Most seedlings stay in trays for about 4-8 weeks before they need to move to bigger pots or the garden. The exact timing depends on the crop, the cell size, and how fast your plants grow. Some seedlings are fine for months while others get cramped in just a few weeks.
I learned to spot trouble early by using clear plastic cups as seed pots one season. Watching the roots press against the walls taught me more than any book. When white roots started circling the bottom, I knew those plants needed more room right away. That visual cue saved many seedlings from becoming stunted.
Knowing when to transplant seedlings comes down to reading what your plants tell you. Growth slows down when roots run out of room to spread. Lower leaves turn yellow because the plant pulls nutrients from old growth to feed new leaves. The whole seedling looks stuck even though you're giving it water and light.
Root bound seedlings suffer long-term damage that shows up weeks later. Roots that circle the pot keep circling after you transplant them. They never spread out into the surrounding soil the way they should. The plant struggles to take up water and food, and you end up with smaller harvests or weaker flowers.
Tomato seedlings push for more space after 4-6 weeks in standard cell trays. Their roots grow fast and thick. Pepper seedlings can wait a bit longer since they grow slower at first. Onions and leeks are the patient types. They can sit in cells for 10-12 weeks without getting cranky about space.
Check root development by tipping the tray and sliding out one seedling with its soil block. Look at the root ball carefully. White roots reaching the edges of the block means it's almost time to move up. Roots wrapping around the outside tell you that you've waited too long. The sweet spot is right before the circling starts.
Bigger cells buy you more time if you're starting early. A 50-cell tray holds more soil than a 72-cell tray, so roots have more room to roam. I use larger cells for tomatoes and peppers that need 8-10 weeks of indoor growth before my last frost date. Small cells work fine for quick crops like lettuce that move outside sooner.
My rule of thumb is to err on the early side. Moving seedlings to bigger pots a week too soon beats waiting a week too long. Those extra few days of root-bound stress can set plants back for the whole season. When in doubt, pot up your seedlings and give them room to grow.
Read the full article: Starting Seeds Indoors: A Complete Guide
