Nasturtiums are popular annual plants cherished for their vibrant flowers and unique, shield-shaped leaves. Both the foliage and blossoms are edible, offering a pleasant peppery flavor that makes them a favorite in culinary gardening. Successfully starting these plants requires understanding their specific seed requirements, which differ significantly from many other garden varieties. This guide provides the necessary details for robust germination and vigorous plant growth.
Addressing the Cold Stratification Requirement
Nasturtium seeds do not need cold stratification, a process that involves exposing seeds to a period of cold, moist conditions to simulate winter. This technique is typically used to break the dormancy of seeds from perennial plants native to regions with cold winters. The chilling period provides a signal that prevents premature sprouting.
Nasturtiums are native to the warmer climates of the Andes mountains and are highly sensitive to frost, classifying them as tender annuals. Their physiology does not require a freeze-thaw cycle to initiate growth. Attempting to cold-stratify nasturtium seeds is unnecessary and may harm the embryo, delaying or preventing germination. The factor that restricts water absorption is a physical barrier, not temperature-related dormancy.
Essential Pre-Planting Seed Preparation
While a cold treatment is not necessary, nasturtium seeds possess a hard and thick outer shell, or seed coat, which causes physical dormancy. This tough exterior prevents water from reaching the embryo inside, a process called imbibition, which is required for germination. Therefore, the most effective pre-planting steps involve manually weakening this outer layer to facilitate water entry.
Scarification
Scarification involves gently scratching or nicking the seed coat. This can be accomplished using a small file, sandpaper, or a pair of nail clippers, applied to the opposite side of the seed’s eye. It is important to break the outermost layer without damaging the interior embryo.
Soaking
Soaking the seeds in warm water is an effective preparation technique, often used in conjunction with scarification. Placing the seeds in warm water for 12 to 24 hours softens the seed coat, allowing for better water penetration. Seeds that float after soaking may be non-viable. Combining a gentle nicking with an overnight soak provides the most reliable way to ensure quicker, more uniform sprouting.
Optimal Environmental Conditions for Germination
Providing the correct environment is necessary for successful germination once the seeds are prepared. Nasturtiums require warm soil to sprout quickly and reliably. The ideal soil temperature range for rapid germination is between 65°F and 75°F (18°C to 24°C).
The seeds should be planted relatively shallowly, typically a depth of about one-half inch (1-1.5 cm) into the growing medium. A well-draining soil is preferred for planting, as consistent moisture is needed for germination, but the seeds will rot if the soil remains waterlogged. After planting, the soil must be kept consistently moist until the seedlings emerge, which typically takes between 7 and 14 days.
Nasturtiums are best started outdoors by direct sowing after the last predicted frost date, as the seedlings are sensitive to cold and do not tolerate transplanting well. If an earlier start is desired, seeds can be sown indoors approximately four to six weeks before the last frost. When starting indoors, use biodegradable pots, such as peat or coir, to prevent root shock when moving the young plants to the garden. The planting site should receive full sun, and the use of overly rich soil should be avoided, as this tends to produce excessive foliage at the expense of flower production.