Kale is a cool-season leafy green belonging to the Brassica family. When starting this vegetable from seed, a common question arises regarding the need for light to trigger germination. Kale seeds do not require light to germinate; they are typically planted beneath the soil surface. The initial emergence of the root, or radicle, is driven by internal biological mechanisms and specific environmental conditions, not sunlight.
Kale Seeds and the Light Requirement
Kale seeds are classified as non-photoblastic, meaning their germination is indifferent to the presence or absence of light. Unlike photoblastic seeds, which rely on light to signal they are near the soil surface, kale seeds possess sufficient internal resources to begin growth in darkness. The seed contains a stored energy supply, primarily starches and oils, which fuels the embryo during the initial stage of development.
This energy reserve, combined with internal hormones, is activated by moisture and temperature cues, not light signals. The primary function of the seed is to initiate the emergence of the root, which anchors the future plant and begins water absorption. Because this phase is metabolically self-sufficient, the seed does not need to wait for a light signal to begin the process.
Essential Environmental Factors for Successful Sprouting
Since light is not a factor for kale germination, the focus shifts to three other environmental conditions necessary for successful sprouting.
Consistent Moisture
Consistent moisture triggers the entire process through a phenomenon called imbibition. This is the physical absorption of water by the dry seed, which causes it to swell, softens the seed coat, and activates the enzymes needed to break down stored food for the embryo.
Temperature
Temperature is important, as kale is a cool-weather crop that prefers a cooler germination environment. The ideal soil temperature range for sprouting is between 60 to 65 degrees Fahrenheit, though they will germinate across a wider range of 40 to 85 degrees Fahrenheit. Maintaining the soil within this optimal cool range encourages quick and reliable germination, typically occurring within three to twelve days.
Planting Depth
Planting depth is a direct consequence of the seed not needing light. Kale seeds are small and should be planted shallowly, typically between one-quarter to one-half inch deep in the soil or seed-starting mix. This depth ensures the seed is enveloped in moist soil while remaining shallow enough for the nascent seedling to push its way to the surface without exhausting its limited energy reserves. Planting too shallowly risks drying out the seed, while planting too deep impedes the seedling’s emergence.
Why Light Becomes Critical Post-Germination
While light is inconsequential for the initial sprouting, it becomes immediately necessary the moment the seedling emerges above the soil line. Germination ends when the radicle appears, and the next phase, seedling growth, is entirely dependent on photosynthesis. The tiny sprout must quickly transition from relying on its internal energy stores to producing its own food using sunlight.
If a strong light source is not provided at this stage, the seedling will begin a process called etiolation. Etiolation is the plant’s desperate attempt to find light, characterized by an abnormal lengthening of the stem, resulting in a spindly, pale, and weak plant. The stem elongates rapidly, driven by hormones, as the seedling sacrifices stem thickness and leaf development to reach the perceived light source.
This stretching and weakening of the stem makes the seedling vulnerable to collapse and disease. To prevent etiolation and promote healthy, stocky growth, seedlings started indoors should be placed under a grow light immediately after they emerge, typically receiving 14 to 18 hours of light per day. This prompt introduction of light allows the plant to focus its energy on developing strong stems and its first set of true leaves.