How to use and calibrate a broadcast spreader

A broadcast spreader throws granules in a wide arc from a spinning plate. Used well, it puts fertiliser, granular pre-emergent or granular wetting agent down evenly across the lawn. Used without calibration, it’s the most common cause of stripes and fertiliser burn. The good news is that calibrating takes about ten minutes, and once you’ve done it for a product you only need to check it each season.

Broadcast or drop spreader?

Broadcast (rotary) spreaders throw granules out to the sides and front from a spinning plate under the hopper. They cover the lawn quickly and blend the edges of each pass, so small errors in spacing don’t show as much. The trade-off is that granules can land on paths and garden beds unless you use an edge guard.

Drop spreaders let granules fall straight down through slots between the wheels. They’re precise along edges and won’t throw product onto paths, but each pass has to meet the last one exactly or you’ll see stripes.

Hand-held spreaders are small broadcast spreaders you crank by hand. They suit small lawns and patch work.

Most home lawns suit a push-along broadcast spreader. This guide focuses on those, but the calibration steps work for any type.

Why calibrate?

Bag labels sometimes give spreader settings, but only for particular spreader models. Your spreader, the size and weight of the granules, how full the hopper is and how fast you walk all change the output. Two products with the same rate can need very different settings, because granules differ in size and density.

Calibration tells you what your setup actually puts out, so you can match the label rate. Without it, you’re guessing, and a guess that’s double the rate can burn a lawn while one that’s half the rate wastes your time and money.

Before you start

1. Read the label. Note the rate, whether to water in, and any warnings about heat, wet leaves or lawn stress.
2. Measure your lawn with the Lawn Area Calculator, so you know how much product you need in total.
3. Work out the total amount with the Product Application Calculator. This also tells you whether you have enough product before you start.
4. Check the spreader. The gate should open and close fully, the plate should spin freely, and the hopper should be clean and dry.

Calibrate in ten minutes

1. Mark out a test area, for example 10 m² on a tarp or a clean driveway. A 5 m × 2 m rectangle is easy to measure.
2. Weigh some product and fill the hopper over the tarp. Note the weight you started with.
3. Set the spreader to the bag’s suggested setting for your model, or a middle setting if there isn’t one.
4. Walk the test area at your normal pace, opening the spreader as you start and closing it as you finish. Walk at the same speed you’ll use on the lawn.
5. Sweep up and weigh what landed in the test area. Alternatively, weigh what’s left in the hopper and subtract it from what you started with.
6. Use the Spreader Calibration Helper. It works out grams per square metre and tells you whether to open up or close down to match the label rate.
7. Repeat at the adjusted setting until you’re within about 10% of the label rate. Note the setting on the bag or in your Lawn Journal for next time.

Put the swept-up product back in the bag. If you calibrated on a driveway, sweep thoroughly so no granules are left to wash into the drain.

Measuring the throw width

To space your passes correctly, you need to know how wide your spreader throws this product.

1. Lay a strip of tarp across your path, or use a clean driveway.
2. Walk across it at your normal pace with the spreader open.
3. Measure from where the granules start to where they stop.

Space your passes so the edge of one pass just reaches the wheel track of the last. That overlap evens out the lighter edges of the throw pattern.

Spreading pattern

• Do the edges first. Walk one or two passes around the perimeter. If your spreader has an edge guard, use it near garden beds, paths and water.
• Fill in the middle in parallel lines, spaced so the throw patterns meet as described above.
• Even out the result: a common technique is to apply half the rate in one direction and the other half at right angles. Set the spreader to deliver half the rate, then cover the lawn twice. Small errors cancel out instead of forming stripes.
• Close the spreader whenever you stop or turn, so product doesn’t pile up.
• Keep a steady pace. If you slow down with the gate open, more product lands in that spot.
• Keep the hopper reasonably full. The output can change as the hopper empties, so top it up rather than running it nearly dry.

Keep it off hard surfaces

Granules on paths and driveways end up in stormwater drains with the next rain. Research on residential stormwater has found that larger and longer rain events carry more fertiliser nitrogen off urban properties. So:

• fill the spreader over a tarp or on the lawn’s edge, not on the driveway;
• use the edge guard near hard surfaces;
• sweep up any strays and put them back on the lawn.

Spreading different products

The same spreader handles several jobs, but each product needs its own calibration.

• Granular fertiliser: feed only while the lawn is actively growing, and water in as the label says. Overlaps are what cause burnt stripes, so take care at the turns. See the granular fertiliser guide.
• Granular pre-emergent: even coverage matters even more, because gaps in the treated layer become patches of weeds. Use the half-rate criss-cross method. See the pre-emergent guide.
• Granular wetting agent: usually needs watering in straight away to reach the soil. See the wetting agents guide.
• Grass seed: many spreaders can sow seed too. Seed is lighter than fertiliser, so the throw is narrower and the setting is different. Calibrate with the seed itself, and use the sowing rate on the bag.

Never mix two products in the hopper unless both labels say you can.

Safety

Even fertiliser can irritate skin and eyes. Wear the gloves and eye protection the label lists while filling the hopper, and wash your hands afterwards. For chemical products such as pre-emergents and grub treatments, follow every safety direction on the label, keep children and pets off the lawn until the label’s re-entry conditions are met, and store the product locked away in its original container.

Weather and timing

• Wind changes the throw pattern and carries fine particles onto paths. Spread on a calm day.
• Wet leaves can hold granules against the grass, where some products can scorch. Unless the label says to apply to wet foliage, spread when the leaves are dry.
• Heat increases the risk of burn from fertilisers. Spread in the cooler part of the day.
• Rain: light rain after spreading can help water a product in, but heavy rain can wash it away. Check the forecast and the label.

Watering in

Follow the label. Most granular fertilisers and many pre-emergents need watering in to move them off the leaves and into the soil. Do it straight away if the label says so. If you can’t water in, choose another day.

Look after the spreader

Rinse and dry it after use, especially after fertiliser, which corrodes metal parts and can clog the gate. Run it empty for a few seconds to clear the plate, then store it with the gate closed, out of the weather. Check the gate linkage each season: if it has stretched or slipped, your old settings won’t match.

Common mistakes

• Filling the hopper on the lawn. A spill burns a patch. Fill it on a tarp or a hard surface and sweep up anything you drop.
• Leaving the gate open while you stop, turn or adjust the spreader.
• Walking at a different pace from your calibration run. The output changes with your speed.
• Trusting the bag’s setting without checking it on your spreader.
• Spreading right up to water, such as ponds, creeks or gutters. Use the edge guard, or stop short.
• Going over the lawn again to use up leftover product. Store it instead.

Troubleshooting

Log it

Record the product, the setting, the area, the amount and the date in your Lawn Journal. Next time you use the same product, the setting is already there, and the journal can warn you if you’re about to re-apply sooner than the label allows.