"Potting-up" is just the term for moving seedlings out of the seedling tray or small container and into a larger container--usually a 4" pot. This is important because when you move the plant into that larger container, you have the opportunity to bury the entire stem so that just a tuft of leaves is sticking out of the planting mix. That may sound weird, but those little hairs on the plant's stem will grow into roots when they are buried and make the plant much stronger than it would be otherwise.
Potting-up is very simple.
First gather up enough 4" pots so you have one for each plant. You can re-use 4-inch pots that you may have lying around, just make sure you disinfect them using a weak bleach solution and rinse them completely before planting in them. You can also find new, clean 4" pots at garden centers or online. I purchased a case of 500 4" pots from Stuebers Distributing in Snohomish, Washington for around $25 about 4 years ago. I use all new pots each year and I'm still a long way from using all those pots and it saves me time to use fresh, new ones rather than washing old ones. The used pots go onto my potting bench to be used for potting up baby plants from my perennial garden.
You will also need several large, water-tight trays that the 4” pots will fit into. I clean, disinfect and re-use trays from year to year. These were also purchased from Stubers Distributing; they are flat bottom trays, about 2” deep and they do not have drainage holes.
You will need to mix up another batch of seed starter mix and water just as you did for starting the seeds. Remember to make it moist enough to pack into a ball, but not wet enough to wring out any of the water when you squeeze it.
Carefully remove each seedling, one at a time. Make sure to handle plants by the root ball or leaves, taking care to protect the fragile stem.
Place plant deep enough into the pot to cover up as much of the stem as possible, leaving a tuft of leaves sticking out of the soil. You can trim off any of the lower leaves that would end up under the starter mix (such as the cotyledon leaves if they are still attached.) Firm the moist soil around the stem.
If the seedling is a bit tall or leggy, you can place it in the pot kind of sideways, curving the stem carefully in order to submerge most of it in the soil.
Now, the potted seedlings go into the water-tight trays and into an unheated greenhouse (if available) or back under the lights indoors with the fan going. Don't water them at all for a couple of days. I learned this lesson the hard way--one year I watered the newly potted-up plants right away and they all died. Very depressing. I found out later that the stem needs to breath a bit and too much water right away kind of suffocated the poor little things. When you do water, do it by adding water to the tray so the plants can take the water up from the bottom and include a little fish emulsion every other week or so.
Congratulations! Your tomatoes have graduated to a larger growing space and are well on their way to your future tomato sauce garden!
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