How to Water a Garden Planter — And the Systems That Will Do It For You

A practical guide to keeping planters watered without standing there with a hose every evening.

A planter is a garden in a box. That's the appeal — you decide the soil, the position, the shape, and you can put one somewhere nothing else would grow. It's also the problem. A border has the whole ground underneath it to draw on. A planter has whatever water is in it, and nothing else.

Which is why the most common reason a planter underperforms isn't the compost, the aspect or the plant choice. It's water. Too little in July, too much in November, and never much thought given to it in between.

This is a guide to getting that right — first by hand, then by letting a system do it. We've named the systems we'd actually recommend and linked to the manufacturers, so you can go and read the spec yourself.

Why planters dry out faster than the ground

Four things are working against you, and they compound.

Limited volume. Our Modern Wooden Garden Planter holds between 70 and 240 litres of compost depending on size. That sounds a lot until you consider that a mature shrub in open ground has cubic metres to root into. When a planter's water is gone, it's gone.

Exposed sides. A planter loses moisture through its surface and its walls, in a way buried soil doesn't. Anything raised on castors or legs loses more again, because air moves underneath it.

Rain shadow. Planters live against walls, under eaves, on covered terraces and beside fences — exactly the places rain doesn't reach. A well-planted trough also acts as its own umbrella: dense foliage sheds rain over the sides rather than into the compost. Gardeners routinely assume a wet week has watered their pots. Usually it hasn't.

Root density. A happy planter is full of roots. Roots displace compost, and compost is what holds the water. By year two or three, the same box holds noticeably less.

The RHS guide to watering containers is the best free reference on the fundamentals, and worth ten minutes of anyone's time.

Get the basics right before you automate anything

Irrigation amplifies whatever your setup already does. If drainage is poor, a watering system will drown the plant faster than you ever could by hand. Sort these first.

Check the drainage. Every planter needs somewhere for surplus to go. Ours are delivered lined, and the liner is there to keep wet compost off the timber — not to hold water in. Before you add any automatic system, satisfy yourself that water poured in the top comes out of the bottom within a minute or two. If it doesn't, you're building a pond.

Don't stand a planter flat on a sealed surface. On paving or decking, lift it slightly — feet, battens, or castors. Water needs an escape route, and timber that sits in a puddle will not last as long as timber that doesn't.

Mulch. Five centimetres of bark, gravel or slate chip over the compost surface cuts evaporation substantially and costs almost nothing. It's the single highest-return thing on this list.

Water long, not often. A short daily splash wets the top few centimetres and encourages shallow roots. A proper soak every few days drives roots down, where the moisture lasts. This principle matters for how you set a timer, too.

Water early or late. Midday watering in summer loses a meaningful share to evaporation before it reaches anything.

The systems, and how each one actually works

There are four genuinely different approaches. They're not competing — plenty of gardens use two.

1. Buried reservoirs: watering from underneath

This is the one most people don't know about, and for deep planters it's often the best answer. Instead of applying water to the surface and hoping it soaks down, you bury a tank below the rootball and let the plant take what it needs.

The mechanism is capillary action. Water moves upward through compost when the pull of dry soil exceeds gravity — the same reason a dry sponge draws water up from a saucer. The compost only draws up what it can hold, which makes over-watering far less likely than with a timer. It does not remove the need for drainage that works.

The Mona Plant System is the best-developed version. Yellow tanks sit under the compost with internal capillary legs, linked by flexible tube, and a single filler pipe brought up to the surface where you can reach it. Mona quote each tank as serving roots roughly 15cm above the reservoir and within about a 40cm spread — which, as it happens, is almost exactly the width of our trough planters.

Their MPS Tank 7 is a good illustration of how well this suits a trough: it's a shallow, linkable unit roughly 55cm long, 15cm wide and 10cm high, holding around 4.5 litres, and several can be chained together off one filler pipe. Lay two or three end to end along the base of a long trough and you have a continuous reservoir under the whole planting — filled from a single tube at one end. Tanks come in a range of sizes, from a few litres to over twenty, in round and rectangular shapes, and a float indicator tells you when to top up. In the UK they're available through Europlanters, made to order.

Ollas are the ancient version of the same idea — documented in Chinese agricultural writing two thousand years ago — and they still work. An unglazed terracotta vessel is buried with its neck above the compost; water seeps through the porous clay into the surrounding soil at whatever rate the soil is dry enough to accept. Henry Watson's Potteries in Suffolk make one from Staffordshire clay holding 1.75 litres — sensible for a single square planter, and no plumbing at all. Terracotta.uk.com make a UK range too.

Blumat, from Austria, splits the difference. A porous clay cone goes into the compost and acts as a sensor: as the soil dries, suction inside the cone opens a valve and draws water from a raised reservoir or bottle through fine tubing. Each cone regulates itself independently, so a mixed planting of thirsty and drought-tolerant plants each gets what it wants from one supply.

Best for: deep planters, trees and shrubs, anywhere without a tap or power. Weakness: you need to install it at planting time — retrofitting means lifting the plant.

2. Drip irrigation on a tap timer

The mainstream option: a 4mm micro-hose run from an outdoor tap through a pressure reducer, with an adjustable dripper staked into each planter, and a battery timer at the tap doing the thinking.

