Cold hardiness & minimum temperature

Is Boissier's Silverbush (Convolvulus boissieri)cold hardy? Hardiness zone & min temp

Also called Boissier's silverbush, Spanish silverbush.

More about boissier's silverbush

About Boissier's Silverbush

Convolvulus boissieri · also called Boissier's silverbush, Spanish silverbush · flowering

Convolvulus boissieri (synonym C. nitidus) is a dwarf, cushion-forming evergreen sub-shrub native to rocky, sun-baked mountain slopes of southern Spain. It produces large, gleaming white to pale pink funnel-shaped flowers against dense, silver-silky foliage, making it a sought-after plant for alpine gardens, troughs, and rock gardens. It requires near-perfect drainage and full sun; it is particularly intolerant of winter wet at the root crown. It is not documented as toxic to pets.

Cold limit: USDA 7-9 · RHS H4 (-10°C to 30°C)

Watch for — Crown rot from winter wet: The primary threat; the rosette crown is extremely susceptible to rotting when moisture sits around it in cold weather — always plant on a slope or with a collar of sharp grit around the crown, or grow in an alpine house.

What boissier's silverbush's hardiness rating actually means

Yes — boissier's silverbush is genuinely cold hardy. Rated RHS H4 and USDA 7-9, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H4 means: Hardy in an average winter across much of the temperate world. On the US scale that maps to USDA 7-9 — 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 −10 to −5 °C. Boissier's Silverbush is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for boissier's silverbush as it gets too cold:

It tolerates winter lows to about −10 to −5 °C once established.
Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root.
First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.

Can boissier's silverbush go outside or overwinter — and where?

Plant it out within USDA 7-9 and it overwinters with little or no help.
It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy.
The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

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 boissier's silverbush can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H4 figure above.

Frost protection for borderline boissier's silverbush

Boissier's Silverbush is right on a hardiness edge in many gardens, so if you are pushing it, these measures buy it the margin it needs:

At the cold edge of its range, mulch the root zone in late autumn to buffer the deepest freezes.
Protect container specimens — pots freeze through far faster than open ground, costing roughly a zone of hardiness.
Shelter new growth from late spring frosts with fleece if a hard night is forecast.

Boissier's Silverbush hardiness — frequently asked questions

Is boissier's silverbush cold hardy?

Yes — boissier's silverbush is genuinely cold hardy. Rated RHS H4 and USDA 7-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Boissier's Silverbush is hardy across USDA 7-9; it belongs in the ground or a frost-proof container, not on a windowsill, and many types actively need a cold winter to perform.

What is the minimum temperature boissier's silverbush can survive?

Minimum survivable temperature is roughly about −10 to −5 °C. Boissier's Silverbush is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is boissier's silverbush?

Boissier's Silverbush is rated USDA 7-9 and RHS H4 — Hardy in an average winter across much of the temperate world.

Can boissier's silverbush survive winter outside?

Plant it out within USDA 7-9 and it overwinters with little or no help. It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy. The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

How do I protect boissier's silverbush from frost?

At the cold edge of its range, mulch the root zone in late autumn to buffer the deepest freezes. Protect container specimens — pots freeze through far faster than open ground, costing roughly a zone of hardiness. Shelter new growth from late spring frosts with fleece if a hard night is forecast.