Calceolaria flowers consist of two separated lips joined by an almost non-existent tube. The upper lip is very much smaller than the lower, and hooded or flat with an inrolled margin. The flowers are borne in few flower clusters, some almost like umbels. One or two of the dwarf alpine species are single-flowered. Despite coming in all shapes and sizes, the flowers in this group are all colored some shade of yellow: lemon, sulfur, or even gold. They sometimes have spotting, usually reddish, inside the throat or pouch.
If the mouth of the flower is closed, plants are pollinated by bumblebees. Open-mouthed flowers are pollinated by smaller bees. The insects that visit the flowers are rewarded with pollen and oil instead of the usual nectar. There are two stigmas in each flower, which, when fertilized, develop into pods containing numerous small seeds.

The Species

Calceolaria uniflora (syn. C. darwinii) is an exceptionally beautiful species with darkish green leaves and flowers a basic orange-yellow with varying amounts of deep garnet-red to bright chestnut brown in the throat and on the outside of the vertical lower lip. The species is widespread in Chile and Argentina, in southern Patagonia and Tierra del Fuego. It grows in coastal and river sands and rocks, scrubland, peaty alpine moorland, clifftops, and steppe, often in very exposed, well-drained sites from sea level to 3,900 feet (1,200 m) in altitude.
One of the best locations for this species is Estancia Stag River in the south of Santa Cruz, Argentina. Estancia is surrounded by an immense grassland area, notofagus woods, and screes. Particularly in the grasslands, but also on exposed ridges, you can spot numerous C. uniflora. Other species like C. biflora, and C. polyrhiza are common in grassland as well. Where C. uniflora and C. polyrhiza grow together some exciting hybrids appear.
Calceolaria biflora is more common in moist grasslands and forest edges. This is a wide-spread species in Patagonia from Tierra del Fuego to the north of Neuquen, with inflorescences of up to four yellow flowers on 15-inch (40 cm) stems. The species is in cultivation and it is possible to grow it in the open garden.
Calceolaria polyrhiza is a species from damp grasslands but appears also on dry, sandy, windswept Patagonian steppe down to the Santa Cruz River and adjacent Andean slopes from sea level to 4,900 feet (1,500 m) in altitude. Often confused with C. biflora, it has inflorescences of two or three yellow flowers on 12-inch (30 cm) stems. It is easy to grow from seeds and pretty hardy in cultivation.
Calceolaria fothergillii is mainly found in the Falklands and rarely in the south of Argentina and Chile. This species prefers open, dryish, heath scrubland from sea level (on cliffs) to moderate altitudes. The small cushions are 2.35 to 4.7 inches (6-12 cm) high and somewhat woody at the base. The leaves are spathulate, dark green, and hairy on both surfaces. The flowers have an elongated, somewhat flattened, obovate lip, typically a fairly deep yellow with rich garnet-red spotting or shading that may cover the whole surface. The species is very similar to C. uniflora and easy to grow in the alpine house.
Calceolaria tenella, from Santa Cruz and the south of Neuquen, is a mat-forming species with hairy leaves with few-toothed margins up to 0.4 inches (1 cm) in diameter. The flowering stems grow to two inches (5 cm) tall. The plant flowers in January or February in its native habitat. The best opportunity to see this species is on wet, vertical cliffs along streams or in bare damp volcanic sand up to 6,500 feet (2,000m) altitude. This rarely cultivated species is attractive with solitary or paired flowering stems bearing up to three clear yellow flowers. In cultivation, choose a low pH potting soil, never allow it to dry out, and keep it well protected from bright sunlight.
Calceolaria dentata is not the right species for a small rock garden but is good for larger gardens with big rocks. It’s an endemic perennial from the north of Neuquen where it mainly grows among cliffs up to altitudes of 6,500 feet (2000 m). It is an easy-to-cultivate species in moist conditions. The dark green leaves and bright yellow flowers on about 20-inch (50 cm) stems make it an attractive species.
If you are traveling in Neuquen, visit Batea Mahuida Volcano near Villa Pehuenia. The summit of this mountain, at about 5900 feet (1800 m), harbors Calceolaria pennellii, one of the most spectacular species I have ever seen. This very compact, rosette-forming alpine species is about three inches (8 cm) high, making mats 20 inches (50 cm) wide. The leaves are elliptic to ovate and glandular-pubescent on both surfaces. The flowers are bright yellow and unmarked; the globose lower lip is very lightly upcurved and 0.8 to one inch (2-2.5 cm) across. This species grows on windswept summits in volcanic sand. I have never seen this plant in cultivation due to the extreme weather conditions of its native habitat, and cultivation will probably always remain a dream.
Calceolaria borsinii is another beautiful species from Chubut and Neuquen, about 8-12 inches (20-30 cm) tall. The leaves are hairy and the flowers are deep yellow. It grows on rocky outcrops and ridges.
Calceolaria corymbosa subsp. montana is from Mendoza and Neuquen, frequently growing in rocky steppe conditions. It grows about 12 inches (30 cm) tall. As with many calceolarias, the bright yellow flowers bloom during December and January.
Calceolaria pinifolia is a real gem from rocky outcrops in Chile from the central Cordillera to the Atacama Desert in the north. This compact, mat-forming species reaches a maximum of six inches (15 cm) high with two-inch (5 cm) long leathery leaves. The flowering stems are erect with two-to-five-flowered cymes. The flowers are yellow with red-freckled throats. It grows in rock crevices and dry gravelly places. This species is in cultivation, and I actually have seedlings popping up, which I am looking forward to seeing mature.
Calceolaria ‘John Innes’ is a natural hybrid, thought to be between C. biflora and C. polyrhiza. It is a perfectly hardy plant for the rock garden. The plant is very low, at six inches (15 cm), and spreads slowly to form a compact mat. The large flowers each have their own four-to-nine-inch (10-23 cm) stem. The color of the flower is mainly a bright yellow with a few brown spots on the pouch. It likes good drainage, but does not like fully drying out. In other words, the soil has to be moisture-retentive and quickly draining. Choose a location such as a well-drained slope in full sun, and mix an ample amount of peat moss to keep the roots moist. If allowed to dry out too much in summer it will go dormant early. However, just because it disappears above ground, don't think that it has died.

