Seedling

Key facts and history

The first true leaves have species-dependent shapes that differ from the round cotyledons, and the cotyledons eventually senesce and fall off.^[4] Eis (1967a) observed that seedling mortality was greater in moist habitats than in dry habitats on both mineral and litter seedbeds.^[3] In dry habitats, surviving seedlings after the first growing season had a much better chance of continued survival than those in moist or wet habitats.^[3] Growth of the root and shoot originates from the seedling's apical meristems.^[4]

Seedling development is influenced by the interplay of temperature and light; for example, with low light around 40 lumens/m², a day temperature of 28 °C and night temperature of 13 °C works well (Brix 1972).^[6] Growth ceases when the daily light period is under 14 hours, but extending it to 16 hours or more with dim light triggers uninterrupted (free) growth.^[6] Once seedlings have entered free growth, extending low-intensity light beyond 16 hours offers minimal additional benefit.^[7] Dry matter production is boosted by long photoperiods with high light intensities ranging from 10,000 to 20,000 lumens/m²; lengthening the photoperiod from 15 to 24 hours can double dry matter growth (Pollard and Logan 1976, Carlson 1979).^[7]

Cutworms, pillbugs, slugs, and snails are among the pests that cause significant damage to seedlings.^[1]

What the cited sources add

The Growfully guide explains that seed leaves store food for the seedling until true leaves can perform photosynthesis, after which the seed leaves fall off.^[5] Once true leaves appear, gardeners may start thinning seedlings by trimming out the weakest ones with scissors, and can begin regular fertilizing after one set of true leaves.^[5] After a plant has four or more sets of true leaves, it can be potted up to larger pots for more robust growth.^[5] The BBC Gardeners' World Magazine guide states that pricking out seedlings should be done once they have their first pair of true leaves, using a pencil or dibber to lever out the roots.^[8]

When transplanting, the seedling should be held by a leaf, never by the stem, because the stem can be easily crushed.^[8] Transplanting deeply helps keep the seedling secure, and the compost should come up almost to the lowest leaves.^[8] If seedlings are not pricked out, they can succumb to damping off disease.^[8]