Potting On Tomato Seedlings in May

Strong Plants, Better Harvests

By May, tomato seedlings sown in early spring will be ready for their next stage of growth. Whether you’re growing them in a greenhouse or on a sunny patio, potting on (also known as potting up) is a key step that helps young plants develop strong roots and become sturdy, productive growers.

Here’s how to do it right—and what to watch for.

Why Pot On?

As tomato seedlings grow, their roots quickly fill the small seed trays or modules they started in. If left too long, they become root-bound, stressed, and slow to grow. Potting them on gives each seedling more space, fresh compost, and the best start before planting in final positions later in the month.

When to Pot On

Look for these signs that your tomato seedlings are ready:

• 2–3 sets of true leaves
• Roots appearing through the drainage holes
• Slowed growth or yellowing leaves

In the UK, early to mid-May is the sweet spot for potting on tomatoes—especially for those destined for the greenhouse or later planting outdoors after the last frost.

Step-by-Step Guide

1. Prepare pots – Use clean 9cm (3½”) pots filled with multi-purpose compost (add a handful of perlite or vermiculite for drainage if desired).
2. Water first – Lightly water your seedlings beforehand to reduce root disturbance.
3. Handle gently – Hold seedlings by the leaves, not the stems, and gently tease them out of their trays.
4. Plant deep – Bury seedlings deeper than before, up to the first set of true leaves. Tomato stems can root along their length, giving you a stronger plant.
5. Firm and water in – Lightly firm the compost around the plant and water thoroughly.
6. Label each plant – Especially if you’re growing multiple varieties.

Aftercare

• Warmth & Light: Keep potted-on tomatoes in a bright, frost-free spot. A greenhouse, cold frame, or sunny windowsill works well.
• Ventilation: Good air circulation prevents damping off and mould.
• Feeding: No need to feed yet—fresh compost has nutrients to last a couple of weeks.

Looking Ahead

By the end of May, once all danger of frost has passed, you can move your tomato plants into their final positions—grow bags, large pots, or borders. They’ll be stronger, sturdier, and better prepared to deliver a bumper crop through summer.

Tomato Potting On Checklist

• Pot on seedlings when roots fill trays
• Use fresh compost and bury deeply
• Keep in a bright, sheltered place
• Water well and label clearly