Cold hardiness & minimum temperature

Is Lobelia erinus 'Cascade Blue' (Lobelia erinus 'Cascade Blue')cold hardy? Hardiness zone & min temp

Also called Cascade Blue Lobelia, Trailing Blue Lobelia.

More about lobelia erinus 'cascade blue'

About Lobelia erinus 'Cascade Blue'

Lobelia erinus 'Cascade Blue' · also called Cascade Blue Lobelia, Trailing Blue Lobelia · flowering

'Cascade Blue' is a trailing edging lobelia smothered in small, deep-blue flowers from late spring to autumn. A tender annual ideal for hanging baskets, window boxes and container edges, it spills attractively over rims. It performs best in cool, moist conditions with sun to part shade and may pause flowering in summer heat.

Cold limit: USDA 10-11 (grown as a half-hardy annual elsewhere) · RHS H2 (10-24°C)

Watch for — Mid-summer flowering pause: Heat stalls bloom and growth; shear back by a third, water well and feed to revive a second flush as temperatures ease.

What lobelia erinus 'cascade blue''s hardiness rating actually means

Lobelia erinus 'Cascade Blue' is half-hardy (RHS H2). 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 H2 means: Tender — survives a frost-free greenhouse or a very mild, sheltered spot. On the US scale that maps to USDA 10-11 (grown as a half-hardy annual elsewhere) — 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 1 to 5 °C — tolerates cold but no real frost. Lobelia erinus 'Cascade Blue' shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for lobelia erinus 'cascade blue' as it gets too cold:

Down to roughly about 1 to 5 °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 lobelia erinus 'cascade blue' go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 10-11 (grown as a half-hardy annual elsewhere) 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 lobelia erinus 'cascade blue' can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H2 figure above.

Frost protection for borderline lobelia erinus 'cascade blue'

Lobelia erinus 'Cascade Blue' 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.

Lobelia erinus 'Cascade Blue' hardiness — frequently asked questions

Is lobelia erinus 'cascade blue' cold hardy?

Lobelia erinus 'Cascade Blue' is half-hardy (RHS H2). 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 10-11 (grown as a half-hardy annual elsewhere) (and sheltered UK gardens) lobelia erinus 'cascade blue' can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature lobelia erinus 'cascade blue' can survive?

Minimum survivable temperature is roughly about 1 to 5 °C — tolerates cold but no real frost. Lobelia erinus 'Cascade Blue' shrugs off cold nights but a real, sustained freeze will kill it.

What hardiness zone is lobelia erinus 'cascade blue'?

Lobelia erinus 'Cascade Blue' is rated USDA 10-11 (grown as a half-hardy annual elsewhere) and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can lobelia erinus 'cascade blue' survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 10-11 (grown as a half-hardy annual elsewhere) 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 lobelia erinus 'cascade blue' 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.