Galvanized and Metal Planters on a Balcony: Heat, Rust and Drainage

A metal planter works on a balcony if it has drainage holes, a pale or reflective finish in full sun, and a coating that resists rust. Galvanized steel is sturdy and protected by zinc; aluminum is lighter and does not rust. For vegetables, add a draining plastic liner so the mix stays off the metal.

Still choosing a material? Compare metal with wooden planters and ceramic and terracotta planters, or go back to the balcony planters hub. Tall metal styles are covered in tall planters.

How hot do metal planters get in sun?

Metal planters warm up and cool down faster than other pots. University of Illinois Extension says the temperature of metal containers fluctuates more than that of non-metal containers, and that dark colors absorb more heat. University of Maryland Extension warns that dark containers can raise the temperature of the potting mix enough to injure tender roots.

Finish matters as much as material. Ask Extension says shiny galvanized metal reflects heat, so it is less likely to overheat than a black container. A matte black or dark powder-coated planter on a south- or west-facing balcony is the riskiest combination.

Ways to keep roots cooler:

• Choose a bright galvanized, white or pale finish for spots in full afternoon sun.
• Group pots so they shade each other’s sides, or set a metal planter behind a larger plant.
• Use a bigger planter. More potting mix takes longer to heat through.
• Check moisture often in hot spells. Metal holds moisture longer than unglazed terra cotta or wood, University of Maryland Extension says, but heat still dries the top layer.

Galvanized steel vs aluminum and painted steel

Most metal planters are one of three types. The right one depends on how heavy a pot you can move and how long it needs to last outdoors.

The American Galvanizers Association explains that zinc acts as a sacrificial coating: it corrodes in place of the steel, and it keeps protecting steel even where a scratch leaves up to a quarter inch of bare metal. That is why a galvanized tub copes with knocks better than a painted one.

Aluminum is the lighter choice for a balcony where you move pots around. For more on weight, see lightweight planters.

Drainage holes in metal planters

Many decorative metal tubs, buckets and troughs are sold without drainage holes. University of Maryland Extension says every container needs adequate holes or slits to drain excess water, and that more holes may be added depending on the material. Without them, water pools at the base and the roots sit wet.

Do not count on gravel to fix a pot with no holes. The same source says rocks, gravel or broken pot shards in the bottom do not improve drainage. If a planter has no holes and you do not want to drill it, use it as a cover pot: plant into a plastic pot with holes, set it inside, and tip out any water that collects.

Drained water has to go somewhere. Set each planter on a saucer or feet so runoff stays off the floor and the balcony below. See planter saucers and drainage for options.

Liners for galvanized planters and edible crops

A liner keeps potting mix off the metal. University of Illinois Extension says containers can be lined with plastic to reduce water loss, and on a metal pot a liner also adds a layer between the roots and a hot wall.

Flowers and ornamental grasses can go straight into the planter, as long as it drains.

Metal planters in winter and on the floor

Metal does not soak up water the way unglazed pottery does. University of Illinois Extension says it is water freezing in pottery’s pores that cracks those pots. Standing water inside a metal planter can still freeze around the roots, so clear drainage holes matter in winter too. Our guide to overwintering container plants covers wrapping pots and covering plants.

Galvanized steel adds its own weight to the wet mix, so a large steel trough is a floor piece, not something to lift onto a stand. Before you line several up, read balcony weight limits for who can tell you what your balcony carries.

For a narrow balcony, metal wall pockets and stands keep the floor clear. See vertical planters and plant stands.