Most comparisons of planter materials are written by people who sell planters, so they compare surfaces. What matters if something is going to live in the container is different: how fast it dries, whether it survives a freeze with wet soil in it, and whether you can move it once it is full.
Unglazed terracotta: the only piece in our range with nothing sealing the clay. Restaged from the maker's photograph.
The three you will actually be choosing between
Large outdoor planters at this scale come down to three materials in practice. Our own range of thirty-nine runs thirty-three glazed pottery, one unglazed terracotta, two fibreglass and one further ceramic, which is roughly the proportion the market offers.
Sealed wall. Holds water longest, heaviest of the three.
Porous. Breathes and dries fastest, weathers with age.
Does not breathe, does not crack, a fraction of the weight.
Glazed pottery
Fired clay with a glaze over it. The glaze is what changes the behaviour: it seals the wall, so a glazed pot holds water far longer than a bare clay one and needs watering less often. For a boxwood, which resents drying out at the root ball, that is the right side of the trade.
It cuts both ways. A sealed wall means water leaves only through the top and through the drainage hole, so a glazed planter with poor drainage stays wet at the bottom for a long time, and boxwood roots in standing water rot. Drainage matters more on a glazed pot than on any other kind.
Weight is the other consideration. A large glazed piece is heavy empty and a serious object full. Site it before you fill it.
On frost: glazed pottery is fired and is not damaged by cold in itself. What damages any pot is water freezing inside the wall or inside the soil mass, which is an argument for drainage and for not letting a planter stand in a saucer of water through winter.
Unglazed terracotta
Bare fired clay with nothing sealing it. It is porous, so the pot itself breathes and the soil dries from the sides as well as the top.
That porosity is a genuine advantage for anything that hates wet feet, and a genuine nuisance for anything that hates drying out. Boxwood is the second. A boxwood in unglazed terracotta in an exposed sunny position will need watering noticeably more often than the same plant in a glazed pot beside it.
Terracotta also weathers rather than staying as it arrived: it picks up a patina, and in cold climates it is the material most likely to spall if it goes into winter saturated. Ours is one piece, the Pithari urn, and it is the right choice for someone who wants the surface to age.
Fibreglass
The practical option, and the one the other two cannot match on weight. A fibreglass planter is a fraction of the weight of a glazed piece the same size, which is what makes it the answer for a roof terrace, a balcony, or anywhere the piece has to be lifted after it is planted.
It does not breathe, so it behaves like glazed pottery for watering. It does not crack in frost. It is the least characterful of the three at close range and the most forgiving in every practical respect, and both of our fibreglass pieces are the large architectural shapes, a cube and a rectangular box, where that reads as deliberate rather than as a compromise.
What the finishes do, and what is not damage
Three things people report as faults are characteristic of the ranges they belong to.
High-gloss glazes develop a fine network of surface lines over time. This is crazing, it happens because glaze and clay body contract at slightly different rates, and it does not weaken the piece. It is worth keeping such a planter away from sudden temperature swings, but a crazed gloss glaze is behaving normally.
Some ranges are finished with bare clay deliberately left at the rim and the base, unglazed for contrast against the glazed body. That is a design decision by the maker, not a missed patch.
And textured glazes carry small pits and craters that imitate natural stone. That is the intended surface. Which of these applies to a given piece is stated on its own page, and the whole question is summarised on the planters category page.
Choosing, in one paragraph
If a living plant is going in it and the position gets sun, choose glazed pottery and make sure it drains. If you want the pot to age and you are prepared to water more often, choose terracotta. If weight is a constraint, or the planter has to be moved once planted, choose fibreglass and stop worrying about the rest.
None of this decides whether the plant fits, which is a separate question with a separate answer in what size planter a topiary needs, and none of it decides how the plant is put in, which is the planting method.
