Lawn renovation: scarifying, coring and topdressing

Over the years, most lawns build up problems you can’t fix with mowing, feeding and watering alone. A layer of dead stems and runners builds up at the surface and makes the lawn spongy. Soil packs down under feet and play. Hollows and bumps appear. Water runs off instead of soaking in.

Renovation is a reset. It removes the thatch, opens up the soil, and smooths the surface, so the lawn can grow back thicker and healthier. It looks drastic for a few weeks, but done at the right time it’s one of the best things you can do for a tired lawn.

The three jobs

You don’t always need all three. Many lawns only need coring, or coring and a light topdress. Scarifying is the most drastic step, and it’s for lawns that have built up a real thatch problem.

Does your lawn need it?

• Sponginess. Push your hand into the lawn. If it feels springy, like a mattress, there’s a thick layer of thatch.
• Scalping. If the mower keeps cutting into brown stems even on a normal setting, the growing points have risen up on a thatch layer.
• Water sitting on top or running off, especially on a slope.
• Compaction. Push a screwdriver into moist soil. If it’s hard to push in, the soil is compacted.
• Thin grass in high-traffic areas, such as paths, play areas and along the fence where the dog runs.
• An uneven surface that scalps on the bumps and grows long in the hollows.

To look at the thatch, cut out a small wedge of lawn with a spade and look at the layer between the green leaves and the soil. A thin layer is normal and even helpful; a thick, matted layer is the problem.

When to renovate

Clemson University Extension advises renovating warm-season lawns in late spring to early summer, while they are actively growing. That’s when buffalo, couch, kikuyu and zoysia recover fastest. University of Maryland Extension makes the same point about coring: aerate only when the grass is growing vigorously, never when it’s dormant.

Avoid renovating:

• going into autumn and winter in cooler zones, when the lawn can’t recover before it slows down;
• in a heatwave or drought, when the lawn is already stressed;
• under tight water restrictions, because the lawn needs regular water to recover;
• soon after a pre-emergent, unless its label says it’s fine. Scarifying and coring disturb the treated layer.

In tropical areas, time renovation for when the lawn is growing strongly and you can water it, rather than by the calendar.

Plan the job

• Mark sprinkler heads and shallow cables, such as garden lighting, with flags so machines don’t hit them.
• Measure the lawn with the Lawn Area Calculator.
• Order topdressing with the Topdressing Calculator. Ask your supplier for the product’s actual density, because sand weighs roughly 1.3 to 1.5 tonnes per cubic metre depending on grade and moisture.
• Book hire equipment. Scarifiers and core aerators are heavy; check you can get them into the yard and onto a trailer.
• Have fertiliser ready, and calibrate your spreader for it.
• Water the lawn a day or two before coring, so the soil is moist but not muddy. Dry soil is hard to core, and very wet soil is messy to work.

Step 1: Lower the mowing height gradually

Over the two or three weeks before you renovate, lower the mowing height a notch at each mow. Never take off more than a third of the leaf at once. A sudden, very low cut scalps the lawn and sets it back further than it needs to.

Step 2: Scarify

1. Set the depth on the scarifier. Start shallow and check the result on a small area. The aim is to pull out thatch and dead runners, not to rip out the living lawn. [CONFIRM: reviewer to confirm scarifying depth guidance by grass type]
2. Make a pass across the whole lawn, in straight lines.
3. Make a second pass at an angle to the first, if the thatch is thick.
4. Rake up the debris. There will be a lot more than you expect.
5. Dispose of it carefully. Runner fragments of couch and kikuyu can take root in garden beds, so put debris in the green waste bin rather than in compost you’ll use in the garden.

Grass type matters

• Couch and kikuyu spread above and below ground, and recover quickly from wear. They cope with firmer scarifying.
• Buffalo spreads only by surface runners, with no underground runners to regrow from. Scarify it more gently, so you don’t remove the runners it grows back from.
• Zoysia grows slowly, so damage takes longer to fill in. Be gentle and give it longer to recover.

For small areas, a hand scarifying rake does the same job with more effort.

Step 3: Core aerate

1. Use a hollow-tine (core) aerator, which pulls out plugs of soil and leaves open holes for air, water and topdressing to get in.
2. Make passes across the lawn, with a second pass at an angle on badly compacted areas.
3. Leave the cores on the surface to dry and break down, then rake or drag them in, or rake them up if you’re topdressing straight away.

Coring lets air, water and fertiliser reach the roots. Water Corporation notes that aeration helps water and air reach roots, particularly on lawns that have dried out. If water-repellent soil is a problem, this is a good time to apply a wetting agent.

Step 4: Feed

Spread a lawn fertiliser at the label rate with a calibrated spreader, and water it in as the label directs. The lawn needs nutrients to regrow. Don’t use more than the label says: a renovated lawn is more easily burnt.

Step 5: Topdress

Clemson University Extension recommends light topdressing: no more than about half an inch (roughly 13 mm) at a time, raked so it sifts down between the leaf blades.

1. Tip small piles across the lawn from a wheelbarrow.
2. Spread them with the back of a rake or a levelling rake.
3. Work the material into the holes and between the leaves with a levelling rake or stiff broom.
4. Check the leaves still show. Grass buried completely can’t get light and may die.
5. Fill deeper hollows over several applications, a few weeks apart, rather than all at once.

How much to order

The Topdressing Calculator does the sums: area times depth gives volume. For example, a 100 m² lawn topdressed 5 mm deep needs 100 × 0.005 = 0.5 cubic metres, plus a little extra for spillage. At an indicative 1.3 to 1.5 tonnes per cubic metre for sand, that’s roughly two-thirds of a tonne to three-quarters of a tonne. Ask your supplier for their product’s actual density, which changes with the grade and how wet it is.

Choosing material

Clemson advises using sand to topdress only where your native soil is sandy, and otherwise matching your existing soil. Very different materials in layers can hold or shed water differently. Ask your landscape supplier about products made for lawn topdressing, and avoid material with weed seeds, stones or clods.

Step 6: Water and wait

• Water well straight after topdressing, to settle the material and wash it off the leaves.
• Keep the soil moist while the lawn regrows, then return to deep, less frequent watering.
• Keep traffic off until the lawn has grown back through.
• Mow once the grass has grown above your mowing height, on a higher setting, and lower it gradually to your normal height.

Recovery is fastest in the peak of the growing season. Expect the lawn to look rough for a few weeks.

Light renovation options

Not every lawn needs the full treatment.

Safety

• Read the hire company’s instructions for each machine, and ask for a demonstration.
• Wear eye and hearing protection, gloves and sturdy closed shoes.
• Keep children and pets away while machines are running.
• Lift with care. Scarifiers, aerators and bags of topdressing are heavy.
• Check for underground services if any part of the job involves digging deeper than the surface.

Common mistakes

• Renovating in autumn or winter, when the lawn can’t recover.
• Scalping the lawn in one go instead of lowering the height gradually.
• Scarifying too deep, especially on buffalo and zoysia.
• Topdressing too thickly and burying the leaves.
• Skipping the feed and water afterwards.
• Renovating straight after a pre-emergent without checking its label.
• Hitting sprinkler heads with the machines.

Log it

Log the renovation in your Lawn Journal, with the date, which jobs you did, the machines’ settings, and how much topdressing and fertiliser you used. Take a photo before, straight after, and a few weeks later. Next time, you’ll know what worked and how long the lawn took to recover.