Romaine lettuce, known for its crisp, upright leaves and mild flavor, is a type of Cos lettuce that thrives as a cool-weather crop. Proper planting depth is one of the most important factors, as the seed’s ability to germinate and the seedling’s ability to develop a healthy root system are directly tied to how deeply it is set into the soil. Understanding the specific needs of Romaine, whether starting from a tiny seed or an established transplant, will ensure a bountiful harvest of crunchy, non-bitter greens.
Planting Preparation and Timing
Romaine lettuce performs best when the soil temperature is cool, ideally between 40°F and 75°F, with optimal germination occurring between 60°F and 72°F. Planting too early or too late can expose the crop to extreme temperatures, which may cause the plant to bolt, making the leaves bitter. Gardeners generally plant Romaine in early spring as soon as the soil is workable, or in late summer for a fall harvest, avoiding the intense heat of mid-summer.
The chosen location should offer full sun exposure during the cooler spring and fall months to promote robust growth. However, if planting extends into warmer periods, partial shade or afternoon shade will help mitigate the effects of high temperatures. Loose, well-drained soil rich in organic matter is preferred, as lettuce has a shallow root system and requires consistent moisture and nutrients. Preparing the bed with compost before planting will set the stage for healthy plant development.
Planting Depth for Romaine Seeds
When starting Romaine lettuce directly from seed, the planting depth must be extremely shallow because the seeds require light to trigger germination. Sowing the seeds too deeply will prevent the necessary light from reaching them, resulting in poor or failed sprouting. The correct depth for Romaine seeds is 1/8 to 1/4 inch beneath the soil surface.
Gardeners should scatter the seeds thinly over the prepared soil bed and then lightly cover them with a fine dusting of potting mix or soil. After covering, gently firm the soil surface to ensure good seed-to-soil contact, which is necessary for moisture absorption. Keep this shallow layer of soil consistently moist until the seedlings emerge, which typically takes between seven to ten days.
Proper Depth and Technique for Transplants
Planting established Romaine seedlings, or transplants, requires a different technique focused on protecting the plant’s delicate crown, which is where the stem meets the roots. The plant should be set into the garden bed at the exact same level it was growing in its original container. The soil line of the transplant’s root ball should be flush with the surrounding garden soil.
Planting too deeply and burying the stem can lead to stem rot, as the constant moisture at the soil surface encourages fungal growth on the buried tissue. Planting too shallowly leaves the root ball exposed, which can quickly dry out the plant, hindering its growth. Before placing the seedling, the root ball should be gently loosened to encourage the roots to spread into the new soil. Dig a hole slightly larger than the root ball, place the plant, and backfill the hole, lightly tamping the soil around the base to eliminate air pockets.
Immediate Care Following Planting
Immediately after planting seeds or transplants, a gentle initial watering is necessary to settle the soil and hydrate the roots. Water carefully, using a fine spray or a gentle stream, to avoid displacing the shallowly planted seeds or washing out the newly set transplants. Consistent moisture at the shallow root zone is important for lettuce, as it encourages leaf production rather than deep root development.
If starting from seed, the seedlings will eventually need to be thinned out to allow the remaining plants enough space to form a full head. The final spacing between individual plants should be approximately 10 to 12 inches. Providing this wide spacing reduces competition for light, water, and nutrients, which leads to a larger, healthier head of lettuce.