Propagation and Cultivation

The germination of most calceolarias is relatively easy. The only problem they pose is due to the small, dust-like size of their seeds. The seeds must be simply sown on the soil surface. Light is often reported as a required factor for germination, but since the seeds must be sown on the soil surface they will receive light naturally.
To germinate calceolarias, prepare a fine substrate with very good drainage, containing 50% vermiculite, sand, or something similar. You must use a fine-textured substrate because if the substrate is too coarse, the seeds will fall deep between the substrate particles and many will not germinate.
Seedlings usually appear in about three weeks. Transplant the seedlings about four to six weeks after germination into individual pots. For some calceolaria species (for instance, C. uniflora), a cold stratification is advised to increase the germination percentage. Seeds of some of the described species are available from the Czech collectors Vojtěch Holubec and Michal Rejzek, the German company Jelitto, or Michail Belov (Chileflora).
Calceolarias are attractive and valuable for their distinctive, late-summer flowering period. Which species are easiest to grow depends on the climate where you are living. Especially in recent years, our climate where I garden in the south of the Netherlands has changed a lot. The last three years we have suffered from extremely dry and hot summers, with temperatures up to 104°F (40°C), while our wet winters have been exceptionally mild with only a few frosts and temperatures often around 50°F (10°C). These weather changes make the cultivation of the more difficult alpine plants a real effort.
The choicest calceolaria species like C. uniflora, C. fothergillii, C. pennellii, C. borsinii, C. tenella, and C. pinifolia are best grown in clay pots in the alpine house or protected raised beds, especially during winter to protect them from excess rain or snow. They all are completely hardy. I use a soil mix of peat, sharp sand, perlite, or vermiculite. Other species like C. biflora, C. polyrhiza, C. dentata, C. corymbosa, and C. ‘John Innes’ can be grown in the open garden in a light, fertile soil that never becomes dry. Calceolarias are often affected by fungus during the winter, and it is very important to control for aphids during the growing season.
I hope you can grow some of the described species and have a lot of slipper flowers in the garden or alpine house.