Seed propagation is a practical and rewarding way to grow new plants, allowing gardeners to cultivate species often unavailable as nursery starts. This method involves germinating seeds in a controlled environment before transitioning the young plants outdoors. Understanding the specific needs of a seed, from dormancy to establishment, significantly increases the success rate. Starting plants from seed is also a cost-effective alternative to purchasing mature plants.
Pre-Sowing Preparation
Preparing the seeds and the growing environment is fundamental for successful germination. Containers, such as plastic trays, cell packs, or biodegradable peat pots, must have adequate drainage holes. The growing medium should be a sterile seed-starting mix, typically composed of fine components like peat moss, coir, vermiculite, and perlite, to ensure proper aeration and moisture retention. Garden soil is avoided because its dense structure impedes root development and it may harbor pathogens.
Testing the viability of older seeds is prudent. The float test involves placing larger seeds in water; those that sink are generally viable, while those that float should be discarded. Some seeds have a natural dormancy mechanism requiring pre-treatment techniques that mimic natural conditions.
Dormancy Breaking Techniques
Scarification involves physically weakening the hard seed coat, often by gently rubbing the seed with sandpaper, to allow water absorption necessary for germination.
Stratification simulates a period of cold, moist winter conditions. This is done by placing the seeds in a damp medium within a refrigerator for a specified duration, which neutralizes chemical germination inhibitors.
The Sowing Process
Once the seeds and medium are prepared, sowing must be handled with care. Fill the containers with the seed-starting mix and lightly tamp down the material to remove large air pockets without excessive compaction. Before planting, thoroughly moisten the medium, preferably by bottom-watering until moisture wicks up to the surface. A general rule for planting depth is to sow the seed two to three times as deep as the seed is wide.
Following this rule, small seeds, like lettuce, should barely be covered with a fine dusting of soil or vermiculite, while larger seeds, like beans, may be planted an inch deep. Proper spacing is important; place only one or two seeds per cell pack to prevent overcrowding and competition for light. After covering the seeds, label the containers immediately with the plant type and sowing date. Initial watering should be gentle, using a fine mist or bottom-watering, to prevent seeds from washing out of position.
Managing Germination and Early Growth
The period following sowing requires precise environmental control for successful germination and establishment. Many seeds require warm soil temperatures, often between 65°F and 85°F, to signal favorable conditions for sprouting. A seedling heat mat placed beneath the trays commonly provides this warmth, accelerating germination for heat-loving varieties like tomatoes and peppers. Once the first seedlings emerge, the heat mat should be removed, as prolonged high temperatures can stress the young plants.
Emerging seedlings, marked by their initial seed leaves (cotyledons), immediately require intense light to prevent “leggy” growth caused by stretching toward a light source. Supplemental grow lights, such as LED or fluorescent fixtures, should be placed a few inches above the plants and run for twelve to sixteen hours daily. Maintaining adequate moisture is important, but overwatering can lead to “damping off,” a fungal disease caused by soil-borne pathogens. This disease causes the stem to rot at the soil line, resulting in the sudden collapse of the seedling. Prevention involves using sterile mix, ensuring good air circulation with a small fan, and avoiding saturation. When true leaves—the second set of leaves characteristic of the mature plant—appear, thin overcrowded cells by snipping the weaker seedlings at the soil line, ensuring the strongest plant thrives.
Transitioning Seedlings to Outdoor Conditions
The final phase involves preparing indoor seedlings for the outdoor environment through “hardening off.” This process is the gradual exposure of plants to direct sunlight, temperature fluctuations, and wind, which are absent indoors. Hardening off typically begins seven to fourteen days before the planned transplant date, which should be after the last expected frost.
Start by placing seedlings outdoors in a shady, sheltered location for only one or two hours on the first day, protecting them from strong winds. Each subsequent day, increase the time spent outside and gradually introduce the plants to more direct sunlight. This slow acclimation causes plant cells to thicken and develop a protective cuticle, reducing the risk of sunscald and transplant shock. Once seedlings have developed at least two sets of true leaves and completed hardening, they are ready for transplanting. Transplant during the cooler parts of the day, such as early morning or late afternoon. Set the plants into the prepared hole at the same depth they were growing in their container, followed by thorough watering to settle the soil and minimize stress.