Cold hardiness & minimum temperature

Is Sweet alyssum (Lobularia maritima)cold hardy? Hardiness zone & min temp

Also called sweet alyssum, sweet alison, carpet of snow.

More about sweet alyssum

About Sweet alyssum

Lobularia maritima · also called sweet alyssum, sweet alison · flowering

A low-growing, mat-forming annual or short-lived perennial smothered in tiny, intensely honey-scented white, pink, or purple flower clusters from spring to autumn. Thrives in full sun to partial shade in well-drained soil. Drought-tolerant once established. Excellent for edging, containers, and hanging baskets; self-seeds prolifically.

Cold limit: USDA 5–9 (annual in most zones; short-lived perennial in zones 9–11) · RHS H3 (10–24 °C optimal; tolerates light frost to −5 °C as a seedling)

Watch for — Summer heat lull (flowering pause): In temperatures above 27 °C (80 °F), sweet alyssum stops flowering and may look ragged. Shear plants back by one-third and water consistently; flowering resumes when temperatures cool in late summer or autumn.

What sweet alyssum's hardiness rating actually means

Sweet alyssum 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 5–9 (annual in most zones; short-lived perennial in zones 9–11) — 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. Sweet alyssum shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for sweet alyssum 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 sweet alyssum go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 5–9 (annual in most zones; short-lived perennial in zones 9–11) 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 sweet alyssum 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 sweet alyssum

Sweet alyssum 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.

Sweet alyssum hardiness — frequently asked questions

Is sweet alyssum cold hardy?

Sweet alyssum 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 5–9 (annual in most zones; short-lived perennial in zones 9–11) (and sheltered UK gardens) sweet alyssum can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature sweet alyssum can survive?

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

What hardiness zone is sweet alyssum?

Sweet alyssum is rated USDA 5–9 (annual in most zones; short-lived perennial in zones 9–11) and RHS H3 — Half-hardy — comes through mild UK winters outside but is killed by a hard freeze.

Can sweet alyssum survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 5–9 (annual in most zones; short-lived perennial in zones 9–11) 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 sweet alyssum 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.