Cold hardiness & minimum temperature

Is Annual Phlox 'Sugar Stars' (Phlox drummondii 'Sugar Stars')cold hardy? Hardiness zone & min temp

Also called Annual phlox, Drummond phlox.

More about annual phlox 'sugar stars'

About Annual Phlox 'Sugar Stars'

Phlox drummondii 'Sugar Stars' · also called Annual phlox, Drummond phlox · flowering

'Sugar Stars' is an annual phlox with distinctive star-shaped, lavender-blue petals edged and centred in white. A bushy, free-flowering Texas native annual, it blooms from late spring through summer in dense fragrant clusters, excellent for borders, containers and cutting. It performs best in cool to mild conditions and loves full sun.

Cold limit: USDA 2-11 (grown as a warm-season annual) · RHS H3 (10-27°C)

What annual phlox 'sugar stars''s hardiness rating actually means

Annual Phlox 'Sugar Stars' 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 2-11 (grown as a warm-season annual) — 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. Annual Phlox 'Sugar Stars' shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for annual phlox 'sugar stars' 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 annual phlox 'sugar stars' go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 2-11 (grown as a warm-season annual) 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 annual phlox 'sugar stars' 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 annual phlox 'sugar stars'

Annual Phlox 'Sugar Stars' 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.

Annual Phlox 'Sugar Stars' hardiness — frequently asked questions

Is annual phlox 'sugar stars' cold hardy?

Annual Phlox 'Sugar Stars' 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 2-11 (grown as a warm-season annual) (and sheltered UK gardens) annual phlox 'sugar stars' can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature annual phlox 'sugar stars' can survive?

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

What hardiness zone is annual phlox 'sugar stars'?

Annual Phlox 'Sugar Stars' is rated USDA 2-11 (grown as a warm-season annual) and RHS H3 — Half-hardy — comes through mild UK winters outside but is killed by a hard freeze.

Can annual phlox 'sugar stars' survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 2-11 (grown as a warm-season annual) 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 annual phlox 'sugar stars' 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.