Repotting guide

When & how to repot Cos Romaine Lettuce (Lactuca sativa var. longifolia 'Cos')

Also called cos lettuce, romaine lettuce.

More about cos romaine lettuce

About Cos Romaine Lettuce

Lactuca sativa var. longifolia 'Cos' · also called cos lettuce, romaine lettuce · edible

Cos, or romaine, is an upright lettuce forming tall, crisp, sweet hearts on sturdy ribs. It is more heat- and bolt-tolerant than soft butterheads, making it a reliable cool-season crop. Grow in full sun in fertile, moisture-retentive soil, water consistently, and harvest whole heads or pick outer leaves cut-and-come-again.

Mature size: 25-35 cm tall, 15-20 cm wide at harvest

How to tell cos romaine lettuce needs repotting

Repotting on a calendar is less reliable than reading the plant. For cos romaine lettuce, watch for these signs:

Roots circling the bottom of the module or pot, or poking out of the drainage holes.
The seedling dries out within a day and growth has visibly stalled.
Roots are white and matted in a tight spiral when you tip the plant out.
It has outgrown its current container for the stage of the season — pot cos romaine lettuce on before it becomes hard root-bound.

For the underlying biology of a pot-bound root system and why it stalls a plant, see our guide to spotting and fixing a root-bound plant.

How often to repot cos romaine lettuce

Pot on seedlings as they grow; not a perennial repot. Cos Romaine Lettuceis grown for one season, so the question is really “how often to pot on” — keep moving it up before the roots circle. Upright, semi-heading habit forming a tall, elongated head of crisp, strongly ribbed leaves around a sweet blanched heart; slower to bolt than loose-leaf and butterhead types..

What size pot to step cos romaine lettuce up to

Pot cos romaine lettuce on gradually — a seedling jumped straight into a huge pot sits in cold, wet, airless soil and stalls. Step up one or two sizes at a time as the roots fill each container, finishing in a large final pot or the ground. The aim is roots that never circle and never check.

Not sure of the exact diameter? Our pot size calculator takes the current pot and root spread and tells you the right next size — it deliberately recommends a single step up, never a big jump.

The best time of year to repot cos romaine lettuce

Pot cos romaine lettuce on through the active growing season, whenever roots fill the current container — there is no single date, just "before it becomes root-bound". Avoid potting on during a cold snap.

Step-by-step: repotting cos romaine lettuce

Pot on before it is root-bound. Check cos romaine lettuce regularly; move it up as soon as roots reach the edge of the cell or pot, not after they have circled.
Step up one or two sizes. Choose the next container up — not a giant one. Cold, wet, unused soil around a small root system stalls seedlings.
Knock it out gently. Support the stem, tip the pot, and ease the rootball out without breaking it. A little teasing of circled roots at the base is fine.
Pot into rich mix. Set it into fresh fertile, free-draining loam, ph 6.0-6.8 at the same depth (tomatoes are the exception — they can go deeper to root along the stem).
Water in and grow on. Water well, keep it in good light, and resume feeding once it is established and growing again.

Aftercare

Water cos romaine lettuce in well and keep it in bright light; a freshly potted-on seedling can wilt for a day while roots settle, so do not overcompensate by drowning it. Do not fertilise for about 1 week — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.

The right soil mix for cos romaine lettuce

Cos Romaine Lettuce wants fertile, free-draining loam, ph 6.0-6.8. Wants rich, moisture-retentive soil with generous organic matter and good drainage. Dig in compost before sowing. Firm, fertile ground supports the upright structure; thin, droughty soil gives small, bitter, quick-bolting heads. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting cos romaine lettuce — frequently asked questions

How often should you repot cos romaine lettuce?

Pot on seedlings as they grow; not a perennial repot for cos romaine lettuce. Cos Romaine Lettuce is a seasonal crop, so you pot it on as a growing plant rather than repotting a perennial. Step seedlings up gradually into fertile, free-draining loam, ph 6.0-6.8 so the roots never circle the cell, ending in a large final container. A root-bound transplant stalls and never fully recovers.

What size pot does cos romaine lettuce need?

When is the best time of year to repot cos romaine lettuce?

Can you put cos romaine lettuce straight into a much bigger pot?

No. Even a fast-growing cos romaine lettuce should only go up one pot size at a time. A vastly oversized pot holds a reservoir of wet soil the roots cannot reach, which stays cold and soggy and rots the roots — the opposite of what you wanted.

Should you fertilise cos romaine lettuce after repotting?

Not immediately. Wait about 1 week after repotting cos romaine lettuce. Fresh mix already contains nutrients, and feeding freshly cut or disturbed roots burns them. Resume your normal feeding routine once you see new growth.