowers on this blog, both native plants (Aquilegia canadensis, Aesculus pavia, Lobelia cardinalis, Lonicera sempervirens, Spigelia marilandica, Silene virginica) and tropicals (Juanulloa mexicana, Macleania sp. aff. smithiana, Macleania pentaptera, Ceratostema glans, Cavendishia capitulata, Bouvardia ternifolia, Bessera elegans, Behria tenuifolia, Columnea microcalyx, Columnea crassifolia). These flowers share obvious characteristics: bright red/orange/magenta color, tubular (or sometimes bell-like) shape, lack of fragrance, and copious nectar. The same features are seen in plants from Africa, Asia, and Australasia where there are no hummingbirds. In those locations, sunbirds (Nectariniidae) and honeyeaters (Meliphagidae) fill much the same ecological niche. However, because sunbirds and honeyeaters are less likely to hover than hummingbirds, bird-pollinated flowers in the old world are usually not pendant and are often attached to a stout inflorescence that allows the birds to perch (see, for example, Cyrtanthus obliquus).

Dermatobotrys saundersii is an epiphytic shrublet from South Africa that is likely pollinated by sunbirds. It is easy to grow from seed but seems not to like hot North Carolina summers. I have been unable to keep plants going for more than a couple of years.

The flowers of Dendrobium chrysopterum from New Guinea bear all the hallmarks of bird pollination. Like similar Dendrobium orchids, they may be visited by honeyeaters.

Reference

1. Taylor, CM (1989). A revision of Hillia subg. Ravnia (Rubiaceae: Cinchonioideae). Selbyana 11: 26-34.

Six on Saturday #73 (April 1, 2023)

It’s been a while since I have had the time to put together a Six on Saturday post. Spring is well underway here in the North Carolina piedmont. The early Narcissus have long finished flowering, and only late-flowering clones like ‘Thalia’ and ‘Golden Bells’ are still in bloom. Azaleas are just getting started. The native pinxter flower, Rhododendron periclymenoides is in full bloom, and the buds are opening on Florida flame azalea (R. austrinum). My hardy Calanthe and Bletilla orchids were hit hard by a freeze after a prolonged frost-free spell, and many of their new growths were turned to mush. It remains to be seen how many flowers they’ll make this year.

Here are six plants from the greenhouse and garden that I haven’t featured before.

1. Rhododendron ‘Aravir’

‘Aravir’ is another of the modern vireya (tropical Rhododendron) hybrids. Its parentage is R. konori x (‘Pink Delight’ x jasminiflorum), which explains its similarity to the Victorian ‘Princess Alexandra’ (R. ‘Princess Royal’ x jasminiflorum). The parentage of ‘Pink Delight’ is unknown, but it was an old Veitch hybrid, so probably similar to ‘Princess Royal’. I got this plant as an unrooted cutting about 2 1/2 years ago, and this is the first time I have seen it flower. I am currently experiencing (hopefully temporary) post-COVID anosmia, so I can’t say much about its fragrance. I can barely detect a scent, which probably means that it is very strongly scented. Like all vireyas, this plant is not frost-hardy. I grow it in the greenhouse in winter and outdoors under shade cloth in summer.

This is one of a group of vireya hybrids with names drawn from the Chronicles of Narnia. ‘Aravir’ refers to the Narnian morning star.

2. Columnea crassifolia

This beautiful epiphytic gesneriad is from Guatemala, so it is a greenhouse plant. The large hummingbird-pollinated flowers are similar to its relative C. microcalyx (syn. gloriosa), but while the stems of C. microcalyx hang limp or creep along a surface, those of C. crassifolia are rigid. This plant flowers most of the winter and on-and-off during the summer. Like R. ‘Aravir’, it goes outside under shade cloth once the danger of frost is past.

3. Taraxacum albidum (Japanese white dandelion)

When growing dandelions for chicken snacks and salad greens, it’s fun to try unusual varieties. Last year we flowered the pink dandelion (Taraxacum pseudoroseum), and this year white dandelions are getting started. Taraxacum pseudoroseum wasn’t very pink, but T. albidum is definitely a more pure white. Since dandelions can be persistent weeds, we keep them in pots and clip off inflorescences before the seeds are mature.

4. Camellia ‘Rosehill Red’

Not much to say about this; it’s a very nice Camellia japonica cultivar. I tend not to like double and semi-double flowers, but these ones aren’t too fussy looking. It is beside Camellia ‘Yuletide’ which flowers in early winter, so I get an extended shot of red color in that part of the garden.

5. Muscari armeniacum ‘Touch of Snow’

These little guys that I planted last autumn are a little difficult to find in the garden, but they’ll become more obvious as they start to form clumps. They make a nice change from the more typical purple grape hyacinths.

6. Hyacinthus ‘Woodstock’

I had a little trouble finding six flowers that I hadn’t featured before, so I’ll slip in these hyacinths which actually flowered a couple of weeks ago. Like the Muscari ‘Touch of Snow’, I planted them last autumn, so this was my first look at the flowers. My wife was the one who picked them out of the catalog, but I very much like the intense magenta color which darkens to purple at the base of the flower. Quite possibly my favorite Hyacinth now.

