Cold hardiness & minimum temperature

Is Ithuriel's Spear (Triteleia laxa)cold hardy? Hardiness zone & min temp

Also called Ithuriel's spear, Grassnut, Triplet lily, Wild hyacinth.

More about ithuriel's spear

About Ithuriel's Spear

Triteleia laxa · also called Ithuriel's spear, Grassnut · flowering

Triteleia laxa is a cormous perennial native to grasslands and open woodlands of California and southern Oregon, producing open umbels of funnel-shaped, pale to deep violet-blue flowers on tall, wiry stems in late spring and early summer as the grass-like leaves die back. It is a tough, drought-tolerant bulb that thrives in full sun to part shade in light, free-draining soil and is well suited to naturalising in gravel gardens, dry borders, and rock gardens. The corms need a dry summer dormancy and should not be overwatered. No formal ASPCA listing for toxicity has been found; treat with caution and classify as mildly-toxic.

Cold limit: USDA 5-9 · RHS H5 (-15–30°C)

What ithuriel's spear's hardiness rating actually means

Yes — ithuriel's spear is genuinely cold hardy. Rated RHS H5 and USDA 5-9, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H5 means: Hardy in most of the UK and in cold winters. On the US scale that maps to USDA 5-9 — 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 −15 to −10 °C. Ithuriel's Spear is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for ithuriel's spear as it gets too cold:

It tolerates winter lows to about −15 to −10 °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 ithuriel's spear go outside or overwinter — and where?

Plant it out within USDA 5-9 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 ithuriel's spear can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Ithuriel's Spear hardiness — frequently asked questions

Is ithuriel's spear cold hardy?

Yes — ithuriel's spear is genuinely cold hardy. Rated RHS H5 and USDA 5-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Ithuriel's Spear is hardy across USDA 5-9; 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 ithuriel's spear can survive?

Minimum survivable temperature is roughly about −15 to −10 °C. Ithuriel's Spear is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is ithuriel's spear?

Ithuriel's Spear is rated USDA 5-9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can ithuriel's spear survive winter outside?

Plant it out within USDA 5-9 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.

What happens to ithuriel's spear below its minimum temperature?

It tolerates winter lows to about −15 to −10 °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.