Seedlings can go outside when they have been properly hardened off and the outdoor conditions are suitable. The process and timing generally work as follows:
- Start hardening off seedlings about 7 to 10 days before you plan to plant them outside. This involves gradually exposing them to outdoor conditions, starting with a protected shaded area for short periods of time and increasing exposure to sunlight and wind each day.
- Begin by placing seedlings outside in a sheltered spot for 1-2 hours and gradually increase the time they spend outside over about a week or two.
- Move seedlings to more direct sunlight gradually, increasing exposure time carefully.
- Once nighttime temperatures consistently stay around 50°F (10°C), seedlings can be left outside overnight for a few nights before transplanting.
- Transplant seedlings into prepared soil when they are hardy enough and after the danger of frost has passed.
- Seedlings are often ready to move outside about 3-4 weeks after sowing, assuming they have a decent root system and several true leaves.
- Timing also depends on the local climate and the type of plant, with heat-loving plants requiring warmer soil (60°F or 16°C) and cold-tolerant ones able to go out earlier.
So, seedlings are generally moved outside after about a month (or longer depending on conditions) and once the weather is warm enough and frost risk is gone, with a hardening-off period starting a week or so earlier to acclimate them safely to outdoor conditions.
