When Should I Pot On My Seedlings to a Bigger Pot?

Potting on — moving a seedling into a larger container before it is planted outdoors — is often skipped or done at the wrong time. A seedling left in a pot it has outgrown will slow down noticeably and may never fully recover its growth potential. Knowing the signs and doing it at the right moment keeps plants growing vigorously right up until transplant day.

Signs a Seedling Needs Potting On

The clearest sign is roots emerging from the drainage holes at the bottom of the pot — the plant has filled its current container and is searching for more space. Other signs include:

• The pot dries out within a day of watering (roots have filled all the available compost)
• Growth has visibly slowed despite good light and temperature
• Leaves are yellowing despite adequate light (root-bound plants cannot access nutrients properly)
• The plant looks disproportionately large for its pot and is at risk of toppling

How Much Bigger Should the New Pot Be?

Move up one size at a time — typically from a 7 cm (3 in) pot to a 9–11 cm (3.5–4.5 in) pot, or from a 9 cm pot to an 11–13 cm pot. Moving to a very large pot in one step can lead to overwatering problems: the large volume of compost around a small root system stays wet for too long and risks root rot.

The Potting On Process

Water the plant an hour before potting on so the root ball holds together. Prepare the new pot with fresh, moist potting compost at the base. Turn the old pot upside down and ease the root ball out — it should come out cleanly with roots visible on the outside. Place it in the new pot so the top of the old root ball sits about 1–2 cm below the rim. Fill around it with fresh compost, firm gently, and water in well. For tomatoes, you can bury the stem deeper than it previously sat — tomatoes root along buried stems, giving the plant a stronger base.

What Compost to Use for Potting On

Switch from seed compost (low nutrients) to a good multipurpose or potting compost (higher nutrients) from the first potting-on stage onward. Seedlings growing their second and third sets of leaves need nitrogen and other nutrients that seed compost cannot supply. Add a small amount of slow-release granular fertiliser to the compost at this stage if your potting mix does not already contain it.

After Potting On: A Brief Recovery

Keep newly potted plants out of direct harsh sunlight for 24–48 hours while roots establish in the new compost. They rarely wilt visibly but a brief settling-in period reduces any minimal stress. Resume normal care within a couple of days.