If you start plants from seed, eventually you’ll have to master the technique of potting up. It’s what you do when a seedling gets too big for the pot or cell it’s growing in. Usually it’s too early to plant outdoors, and the seedling still needs a few more weeks of coddling indoors. Not all seedlings require potting up, but many do.
A good example is tomatoes, which are started in small pots about eight weeks before the last frost date in spring. After growing indoors for four to five weeks, the seedlings need to be potted up into something bigger, where they can continue to grow until it’s time to plant them outdoors. Choose a pot that has about three times the volume of the original. Avoid waiting too long, because root crowding can stunt plant growth and make it difficult for plants to recover after transplanting.
Why not just start with a big pot in the first place? You could, but most seedlings grow better in pots that are smaller, about 2″ square. With smaller pots, it’s easier to provide consistent moisture and fertilizer.
Another time you might do potting up is when you sow seeds in flats. These containers come in many sizes, but most are about 6×6″ and 2″ deep. They’re handy for small seeds, such as lettuce. You sprinkle the seeds onto the surface of the flat, cover with a thin layer of seed-starting soil (Germinating Mix) and let them germinate.
Once the seedlings have a couple sets of true leaves, you can gently take the soil out of the flat and dislodge the seedlings for potting up into individual pots. This process is called “pricking out.” Lee Reich, author of Weedless Gardening, takes you through the process in the video below. Watch and learn: