Kale is a popular cool-weather crop known for its high nutritional content, including Vitamin C, Vitamin K, and various antioxidants. This resilient leafy green is easy to cultivate and provides a long harvest, often extending into the winter months. Successfully growing kale begins with understanding the precise requirements for starting it from seed. Mastering the initial stages, from planting depth to post-germination care, ensures a strong and flavorful crop.
Planting Depth and Timing
The size of the seed dictates the planting depth; for kale, a shallow placement is necessary for successful emergence. The recommended depth for sowing kale seeds is consistently between one-quarter and one-half inch (approximately 6 to 12 millimeters). This shallow depth is necessary because the small seed contains limited endosperm, the stored energy source for the young seedling. Planting too deep would exhaust this energy before the sprout could reach the soil surface and begin photosynthesis.
After placing the seed, cover it lightly with a fine, sterile medium like vermiculite or sifted seed-starting mix. This medium helps maintain moisture without compacting the soil. Kale seeds germinate reliably across a wide range of temperatures, but the ideal soil temperature for the quickest sprouting is between 60°F and 65°F. Germination can occur with soil temperatures as low as 40°F, but the process will be significantly slower.
Timing your planting correctly is determined by local frost dates, as kale thrives in cool conditions. For a spring harvest, seeds should be started indoors approximately four to six weeks before the last expected frost date. Direct sowing outdoors can begin two to three weeks before the last frost, provided the soil is workable.
For a fall and winter harvest, which often produces the sweetest leaves after a light frost, seeds should be started in late summer. This means sowing seeds outdoors about six to eight weeks before the first anticipated fall frost. Starting a fall crop indoors during the summer heat can be beneficial, allowing tender seedlings to establish themselves in a controlled environment before being transplanted into the cooling garden soil.
Seed Starting Methods
Gardeners can begin kale seeds either indoors under controlled conditions or by direct sowing into the garden bed. Starting seeds indoors is advantageous because it allows for a head start on the season and greater environmental control, leading to earlier harvests. When starting indoors, use a sterile, soilless seed-starting mix in trays or individual cell packs to minimize the risk of damping-off disease.
Once the seeds are sown at their shallow depth, providing adequate light is the next concern. Seedlings require intense light exposure for 14 to 16 hours per day to prevent them from becoming tall and spindly, a condition known as etiolation. This is best achieved using fluorescent or LED grow lights placed only two inches above the seedlings, moving the lights up as the plants grow.
Direct sowing is a simpler method that eliminates the need for transplanting, which can sometimes shock the seedlings. Site preparation involves loosening the soil and incorporating organic matter, such as nitrogen-rich compost, which supports the heavy foliage growth of kale. Seeds are placed at the same shallow depth and spaced approximately one inch apart in rows that are 18 to 30 inches wide.
Before transplanting indoor-started seedlings outdoors, they must undergo a process called hardening off over one to two weeks. This involves gradually exposing the plants to increased sunlight, wind, and temperature fluctuations to acclimate their tissues to the harsher outdoor environment. Failure to harden off can result in severe leaf burn, stunted growth, or the death of the seedlings.
Post-Germination Care
Once the kale seeds have germinated, attention must turn to managing the density of the new seedlings. Thinning is a necessary step, as overcrowding will stunt growth and promote disease by restricting air circulation. Seedlings should be thinned to their final mature spacing, typically 12 to 18 inches apart, once they have developed their first set of true leaves.
To thin the seedlings, use a small pair of scissors or nail clippers to snip the weaker plants at the soil line, rather than pulling them out. Cutting the stem avoids disturbing the root systems of the remaining, stronger plants, which is important in small containers or closely spaced rows. The thinned seedlings can often be eaten as microgreens, provided they are not diseased.
Consistent moisture is important during the early stages of growth to support the development of leafy tissue. The soil should be kept evenly moist but never saturated, as waterlogged conditions can lead to root rot. Aim to provide the equivalent of one to one and a half inches of water per week, adjusting for rain and humidity.
Maintaining the proper temperature is important for flavor and preventing premature flowering. Kale is a cool-season plant, and temperatures consistently above 75°F can cause the leaves to become tough and bitter. If growing indoors, ensure strong light continues to encourage compact growth. If outdoors, provide midday shade if temperatures are exceptionally high.