Coleus (Plectranthus scutellarioides) is a popular ornamental plant grown from seed, cherished for its vibrant, patterned foliage. Correctly identifying the young plants is necessary for successful cultivation, especially in crowded seed-starting trays. The earliest stages of growth look very different from the mature plant, so knowing what to look for ensures you are nurturing your intended seedlings and not stray weeds.
The Appearance of Cotyledons
The very first leaves that emerge from the soil are the cotyledons, or “seed leaves,” which are part of the embryo inside the seed. Coleus is a dicot, producing two cotyledons that unfurl shortly after germination. These initial leaves look nothing like the colorful, mature foliage the plant is known for.
Cotyledons are typically small, simple, and round or oval in shape, with smooth edges. They appear a solid, uniform green color, lacking the distinctive patterns of mature coleus. Their sole function is to provide the seedling with initial energy until the true leaves can begin photosynthesis efficiently. They are temporary and will eventually yellow and drop off.
Transition to True Coleus Leaves
The true leaves are the next set of leaves to appear, emerging from the center of the cotyledons. These leaves are the miniature version of the adult foliage and structurally distinct from the seed leaves. The first true leaves are usually pointed at the tip and display the serrated, or toothed, margins characteristic of the Lamiaceae family.
The true leaves emerge in an opposite arrangement and soon begin to hint at the vibrant coloration and patterning of the specific cultivar. This initial color may be subtle, often a pale green with a faint blush of purple or red along the edges. The intensity of this color is directly influenced by the amount of light the seedling receives.
As the plant grows, the stem will begin to develop the square shape common to mint family members, which becomes more apparent after the first few sets of true leaves emerge. The development of these textured, patterned leaves signals the seedling’s shift to self-sufficiency. Once the seedling has developed two full sets of true leaves, it is considered established and ready for transplanting.
Essential Care for Seedlings
Light Requirements
Once the true leaves are visible, the seedling needs focused care to thrive and develop its full color potential. Coleus seedlings require bright, indirect light, often provided by grow lights positioned three to four inches above the foliage for sixteen hours a day. Insufficient light causes the plant to become “leggy,” with long, weak stems stretching for a light source. Too much direct, intense sun can scorch the tender leaves.
Moisture and Thinning
Moisture management is necessary for healthy root development, requiring the soil to be consistently moist but never waterlogged. Overwatering can lead to root rot, while letting the soil dry out excessively will stunt growth. Watering from the bottom helps keep the foliage dry and encourages roots to grow downwards. Thinning crowded seedlings should happen once the true leaves appear, removing the weakest plants to reduce competition for light and nutrients.
Transplanting
When the seedlings have developed at least two sets of true leaves and are sturdy enough to handle, they are ready for transplanting into individual pots. Repotting allows for continued vigorous growth and prevents the roots from becoming tangled and restricted. After transplanting, a gentle, half-strength liquid fertilizer can be introduced. This is necessary because the tiny plants have exhausted the minimal nutrients in the seed-starting mix.