Jim at Garden Ruminations is the host of Six on Saturday. Head over there to see his Six for this week and find links to the blogs of other participants.

Macleania pentaptera

Currently flowering in my greenhouse for the first time: Macleania pentaptera, another one of the neotropical epiphytic “blueberries” (a misonomer; this species, like M. smithiana, has white berries).

M. pentaptera is endemic to Ecuador, where it grows in primary rainforest at 150-2100 meters above sea level and is pollinated by hummingbirds. When I was researching this species before buying a plant, the lower limit of its altitude gave me hope that it might tolerate hot North Carolina summers. So far, that hope seems to have been borne out. The small rooted cutting that I purchased in 2020 has grown vigorously, with no evidence of damage during even the hottest weather. In that respect, it seems to very similar to the Macleania sp. aff. smithiana that I previously featured (see that post for more on the genus). At least four other Macleania species (M. coccoloboides, M. cordifolia, M. insignis, and M. rupestris) are also occasionally available from nurseries in the USA, but they are all cloud forest species and are unlikely to be heat tolerant. The one plant of M. cordifolia that I tried to grow did not survive even one summer. However, all of these species are so fantastic that I will almost certainly try again.

Ceratostema glans

Ceratostema glans flowers attract many tiny ants, but they are almost certainly pollinated by hummingbirds in nature.

Ceratostema glans is another one of the neotropical “blueberries”, mostly epiphytic members of the Ericaceae which have edible berries (though they may not be blue). This species doesn’t seem to be as floriferous as the Macleania sp. aff. smithiana, but with its unusual flowers and small, semi-succulent leaves on arching stems, it is a very pretty little shrub.

According to Luteyn and Pedraza-Peñalosa, “Blueberry Relatives of the New World Tropics (Ericaceae)“, C. glans grows at 1400 m above sea level in primary rainforest in Ecuador. I purchased this plant from Ecuagenera about a year ago, and it tolerated its first North Carolina summer very well, flowering in April and October and thriving outside under shade cloth in even the hottest months. I have high hopes that this plant will be a long term success.

Bucket Orchids

Coryanthes macrantha flower

A seedling of Coryanthes macrantha recently flowered in my collection for the first time, and it is easy to see why these plants fascinate orchid growers. The genus Coryanthes is a group of about 65 species (per the Kew World Checklist of Selected Plant Families) from Mexico, Central America, and South America. The plants have large pseudobulbs and broad, plicate leaves which are deciduous after several years. They grow epiphytically, generally rooting into arboreal ant nests, and their inflorescences hang down below the pseudobulbs. Due to the lax inflorescence, they are generally best grown in baskets–in a pot, the inflorescence may become lost in the potting mix, its mummified remains only found when the plant is repotted.

The flowers of Coryanthes orchids are truly bizarre, and at first glance can be difficult to interpret in terms of typical orchid anatomy. The flower is dominated by the labellum, the distal end (epichile) of which is thick and waxy and modified into a bucket structure. The proximal end (hypochile), which connects to the base of the column, is fleshy, bulbous, and often rather phallic in appearance. The other two petals are small, strap-like, and generally held parallel to the column. The sepals are like crumpled tissue paper and are held above the rest of the flower in a messy clump. Two nectar glands at the sides of the column drip watery liquid into the bucket.

Coryanthes thivii flower with a drop of liquid about to fall from the column glands into the bucket.

All of this baroque anatomy is in aid of pollination. The hypochile secretes aromatic compounds which are collected by male euglossine bees. The male bees use the fragrances to attract females, so the Coryanthes flower is reponsible for reproduction of both plant and insect. While scratching around to collect the fragrance, some of the bees slip and fall into the bucket. With wet wings, the bees cannot fly out, so they are forced to climb to the end of the bucket where they can squeeze out through a small opening formed by the bucket walls and the end of the column. While pushing past the column, the bees pick up or deposit pollen packets.

Coryanthes thivii flower with the bucket bisected to show the column and exit route.

Over the years, I have owned four Coryanthes species, and I find them easy to grow and flower–for a while. With adequate fertilization and an acidic, moisture retaining substrate like long-fiber sphagnum, the plants grow very fast; my C. thivii seedling bloomed less than a year out of flask and C. elegantium in about two years. Eventually, though, growth slows down and the plants stop producing new pseudobulbs. They hang on for months or years, but eventually the old pseudobulbs die back and that’s the end. I’m not sure whether the plants are naturally short-lived (perhaps their ant-nest habitat is precarious and there’s no selective advantage to long lifespan), or perhaps my plants are missing some critical micronutrient. I’m currently growing C. macrantha and C. leucocorys (not yet flowered), so it remains to be seen if they follow the same trajectory as C. thivii and C. elegantium.

Coryanthes elegantium flowers

Coryanthes thivii again