Nierembergia is a genus of flowering plants in the Solanaceae family — the family that contains plants as diverse as the poisonous nightshade and edible plants, like potatoes and tomatoes. The common name of Nierembergia is cupflower. These plants are grown in rock gardens, borders, and hanging baskets for their one-inch wide (2.5 cm) flowers that are produced in purple, lavender, blue, or white, depending on the cultivar. Cupflowers are perennials in warmer climates, but are grown as annuals in many parts of the world.
Once cupflower plants have become established, they are highly drought-tolerant. They can even be grown next to cement and in other situations where the soil becomes hot. Nierembergia is sold as a component of xeriscape gardening, in which water usage is kept to a minimum. Resistance to deer is another benefit of this plant.
The particular species available for sale can vary, since the taxonomy is confusing. Nierembergia caerulea, scoparioa, hippomanica, linariifolia and frutescens have all been listed as names for the same plant. There are also a number of hybrids available, ranging from 6 to 16 inches (15-40 cm) tall. Generally, the plants form mounds that spread up to 30 inches (75 cm) wide.
Plants can be obtained from garden centers or online retailers, or they can be grown from seed. Seeds should be sown 8 to 10 weeks before the last frost date is predicted. They should be kept slightly moist at 70°F (21.1°C). Once the seedlings have started to grow, they should be watered only when dry.
When purchased plants or seedlings are ready to be planted outside, they should be hardened off. This involves gradually exposing them to the cooler temperatures of the outdoors. If planted in the ground, they should be planted 6 to 12 inches (15-30 cm) apart, in full sun or partial shade. Good drainage is important. Some sources advise rich soil, while others consider them to be good annuals for growing in poor quality soil.
Often, cupflowers are grown in hanging baskets, where they can trail over the sides. In this case, they should be planted 3 to 6 inches (7.5-15 cm) apart. Growth in hanging baskets is a little different than that in the ground. The nutrient supply is limited, and the constant drainage rapidly depletes nutrients from the soil. Plants in hanging baskets should be fertilized more frequently than those in the ground.
It is strongly recommended to trim off the dead flowers. This will keep the plants blooming longer. After the cupflowers have finished producing flowers, trimming the stems back will cause them to live longer.