Nemesia, a genus of plants native to South Africa, is prized by gardeners for its vibrant, two-lipped flowers and often delightful fragrance. The plant’s prolific blooming habit and compact size make it a favorite for containers and garden borders throughout the warm season. However, the exact classification of Nemesia often leads to confusion for new gardeners seeking to understand its longevity, stemming from a difference between the plant’s true nature and how it is typically cultivated across various climates.
The Botanical Reality of Nemesia
Nemesia is botanically classified as a genus containing both annual species and perennial species, as well as sub-shrubs. For instance, Nemesia strumosa is commonly recognized as a true annual, completing its entire life cycle from seed to bloom to death within a single growing season. In contrast, the most widely available cultivated varieties and hybrids, such as those derived from Nemesia caerulea or Nemesia fruticans, are technically classified as “tender perennials”.
A tender perennial is a plant that possesses the genetic capability to live for more than two years, but it is highly susceptible to frost and freezing temperatures. These plants do not possess the hardiness mechanisms necessary to survive the sustained cold of winter outside of their native, milder habitats. Therefore, Nemesia’s tender nature means it is overwhelmingly sold and grown as an annual in regions that experience a significant winter freeze.
The distinction is based on the plant’s inherent biology versus the external conditions it faces. Treating Nemesia as a perennial requires deliberate intervention, but allowing it to succumb to winter cold means it behaves functionally as a seasonal annual. This lack of cold tolerance converts its perennial potential into an annual reality for most gardeners.
Climate Dictates the Life Cycle
The fate of a Nemesia plant is almost entirely determined by the local climate, specifically the severity of the winter season. The plant’s natural hardiness range is generally limited to USDA Hardiness Zones 9 through 11, where winter temperatures rarely drop below 20°F to 30°F. In these warm, frost-free regions, Nemesia can survive in the ground year-round, behaving as a true perennial or even a winter annual, blooming during the cooler months.
Outside of these mild zones, where the ground consistently freezes, Nemesia cannot survive the winter outdoors. Sub-freezing temperatures cause the plant’s cellular structure to break down, leading to its death. Consequently, gardeners in colder climates must treat the plant as an annual, replanting it each spring after the danger of the last frost has passed.
The 50°F threshold is relevant because Nemesia growth slows considerably, and the plant often enters a dormant state when temperatures fall below this point. This temperature sensitivity means the plant performs best in cool-season weather and often declines or stops blooming during the intense heat of mid-summer. If the winter is cold, Nemesia will not survive without protection.
Cultivating Nemesia for Optimal Blooms
To maximize the blooming period of Nemesia, it is important to provide specific environmental conditions that mimic its preference for cooler, but sunny, weather. The plant requires a location that receives full sun for a minimum of six hours daily to encourage the most prolific flower production. However, in regions with very hot summers, providing some partial shade during the intense afternoon hours can help prolong the bloom period and prevent the plant from flagging.
Nemesia thrives in soil that is consistently moist but possesses excellent drainage, as the roots are highly susceptible to rot in waterlogged conditions. Incorporating materials like perlite, vermiculite, or rich organic compost into the planting medium helps achieve this necessary balance of moisture retention and aeration. Applying a balanced, water-soluble fertilizer every two weeks during the active growing season ensures the plant has the nutrients needed to support its heavy flowering.
Continuous and profuse blooming is strongly encouraged by regular maintenance practices focused on removing spent flowers. This process, known as deadheading, prevents the plant from diverting energy into seed production. If the plant becomes leggy or stops blooming due to summer heat, a more aggressive technique called “shearing” is effective. Cutting the plant back by about one-third of its height can stimulate a flush of new, bushier growth and a renewed round of flowering when cooler temperatures return.
Methods for Overwintering
For gardeners determined to preserve Nemesia outside of its natural perennial zones, the most reliable method involves treating the plant as a container specimen and bringing it indoors before the first hard frost. This requires moving the potted plant to a cool, frost-free location.
An ideal overwintering environment is a basement, garage, cool greenhouse, or enclosed porch where temperatures remain between 50°F and 65°F. Before moving the plant, it is beneficial to cut the foliage back significantly, often shearing it down to just a couple of inches above the soil line, to prepare it for dormancy. During this dormant period, watering must be drastically reduced, only providing a minimal amount when the soil is completely dry to prevent root rot.
Another popular method is to take cuttings from the healthiest non-flowering shoots in late summer or early fall. These small cuttings can be rooted in a sterile potting mix and kept in a warm, bright, indoor location over the winter months. This technique bypasses the challenge of keeping the entire mature plant alive and provides small, new plants ready to be hardened off and planted out the following spring, effectively preserving the desired variety.