A seedling is a young plant that has emerged from a seed and started to grow on its own. It has a root system, a stem, and at least one set of true leaves. The seedling stage is the most vulnerable period in a plant's life.

Seedlings depend on light, water, and nutrients in the right balance. Too little light and they stretch and weaken. Too much water and they damp off, a fungal disease that rots the stem at soil level. Too little and they wilt. Weeds compete fiercely at this stage, and insects can strip a tray of seedlings in a day. Transplanting is usually done when the seedling has several true leaves and a sturdy stem.

Common problems

• Damping off from soil-borne fungi
• Legginess from insufficient light
• Aphids, whiteflies, and thrips in greenhouse trays
• Transplant shock when roots are disturbed

Growers harden off seedlings before transplanting by gradually exposing them to outdoor conditions. Skipping this step can set a crop back by weeks, or kill it outright on a windy, sunny day.