Quick answer: a metal planter freezes from the sides inward and reaches solid freeze two to three weeks earlier than the same plant in plastic or wood. That alone rarely kills anything, because most hardy perennials tolerate frozen soil. What kills them is freeze-thaw cycling while sitting wet. Sink the pot into the ground or bury it under 6 to 8 inches of mulch, keep it out of winter sun, and stop watering only once the soil is actually frozen.
Why a metal pot freezes differently
Metal moves heat far faster than soil or plastic, so the walls of an iron planter pull cold straight into the root zone instead of insulating it. In practice, the outer 2 inches of mix in a metal pot can be frozen hard while the core is still workable, and the whole profile hits solid freeze weeks before a same-sized plastic pot does.
That is not automatically bad. Frozen is not the same as damaged. The damage comes from the cycle: every time sun warms the metal, the outer soil thaws and refreezes, and each pass breaks fine feeder roots. Ten cycles do more harm than one hard freeze, which is why the sunniest spot in the garden is the wrong place for a metal container in January.
Which plants actually survive it
The number that matters is root hardiness, usually two zones colder than the air-hardiness figure printed on the tag. A plant labelled zone 5 is generally described as surviving zone 4, and that two-zone cushion is your entire margin.
In a container above ground, subtract roughly two zones again, because the roots no longer benefit from the ground’s stored heat. So a zone 6 perennial in a metal pot left out through zone 5 conditions is genuinely at risk. The reliable group is evergreens, sedums, sempervivums, hardy herbs such as thyme and oregano, and compact woody shrubs like Japanese holly. Tender perennials from zone 8 upward, and all annuals, should be lifted before the first frost rather than gambled on.
Three ways to protect the roots
The best option is heeling in. Dig a hole in a border or vegetable bed, drop the whole pot in so the rim sits just above grade, and backfill. Ground soil holds the root zone near 32°F all winter regardless of air temperature, and the pot lifts straight out in spring.
The second is a mulch collar. Group pots together against a wall, then pile 6 to 8 inches of leaves or bark over the top and around the sides, leaving a gap at the crown so nothing smothers it. Grouping matters more than the mulch itself: three pots touching share warmth, while one pot standing alone in the open does not.
The third is an unheated garage or a windbreak. An attached garage that stays above roughly 25°F is close to ideal, but only if you water occasionally, because the plant is dormant rather than dead.
Watering, drainage, and the thaw
Dormant does not mean dry. A plant that enters winter with dry soil desiccates from the roots up, especially an evergreen that keeps transpiring through mild spells. Water thoroughly in late autumn, then stop as soon as the surface is frozen. Never water into a frozen pot — the water sits, freezes, and expands.
Check drainage before the freeze, too. A pot with a blocked hole lets water pool at the bottom, and that ice jack lifts the whole root ball. It is the same failure that splits metal bird baths in winter, and both are worth reading together before the first hard frost.
For the wider seasonal routine, our fall checklist for metal garden decor walks through what to lift, cover, and store. Surface rust on the planter itself is not a problem for the plant, and the wheelbarrow planter is a good example of a container built to be left out through winter.