Hozelock's Easy Drip Micro Kit is the straightforward UK starting point — 15 metres of 4mm hose, 15 push-fit micro drippers, T-pieces and a pressure reducer. No timer in the box, deliberately, so you can add whichever controller you like. Kits combine, so a small system grows.

Gardena's Micro-Drip System is the more engineered equivalent, with starter sets specifically configured for plant pots and a wide component range if you want to design something precise.

Best for: several planters in a run, near a tap, where you want it to happen without you. Weakness: it will keep watering in a downpour unless you pay for a rain sensor, and mains restrictions apply.

3. Solar-powered, water-butt fed

Worth its own category, because it solves two problems at once: no tap needed, and no mains water used.

Irrigatia's SOL-C24L runs a small pump from a solar panel, drawing from a water butt or tank. It cycles roughly every three hours and — this is the clever part — adjusts how long it runs according to the light hitting the panel. Bright, hot, drying weather means more water; a dull wet week means less. Irrigatia rate it for up to 24 twenty-litre pots, with drippers up to 5 metres above and 60 metres from the source, and claim water savings of up to 90% against a hose.

Given that hosepipe restrictions have become a regular feature of British summers, a system that runs entirely on stored rainwater is not a small consideration. Check your own supplier's current restrictions before you plan around a tap.

Best for: gardens with a water butt, no convenient outdoor tap, or an eye on restrictions. Weakness: the butt has to have water in it, which in August is not guaranteed.

4. Self-watering inserts and wicking

The pot-scale version of a buried reservoir: a false floor creates a water chamber beneath the compost, and a wick or column of compost draws moisture up. Fill via a tube, watch a float gauge.

Excellent for smaller planters and herbs. Less suited to large timber planters, where the fixed reservoir depth is small relative to the volume of compost and you'd do better with a Mona tank or an olla.

Choosing between them

System	Water source	Power	Best suited to	Effort to fit
Mona tanks	Filled by hand	None	Deep planters, trees, shrubs	Fit at planting
Olla	Filled by hand	None	Single planters, veg, herbs	Minutes
Blumat	Raised reservoir	None	Mixed plantings, holidays	Low
Hozelock / Gardena drip	Mains tap	Battery timer	Runs of planters near a tap	A half-day
Irrigatia solar	Water butt	Solar	No tap, or water restrictions	A half-day
Self-watering insert	Filled by hand	None	Small pots, herbs	Minutes

Fitting irrigation to a timber planter

Most irrigation guidance assumes a plastic pot. A few things are worth knowing when the planter is wood and lined.

Bring the pipe over the rim, not through the side. It's tempting to drill a neat hole. Resist it. Running 4mm hose up the back corner and over the top edge is invisible from any normal viewing angle, keeps the liner intact, and leaves nothing to seal or re-seal in five years. If you must go through, go through the back face high up and above the compost line.

Never puncture the liner below soil level. The liner keeps saturated compost off the timber. A hole low down defeats the whole point of it and puts constant moisture against end grain.

Keep drippers off the timber. Aim them into the compost, not at the inside face, and make sure nothing drips down the outside — repeated wetting in one spot will mark a painted finish long before it does anything structural.

Depth is your friend. Our planters are 40 to 48cm deep for a reason: root room. That depth is also what makes sub-irrigation viable, because there's genuine space to bury a reservoir beneath the rootball. In a shallow pot there isn't. If you're planting a tree in a Large Square Planter or filling a Tall Garden Planter for screening, put a tank in while it's empty. You will not want to do it later.

Allow slack for castors. If your planter is on wheels and you move it with the seasons, leave a generous loop of feeder pipe, or fit a quick-release connector.

Group by thirst, not by looks. One timer waters everything on the line at the same rate. If a lavender and a hydrangea share a circuit, one of them is unhappy. Either match the planting across a run, or use adjustable drippers and set them individually.

Common questions

Do I still need to water by hand? In the first few weeks after planting, yes — new roots haven't reached the reservoir or the dripper zone yet. Water in thoroughly at planting whatever system you fit.

How much water does a large planter need in summer? As a working rule, a well-planted 175-litre trough in full sun in a British July wants something in the region of 10–15 litres every two or three days — and daily in a heatwave. It varies enormously with planting and weather, so check rather than guess: push a finger or a bamboo cane in to about a third of the planter's depth. In a 48cm planter the top few centimetres are dry almost all summer and tell you nothing useful; it's what's happening at 15cm that matters.

Can I leave a system running while I'm away? That's largely the point. Test it for a full week before you go, though, not the morning of.

What about winter? Turn timers off and drain the lines before the first frost. Planters need far less water in winter, and the greater risk in a British winter is waterlogging, not drought.

The short version

Sort drainage and mulch first. Then match the system to the planter: buried tanks for deep planters with permanent planting, drip on a timer for a run of seasonal colour near a tap, solar from a butt if there's no tap or a hosepipe ban.

All of our planters are made in the UK from FSC-certified pressure-treated timber, delivered fully assembled and lined, in depths that give roots — and reservoirs — room to work. You can see the full range of wooden garden planters here, or talk to us about a bespoke size if you're planning an irrigated run along a wall or terrace.