Running Inline Dripline Through a Planted Bed

Litres an Hour Over a Square Metre Is Millimetres an Hour

The conversion that makes drip schedulable is a definition rather than a formula. A litre poured evenly over a square metre stands one millimetre deep. So an emitter rated in litres per hour, divided by the area of bed that emitter is responsible for — its spacing along the tube multiplied by the spacing between laterals — is a depth per hour, in exactly the units a rotor or a spray head is rated in and exactly the units a controller schedule is written in. In imperial the same identity reads one gallon per square foot equals 1.6 inches of depth, which is where the awkward-looking inch-per-hour figures on the right of the table below come from.

The table is arithmetic, not test data. It is worth reading for the shape of it rather than the individual rows: a drip grid lands somewhere in the low teens of millimetres per hour across most sensible product choices, and the choices that look very different on paper — a lighter emitter on a tight grid, a heavier one spread out — land close together. That is the useful property. It means the schedule is not especially sensitive to which of two similar products the wholesaler has in stock, and it means a grid that comes out at double or half the band is telling you the spacing has drifted from what was intended.

One caution about which area the rate belongs to. The figure is the depth applied over the ground the grid actually covers, which is the rectangle bounded by the outermost laterals and the outermost emitters — not the bed outline on the plan. Six runs at four hundred millimetres put their outermost laterals two metres apart on a two and a half metre bed, and the five hundred millimetres left over at the margins receives whatever the bulbs spread into it and nothing else. Work the run time against the gridded area and treat the margins as planting positions to keep away from, not as ground the valve is watering.

It also settles the question of whether this bed can share a station with anything else. It cannot. The grid above applies water several times more slowly than a fixed spray zone in the same garden — compare it against the precipitation rate on the head manufacturer's performance chart rather than against a remembered figure — and it applies it in a different pattern, so a valve carrying both is being asked to run two schedules at once and will be set to whichever half complains first.

How Much Tube One Valve Will Carry

Drip flow accumulates by the metre, which is the one hydraulic habit that separates it from everything else on the manifold. Emitter discharge divided by emitter spacing gives litres per hour for every metre of tube on the bed — 1.6 litre emitters at three hundred millimetres draw 5.3 litres per hour per metre — and the zone total is that figure times the metres laid. Seventy-two metres of tube across a thirty square metre border comes to something near 380 litres per hour, or a little over six litres a minute, which is a small enough number that installers stop checking it and then group four beds onto one station without ever adding them up.

Two limits bind before the flow does. The first is the maximum run length for the product, which every inline dripline data sheet publishes as a table against emitter spacing and inlet pressure — the technical literature for Netafim Techline, Rain Bird XFS, Toro DL2000 and Rivulis inline products all carry one, and the numbers differ enough between them that a habit formed on one brand will overrun another. Pressure-compensating tube holds its rated discharge across a working band and so tolerates a long lateral and a fall along it; non-compensating tube does not, and on a sloping bed its bottom emitters simply discharge more than its top ones. The second limit is the head assembly, which on a drip station is not optional: filtration at the mesh the data sheet calls for, a fixed-outlet pressure regulator chosen at the pressure the product's discharge chart is drawn at, and air relief at the high point so the tube does not draw soil and fines back through the emitters as it drains.

Protection of the supply is a separate matter decided by somebody else, and it should be settled before the trench is opened rather than at handover. The International Plumbing Code deals with it in Section 608, Protection of Potable Water Supply, and in the United Kingdom the Water Supply (Water Fittings) Regulations 1999 and the accompanying Water Regulations Guide treat a garden irrigation system as a high-category risk that a check valve does not answer. The assembly type is the purveyor's call in both cases; the installer's job is to have asked.

The Eleven Litres Already Inside the Tube

A nominal sixteen millimetre dripline has a bore somewhere near thirteen and a half to fourteen millimetres, which the data sheet gives exactly and which works out at roughly a sixth of a litre for every metre of tube. The seventy-two metre bed above therefore holds about eleven litres standing in it, and that volume has to be pushed out to the far end before the last emitter on the run discharges anything at all. On a fifteen-minute cycle it is a rounding error. On a five-minute daily cycle — the schedule an anxious client tends to arrive at — a meaningful fraction of the run is spent filling tube, and the end of the bed is systematically drier than the start for reasons no amount of digging near the valve will reveal.

The same volume leaves again when the valve closes, unless the tube carries integral check valves, and on a bed with any fall it leaves through the lowest emitters. That is the origin of the wet patch at the bottom of a sloping border and of the plant that drowns while the rest of the bed is fine. It is also why the tube gets flushed rather than simply capped: dripline is commissioned by running every lateral open-ended until what comes out is as clean as what goes in, before the end closures go on, and it is flushed again on the annual visit. The microirrigation design literature puts a minimum flushing velocity at the tail of a lateral near a third of a metre per second; on a garden bed, opening the ends in turn and watching the water clear is the same instruction in a form you can carry out on your knees.

Mulch Goes Over the Tube, and Barely Changes the Order

The sequence is fixed and it is the part that gets reversed on busy sites: plant, set out and pin the grid, pressure-test it, flush it, photograph it, and then mulch. Pushing tube underneath an existing mulch blanket is slower, leaves the laterals wandering at whatever depth the fork found, and puts the emitters discharging into bark rather than into soil. The tube belongs on the soil surface, pinned every metre or so and closer on the curves, with the mulch laid over the top of it.

That position is worth defending against the two shortcuts either side of it. Buried in the soil, standard dripline is exposed to root intrusion and needs the root-inhibiting product made for subsurface use rather than the one on the shelf. Laid on top of the finished mulch, it bakes in ultraviolet, lifts as the tube warms and curls, gets dragged out by a rake and is chewed by anything with teeth. Under fifty to seventy-five millimetres of bark it is dark, cool, held down by the weight above it and out of the way of everything except a spade.

The mulch quantity is measured from the soil surface, not from the top of the tube, and the tube takes almost nothing out of it. At laterals four hundred millimetres apart there are two and a half metres of tube on every square metre of bed; sixteen millimetre tube has a cross-sectional area near two hundred square millimetres, so the tube displaces around half a millimetre of depth across the bed. Against a seventy-five millimetre spread that is well under one per cent, and a fifteenth of what a ten per cent settling allowance already carries. Do not deduct it, and do not let anybody spend the afternoon working out how to.

So the order is simply the bed: thirty square metres at seventy-five millimetres is two and a quarter cubic metres, near enough three cubic yards once the settling allowance is on, and the practical question left is what depth the client is buying rather than what the tube did to it. There is one genuine drip-specific argument for going to the deeper end of the mulch band here. Under a spray zone, a dry bark blanket intercepts the first few millimetres of every cycle and the plants get what is left; under drip, the water is discharged at the soil surface beneath the mulch and moves straight into the profile, so the blanket costs nothing and pays back the evaporation it suppresses. Keep it pulled clear of stems and trunk flares as ANSI A300 (Part 2) directs, specify the material against AS 4454 where mulch quality is being written into a contract, and take BS 8545 as the reference for establishment planting where trees are in the bed.

Stone over a drip grid deserves one flag before it is agreed. The grid is retrievable from under bark with a gloved hand; from under fifty millimetres of slate chipping over a barrier it is an afternoon's excavation, and every repair for the life of the bed pays that cost again. If the client wants stone, lay the tube on the soil, fix it properly, cut the barrier over it rather than under it, and photograph the layout with a tape in shot before anything covers it.