Nasturtiums (Tropaeolum majus) are vibrant, easy-to-grow annual plants cherished for their bright, trumpet-shaped flowers and rounded, lily pad-like foliage. These versatile plants are popular not only for their ornamental appeal but also because their leaves and blossoms are edible, offering a peppery zest to salads. Starting nasturtium seeds indoors is a common practice for gardeners looking to gain a significant head start on the growing season. This method is particularly beneficial in regions where the summer months are short, allowing the plants to reach maturity and flower earlier than direct-sowing would permit.
Determining the Ideal Start Date
The precise timing for starting nasturtium seeds indoors relies entirely on the average last frost date specific to your geographic region. Gardeners should consult local resources to identify this date, which acts as the reference point for all spring planting. Nasturtiums are tender annuals and cannot tolerate any freezing conditions.
The optimal window for indoor sowing is typically two to four weeks before the last expected frost. This short timeframe ensures the young plants develop strong stems and a moderate root system suitable for transplanting without becoming overly large. Waiting until all danger of freezing temperatures has passed is necessary, as even a light frost can severely damage or kill the young seedlings.
Essential Preparation for Seed Starting
Successful indoor starting begins with selecting a sterile, fine-textured seed starting mix that drains well. Nasturtium seeds possess a hard outer coat, which naturally delays water absorption and germination. To accelerate this process, a pre-sowing treatment is necessary to soften the seed coat.
Gardeners can lightly scratch the seed coat with fine-grit sandpaper or a small file, a technique known as scarification. Alternatively, soaking the seeds in room temperature water overnight for approximately eight to twelve hours can effectively break dormancy. This physical treatment allows moisture to penetrate the embryo more quickly, leading to a faster and more uniform rate of sprouting.
The choice of container is equally important, as nasturtiums are sensitive to root disturbance when moved. Using biodegradable pots made from peat or coir, or deep cell trays, minimizes transplant shock. These materials allow the entire root ball to be placed directly into the garden soil.
Sowing and Early Seedling Care
Once the seeds are prepared, they should be planted relatively deep into the prepared starting mix. A planting depth of approximately one-half to one inch is sufficient to anchor the developing root system. Place one treated seed per biodegradable pot or cell to avoid the need for thinning, which can disrupt the roots of the remaining seedling.
The soil temperature should be maintained in the range of 65–70°F (18–21°C) to encourage optimal germination, which typically occurs within seven to ten days. The medium must be kept consistently moist but never saturated, as waterlogged conditions can lead to fungal diseases like damping off.
Immediately after the seedlings emerge, they require intense light to prevent the stems from becoming elongated and weak, a condition known as legginess. Providing twelve to sixteen hours of strong, broad-spectrum light daily from a dedicated grow light source is generally necessary in an indoor setting. Natural window light is often insufficient, especially during late winter and early spring.
After the sprouts have developed their first true leaves, the ambient temperature can be slightly lowered to 60–65°F (15–18°C) to promote stocky growth. If multiple seeds were sown in one container, the weaker seedlings must be carefully thinned by snipping them at the soil line rather than pulling them out. This selective removal ensures the strongest plant has adequate resources and prevents root entanglement.
Preparing Seedlings for the Garden
Before nasturtium seedlings can be moved permanently outside, they must undergo a gradual process of acclimatization known as hardening off. This conditions the tender indoor-grown plants to outdoor elements, including direct sun, wind, and fluctuating temperatures. Without this step, the sudden shock of the new environment can cause damage and stunted growth.
The hardening off process should span seven to ten days, beginning with placing the seedlings outside in a sheltered, shaded location for only one or two hours each day. Gradually, the duration of outdoor exposure is increased, and the plants are moved into brighter sunlight over the course of the week. This slow introduction allows the plant cells to develop thicker walls and a protective cuticle layer.
The final transplanting should be done after the hardening off period is complete and the outdoor soil temperature has consistently warmed. Since nasturtiums resent having their roots disturbed, biodegradable containers simplify this step by allowing the entire pot to be buried.