Capillary matting in a greenhouse

What capillary matting does well

A capillary mat is a water-transport surface, not a pump or a universal self-watering guarantee. It can help a group of similarly sized pots take up moisture from below while leaving leaves dry. This can be useful on a greenhouse bench where top watering every small pot is fiddly. The RHS houseplant growing guide describes capillary watering as one option for a collection of pots; the same physical principle applies in a greenhouse when the setup is suitable.

For large tomato growbags, deep floor beds and containers with closed bases, drip irrigation or careful hand watering is usually a better fit. The drip guide covers those layouts; capillary matting is primarily a bench-top method for pots and trays.

Build a level, clean bench

Measure the usable staging area, including any lip or obstruction. Put the mat on a flat surface with any waterproof tray or lining the bench manufacturer requires. Uneven support creates dry high spots and wet low spots. Make sure the water source—manual top-up, wicking reservoir or a designed tray—keeps the full mat evenly damp without leaving a stagnant pool over the pot bases. A wide mat may require more than one feed point.

Use clean containers with unobstructed drainage holes. Place the pot bottom directly on the mat; tall feet, a gravel layer beneath the pot or a raised tray can interrupt contact. Moisten dry compost from above at the start if needed so the capillary path is established, then check that moisture reaches the root ball. Do not assume all potting mixes wick at the same rate.

Test the moisture pattern

After a day, lift pots at both ends and the centre of the bench. Feel the mat and check compost below the surface. If one pot is dry, inspect its drainage and contact before adding more water to the whole mat. If several are saturated, reduce the reservoir input or separate those plants onto another bench. Mature plants in hot weather will use water faster than young plants on a cloudy day; an unchanged reservoir can empty unexpectedly.

Root growth into the mat, algae and mineral-salt deposits can develop with long use. Lift and rotate pots as appropriate, clean trays and replace worn matting. Follow plant-specific watering needs; constant wetness is not desirable for every crop. The greenhouse hygiene guide covers keeping benches and reusable equipment clean.

Use it responsibly for absences

Calculate how much water a full bench uses over representative days and test the reservoir as it nears empty. Protect the supply from tipping, leaks and contamination. A mat may keep small pots more even for a short absence, but a hot spell can change demand dramatically. The watering-while-away guide explains backup checks. For sensitive seedlings, use fresh mains water rather than assuming stored rainwater is suitable: the RHS water-storage guidance advises avoiding water-butt water for seedlings because of damping-off risk.