How to understand planting out
A common question among gardeners growing plants from seeds is, ‘How do I know when my seedlings are ready for the garden?’ This is an important consideration because timing plays a crucial role in their long-term development.
Transplanting too early can leave seedlings vulnerable to harsh conditions, while waiting too long may cause them to become root-bound in their containers. There’s no strict rule about the ideal height for transplanting, as different plants vary in size. Additionally, light exposure affects growth—seedlings grown in low light may become tall and leggy but not necessarily ready for the garden.
The best way to determine if a seedling is ready for transplanting is to check the number of true leaves it has developed.
What is a true leaf?
When you plant a seed, the first leaves to emerge are the cotyledons. These look different from the plant’s later leaves and serve as a temporary food source for the young seedling. Shortly after, the true leaves begin to grow. Unlike cotyledons, true leaves generate energy through photosynthesis, which sustains the plant throughout its life. Ensuring your seedling has enough true leaves before transplanting is essential for its healthy development
A good rule of thumb is to wait until a seedling has three to four true leaves before transplanting it into the garden—after it has been hardened off (the process of gradually acclimating indoor-grown seedlings to outdoor conditions before transplanting them into the garden. This helps them adjust to temperature fluctuations, direct sunlight, wind, and other environmental factors they haven’t experienced indoors).
How to harden off seedlings
Here in the UK, the hardening-off process needs to account for unpredictable weather, including chilly nights, wind, and rain.
- Days 1-2: Place seedlings outside for 1-2 hours in a shady, sheltered spot (e.g., against a house wall or under a table).
- Days 3-4: Increase to 3-4 hours with some morning sunlight exposure.
- Days 5-6: Extend to 5-6 hours in more direct light.
- After about 7-10 days, leave seedlings outside overnight if the temperature stays above 10°C.