The garden nasturtium, or Tropaeolum majus, is a widely cultivated annual known for its vibrant, peppery flowers and leaves, which are entirely edible. This plant is often considered one of the easiest to grow, offering a rewarding experience for new gardeners. While the process is straightforward, achieving high germination rates requires a basic understanding of the seed’s natural defenses. The large, round seeds are protected by a very hard outer shell, which must be compromised to allow water absorption and initiate the sprouting process.
Essential Pre-Planting Preparation
The thick, tough coat surrounding the nasturtium seed is a natural mechanism designed to prevent premature germination in dry conditions. This hard exterior resists the intake of water, which is necessary to activate the embryo inside. To overcome this natural dormancy and speed up sprouting, gardeners employ simple techniques to manually break the barrier.
One of the most common methods is soaking the seeds in lukewarm water for at least eight to twelve hours, or overnight. This saturation softens the seed coat, allowing moisture to penetrate and activate the germination process. An alternative method is scarification, which involves physically weakening the seed’s exterior. This is achieved by gently nicking the seed with a small file or lightly rubbing one side against coarse sandpaper.
Scarification creates a tiny opening for moisture without compromising the embryo, often resulting in quicker and more uniform sprouting than soaking alone. The seeds should be planted immediately after treatment. Once the protective layer is breached, the seed is vulnerable and needs to be placed into the soil environment promptly.
Planting Techniques and Initial Sprouting Care
Nasturtium seeds can be started indoors to gain an early start on the growing season, typically four to six weeks before the last expected frost date. For indoor planting, using peat pots or other biodegradable containers is highly recommended to prevent the transplant shock that nasturtiums are particularly sensitive to. When direct sowing outdoors, wait until all danger of frost has passed and the soil has warmed up.
The seeds should be planted about one-half to one inch deep into the prepared soil, as they require darkness to properly germinate. Nasturtiums prefer a cooler soil temperature for sprouting, with an ideal range of 60 to 70 degrees Fahrenheit. The planting medium must be kept consistently moist throughout this period, but never saturated, to avoid fungal issues.
Seedlings will usually emerge within seven to fourteen days if the temperature and moisture conditions are favorable. Once the young plants have broken the surface, they need access to ample light to prevent them from becoming leggy and weak. A sunny south-facing window or a dedicated grow light system, providing 12 to 16 hours of light daily, will encourage stocky, healthy growth.
Moving Seedlings to the Garden
Seedlings started indoors must undergo a gradual process called hardening off before they are moved to the garden. This transition acclimates the young plants to outdoor elements, such as direct sunlight, wind, and fluctuating temperatures. Begin by placing the pots in a sheltered, shaded outdoor location for just a few hours each day.
Over a period of seven to ten days, slowly increase the duration of their outdoor exposure, introducing them to progressively more sunlight and wind. This slow introduction toughens the plant’s cell structure and significantly reduces the risk of shock or leaf scorch. On nights when temperatures are expected to drop below 50 degrees Fahrenheit, the seedlings should be brought indoors.
Once fully hardened off, the nasturtiums can be transplanted into their final garden location after the soil has warmed and the threat of frost is gone. These plants perform best in full sun to partial shade and thrive in poor to average soil. Avoid planting them in overly rich soil, as high nitrogen content encourages excessive foliage growth at the expense of flowers.