Lumpy lawns are unsightly, can be difficult to mow and are potentially unsafe. The causes of lumpy lawns include contraction and expansion of soil during the freeze-thaw cycle, turf thinning as the lawn ages, worm castings and mole hills. You might be able to remedy small lumps in your lawn manually but, if you have a larger lawn or a severely uneven surface, you may need to employ mechanical means.
Manual lawn treatment
If your lawn has only a few lumpy spots, caused by the freeze-thaw cycle or by moles, use a shovel and garden rake to level them out.
Check for mole tunnels running underground. Their evidence will be in the form of a winding trail of raised dirt and soil. Smooth these areas down with a rake and firm them gently with your foot.
After flattening these areas, you may find that you have low spots. Fill them in with soil and smooth over with a garden rake.
Mechanical lawn treatment
For an overall smoothing treatment for a lumpy lawn, aerate the soil with a mechanical aerator. The aerator drills or punches thousands of tiny holes in the soil, allowing oxygen, water and nutrients to penetrate the soil more easily. Aeration also loosens the soil in preparation for raking.
Use a power rake to break up thatch and lumpy spots. A power rake uses sharp knives to slice through the lawn thatch and breaks down lumps and cores left by aeration. Power raking and de-thatching are performed by the same machine but serve different functions. De-thatching digs deeper into your lawn than power raking and rips out excess thatch. While de-thatching can be helpful as a one-time treatment for lawns with an exceptionally deep layer of thatch, power raking is a gentler method, allowing your lawn to recover and thicken more quickly.
Rake away loose debris brought up by the power rake, and smooth remaining areas by adding soil or levelling with a garden rake.
Lawn seeding and treatment
Once your lawn has been smoothed, sow grass seed or sod to create a root system that will hold the soil in place. Water regularly until the seed or sod is established, but be careful not to over water.
Eliminate the sources of large holes, such as moles. You can buy effective repellents at garden centres and nurseries.
Do not eliminate worms from your garden. Although their castings can be a nuisance, worms contribute to the health of your lawn through natural aeration and by enhancing the thatch decomposition process. The presence of many worm castings in your garden indicates good soil health. Use a garden rake to smooth out their castings if their numbers become problematic.
Use an appropriate fertiliser on your lawn to help it thicken and form strong root systems.
Do not use a roller to smooth your lawn. While a heavy roller will flatten the lumps, it will also compact the soil, making it more difficult for water and nutrients to penetrate the ground.