Cold hardiness & minimum temperature

Is Cos Romaine Lettuce (Lactuca sativa var. longifolia 'Cos')cold hardy? Hardiness zone & min temp

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.

Cold limit: USDA Annual; grown in zones 2-11 as a cool-season crop · RHS H3 (10-21°C)

What cos romaine lettuce's hardiness rating actually means

Cos Romaine Lettuce is half-hardy (RHS H3). It survives a mild winter outdoors in a sheltered spot, but a hard frost kills it — so in colder zones it is lifted, potted, or grown as a tender plant. Its RHS rating of H3 means: Half-hardy — comes through mild UK winters outside but is killed by a hard freeze. On the US scale that maps to USDA Annual; grown in zones 2-11 as a cool-season crop — the zones where it can be left outdoors year-round.

New to these scales? The USDA hardiness zone map explained covers how the zone numbers work, and you can find your own zone with the zone finder.

Minimum temperature — and what happens below it

Minimum survivable temperature is roughly about −5 to 1 °C — a light, short frost only. Cos Romaine Lettuce shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for cos romaine lettuce as it gets too cold:

Down to roughly about −5 to 1 °C it copes, especially if dry and sheltered.
A sustained hard frost collapses the top growth; whether it returns depends on whether the roots, crown or tubers froze.
Wet cold is far more lethal than dry cold for this plant — soggy, frozen soil is the usual killer.

Can cos romaine lettuce go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA Annual; grown in zones 2-11 as a cool-season crop or a frost-free UK microclimate.
In colder zones, grow it in a pot you can move under cover, or lift its tubers/roots and store them frost-free over winter.
A south-facing wall, free-draining soil and a dry winter position can push it a full zone hardier than the books suggest.

Work back from your local frost dates with the frost-date calculator: the last spring frost and first autumn frost are what really decide when cos romaine lettuce can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H3 figure above.

Frost protection for borderline cos romaine lettuce

Cos Romaine Lettuce is right on a hardiness edge in many gardens, so if you are pushing it, these measures buy it the margin it needs:

Mulch the crown or root zone deeply with bark, straw or leaf-mould before the first hard frost.
Move container plants against a warm wall or into an unheated but frost-free porch or greenhouse.
Fleece the top growth on the coldest nights, and keep it on the dry side — dry roots survive cold far better than wet ones.
Lift dahlia-type tubers or tender crowns after the first light frost blackens the foliage and store them somewhere cool but frost-free.

Cos Romaine Lettuce hardiness — frequently asked questions

Is cos romaine lettuce cold hardy?

Cos Romaine Lettuce is half-hardy (RHS H3). It survives a mild winter outdoors in a sheltered spot, but a hard frost kills it — so in colder zones it is lifted, potted, or grown as a tender plant. Borderline outdoors. In its mild end of USDA Annual; grown in zones 2-11 as a cool-season crop (and sheltered UK gardens) cos romaine lettuce can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature cos romaine lettuce can survive?

Minimum survivable temperature is roughly about −5 to 1 °C — a light, short frost only. Cos Romaine Lettuce shrugs off cold nights but a real, sustained freeze will kill it.

What hardiness zone is cos romaine lettuce?

Cos Romaine Lettuce is rated USDA Annual; grown in zones 2-11 as a cool-season crop and RHS H3 — Half-hardy — comes through mild UK winters outside but is killed by a hard freeze.

Can cos romaine lettuce survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA Annual; grown in zones 2-11 as a cool-season crop or a frost-free UK microclimate. In colder zones, grow it in a pot you can move under cover, or lift its tubers/roots and store them frost-free over winter. A south-facing wall, free-draining soil and a dry winter position can push it a full zone hardier than the books suggest.

How do I protect cos romaine lettuce from frost?

Mulch the crown or root zone deeply with bark, straw or leaf-mould before the first hard frost. Move container plants against a warm wall or into an unheated but frost-free porch or greenhouse. Fleece the top growth on the coldest nights, and keep it on the dry side — dry roots survive cold far better than wet ones. Lift dahlia-type tubers or tender crowns after the first light frost blackens the foliage and store them somewhere cool but frost-free.