One of our Budsistas, Che Redz, asked me how one would know when it's time to transplant their seedling from the cup it was planted, into a pot.
The seedling only has it's first set of true leaves past the cotyledons (the first two leaves that emerge from the soil). I doubt its roots have had an opportunity to fill in the cup yet. And it is only a couple of inches tall. I'm not ready to transplant unless roots are poking out through the bottom of the container and the plant has developed a couple of nodes. I want it to look like a strong little plant, ready to take on the world. Putting a tender baby plant into a giant pot can be too much of a good thing. It can put unnecessary stress on your plant as the roots try to fill in the too-large pot.
Here are some tips for building a strong seedling:
Drainage holes - We were lucky to catch this before it got out of hand. When I asked Che Redz about drainage, I heard crickets. What's the number one rule in Plant Club? Thou shall never put a plant in a container with no drainage holes.
Add a fan - Air movement causes the plant to sway back and forth, and like you, it strengthens its core. We don't want lazy plants. Toughen up those plant abs! Plus, air circulation is good for keeping bad fungal growth, like powdery mildew, down.
Water regularly - Infrequent watering can put stress on young plants, and kill off tender root systems. Plus, if you're growing in living soil you run the risk of killing off your microbial life by allowing your soil to dry out.
When it's time to transplant your plant, the root system should be developed enough in the original container, so that the plant is ready to successfully transition into its new bigger home, with minimal stress to the roots.