Containers, Drainage And Drying Out

Container growing concentrates every watering problem into a small volume. What the pot is made of and how it is filled decide how forgiving it will be.

A pot is a restricted water reservoir

An open bed has depth and continuity, so water drains away and moves upward from below. A container has neither, and its water content swings between extremes.

This is why the same plant needs daily attention in a pot and almost none in the ground, and why container failures are nearly always water-related.

Small containers swing fastest, which makes them the hardest to keep alive despite being the easiest to buy and place.

Material changes the drying rate

Unglazed terracotta is porous, so water evaporates through the walls as well as from the surface, and plants in it dry out considerably faster.

Glazed ceramic, plastic and metal are impermeable, so they hold moisture longer, which suits thirsty plants and is a liability for anything prone to root rot.

Metal and dark plastic additionally absorb heat in sun, which raises the root temperature and increases water demand well beyond what the plant would face in the ground.

Crocks at the bottom do not improve drainage

Water does not move readily from a fine-textured compost into a coarse layer beneath it, because the difference in pore size holds it back until the compost is saturated.

Adding gravel therefore raises the saturated zone rather than lowering it, and it reduces the volume available for roots at the same time.

What genuinely matters is a free drainage hole and a compost with enough structure, plus standing the pot clear of the surface so water can leave.

Compost that has dried out repels water

Peat-based and coir composts become hydrophobic when they dry completely, so water runs down the gap between the compost and the pot wall and out of the bottom.

The plant then appears watered while the root ball stays dry, which is a common and puzzling cause of decline in a pot that is being attended to daily.

Standing the pot in water until the surface darkens rewets it properly, after which normal watering resumes working.

Grouping, mulching and scale

Pots grouped together shade each other and raise local humidity, which reduces water loss noticeably compared with the same pots spread across a terrace.

A surface mulch of gravel or bark slows evaporation from the top, which is where a large share of the loss occurs in a shallow container.

Choosing a larger pot than seems necessary buys thermal and moisture stability, and for anything intended to stay outdoors permanently that stability matters more than the appearance of the pot.